Plant Disease

Published by American Phytopathological Society
Online ISSN: 0191-2917
Publications
A, Probability of Sclerotinia stem rot (SSR) prevalence, caused by Sclerotinia sclerotiorum, in no-or minimum-till (even and odd passes) soybean fields with July precipitation, using the parameter estimates in Table 4. Seed treatment and manure application were equal to zero (no application ). B, Difference in expected yield between no-and minimum-till fields with or without manure application, using the parameter estimates of Iowa yield quantification (Table 4). For the calculations , July, August, and September precipitation was considered equal to 6 cm, average air temperature of July and August was equal to 20°C, clay content was equal to 30%, and seed treatment and SSR incidence were equal to 0.  
List of variables used in soybean Sclerotinia stem rot (caused by Sclerotinia sclerotiorum) prevalence model and yield quantification in the north- central region of the United States
A, Probability of Sclerotinia stem rot (SSR) prevalence, caused by Sclerotinia sclerotiorum, and B, soybean yield in no-till fields in Minnesota with herbicide application, using the parameter estimates in Table 7. C, Probability of SSR prevalence, caused by Sclerotinia sclerotiorum, and D, soybean yield in no-till fields in Minnesota without herbicide application, using the parameter estimates in Table 7. For all calculations, June, July, August, and September precipitation was considered equal to 12 cm, clay content was equal to 30%, SSR incidence was equal to 60%, seed treatment, fertilizer, and manure applications were equal to 0.  
Article
Regional prevalence of soybean Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum,was modeled using management practices (tillage, herbicide, manure and fertilizer application,and seed treatment with fungicide) and summer weather variables (mean monthly air temperature and precipitation for the months of June, July, August, and September) as inputs. Logisticregression analysis was used to estimate the probability of stem rot prevalence with disease datafrom four states in the north-central region of the United States (Illinois, Iowa, Minnesota, and Ohio). Goodness-of-fit criteria indicated that the resulting model explained well the observedfrequency of occurrence. The relationship of management practices and weather variables withsoybean yield was examined using multiple linear regression (R2 = 0.27). Variables significant to SSR prevalence, including average air temperature during July and August, precipitationduring July, tillage, seed treatment, liquid manure, fertilizer, and herbicide applications, werealso associated with high attainable yield. The results suggested that SSR occurrence in the north-central region of the United States was associated with environments of high potential yield. Farmersï¾’ decisions about SSR management, when the effect of management practices ondisease prevalence and expected attainable yield was taken into account, were examined.Bayesian decision procedures were used to combine information from our model (prediction)with farmersï¾’ subjective estimation of SSR incidence (personal estimate, based on farmersï¾’previous experience with SSR incidence). MAXIMIN and MAXIMAX criteria were used to incorporate farmersï¾’ site-specific past experience with SSR incidence, and optimum actions were derived using the criterion of profit maximization. Our results suggest that management practices should be applied to increase attainable yield despite their association with high disease risk.
 
A, Healthy safflower leaf and B, Cercospora beticola (C2)-infected safflower leaves. Lesions are characterized by light-brown round to irregular spots. The dark-brown to black border is noticeable around some of the disease lesions which were positively shown to contain C. beticola. C, Fructifications in leaf spot caused by C. beticola (C2) in safflower. 
Article
Safflower is an oilseed crop adapted to the small-grain production areas of the western Great Plains, including the Northern Plains Area (NPA). In the NPA, safflower production is being evaluated for potential rotation with sugar beet. Safflower is susceptible to Cercospora carthami, whereas sugar beet is susceptible to C. beticola C. carthami has not been observed on safflower in the NPA but C. beticola is ubiquitous on sugar beet. Observation of unusual leaf spots on irrigated safflower cv. Centennial at Sidney, MT prompted this investigation of safflower as a potential alternate host of C. beticola. Safflower plants were inoculated with four isolates of C. beticola (C1, C2, Sid1, and Sid2) and incubated in growth chambers; leaf spot symptoms appeared between 3 and 4 weeks later. Polymerase chain reaction (PCR) amplification of extracts from lesion leaf tissue with C. beticola-specific primers produced fragments comparable with amplified fragments from purified cultures of control C. beticola. PCR assay of cultures of single spores from diseased safflower leaf lesions also produced fragments comparable with fragments from C. beticola cultures. Antibody that was raised from isolate C2 also bound to antigens from the single-spore cultures of the four C. beticola isolates. Inoculum from single-spore cultures from infected safflower also infected sugar beet and produced typical Cercospora leaf spot symptoms. Assay of these leaf lesions by PCR resulted in amplification of target fragments with the C. beticola-specific primers. Our results demonstrate that safflower is a new host of C. beticola.
 
Phylogenetic tree constructed by the Neighbor-Joining method of 16S rRNA gene sequences from 27 phytoplasmas and Acholeplasma laidlawii, employing A. laidlawii as the outgroup. The numbers on the branches are bootstrap (confidence) values. Phytoplasma strain designations are in parentheses.
Article
A previously undescribed phytoplasma, Erigeron witches'-broom phytoplasma, was detected in diseased plants of Erigeron sp. and Catharanthus roseus exhibiting symptoms of witches'-broom and chlorosis in the state of Sao Paulo, Brazil. On the basis of restriction fragment length polymorphism (RFLP) analysis of 16S rDNA amplified in the polymerase chain reaction (PCR), Erigeron witches'-broom phytoplasma was classified in group 16SrVII (ash yellows phytoplasma group), new subgroup VII-B. Phylogenetic analysis of 16S rDNA sequences indicated that this phytoplasma represents a new lineage that is distinct from that of described strains of ash yellows phytoplasma. Erigeron witches'-broom phytoplasma is the first member of the ash yellows phytoplasma group to be recorded in Brazil.
 
Article
Phytoplasmas (mycoplasmalike organisms, MLOs) associated with mitsuba (Japanese honewort) witches'-broom (JHW), garland chrysanthemum witches'-broom (GCW), eggplant dwarf (ED), tomato yellows (TY), marguerite yellows (MY), gentian witches'-broom (GW), and tsuwabuki witches'-broom (TW) in Japan were investigated by polymerase chain reaction (PCR) amplification of DNA and restriction enzyme analysis of PCR products. The phytoplasmas could be separated into two groups, one containing strains JHW, GCW, ED, TY, and MY, and the other containing strains GW and TW, corresponding to two groups previously recognized on the basis of transmission by Macrosteles striifrons and Scleroracus flavopictus, respectively. The strains transmitted by M. striifrons were classified in 16S rRNA gene group 16SrI, which contains aster yellows and related phytoplasma strains. Strains GW and TW were classified in group 16SrIII, which contains phytoplasmas associated with peach X-disease, clover yellow edge, and related phytoplasmas. Digestion of amplified 16S rDNA with HpaII indicated that strains GW and TW were affiliated with subgroup 16SrIII-B, which contains clover yellow edge phytoplasma. All seven strains were distinguished from other phytoplasmas, including those associated with clover proliferation, ash yellows, elm yellows, and beet leafhopper-transmitted virescence in North America, and Malaysian periwinkle yellows and sweet potato witches'-broom in Asia.
 
Article
Phytoplasmal diseases have long been suspected to occur in several potato-growing regions in Russia on the basis of symptoms and the presence of insect vectors. Symptoms resembling stolbur are most prevalent, but round leaf disease, potato witches'-broom, and potato purple top wilt also occur (1). The phytoplasma etiologies of these diseases have never been verified by molecular means. During the summer of 2006, 33 potato plants exhibiting various symptoms including purple top, round leaves, and stolbur-like symptoms characterized by purple top, stunting, bud proliferation, and formation of aerial tubers were randomly collected from the Volga River Region, Central Region, and Northern Caucasian Region in Russia. DNA extracts were prepared from 1.0 g of petioles and leaf mid veins according to a modified procedure with the Qiagen DNeasy Plant Mini Kit (Qiagen, Valencia, CA) as previously described (2). A nested PCR with primer pair P1/P7 in the first amplification followed by R16F2n/R16R2n in the second am...
 
Article
Plants of commercial strawberry (Fragaria × ananassa Duch., cv. Camarosa) exhibiting extensive fruit phyllody (development of leafy structures from achenes) were observed in a winter greenhouse production facility in West Virginia. In July 2001, 95 dormant, cold-stored plants were purchased from a California strawberry nursery, potted and grown in this West Virginia facility. Five of the plants developed fruits with phylloid growths. These fruits were assessed for phytoplasma infection using nested polymerase chain reactions (PCRs) in which initial ribosomal (r) DNA amplification was primed by phytoplasma-universal primer pair P1/P7 (2), and rDNA reamplification was primed by primer pair R16F2n/R16R2 (1). Amplification of phytoplasma-characteristic 1.2-kbp 16S rDNA in the nested reactions primed by R16F2n/R16R2 confirmed that the symptomatic plants were infected by a phytoplasma, termed strawberry phylloid fruit (StrawbPhF) phytoplasma. No phytoplasma DNAs were amplified from healthy plants. Restriction f...
 
Article
Pterocarya stenoptera C. DC., commonly known as Chinese wingnut, is a fast-growing deciduous tree with tough bark and attractive foliage. Because of its tolerance of compact and nutritionally poor soil, drought, and heat, Chinese wingnut is an important component of the biological diversity in natural ecosystems and is a favorable shade tree in China. Chinese wingnut has also been used as a rootstock for walnuts because of its high resistance to soilborne Phytophthora spp. In the spring of 2004, a disease characterized by witches'-broom symptoms was observed affecting Chinese wingnut trees growing in suburban Taian, Shandong, China. The diseased trees developed dense clusters of highly proliferating branches with shortened internodes, leaves on the affected branches were significantly smaller, and some branches and twigs suffered dieback. Phytoplasma infection was suspected as the cause of this Chinese wingnut witches'-broom (CWWB) disease because the disease occurred in an area where phytoplasmal diseases, such as paulownia witches'-broom (PaWB) and jujube witches'-broom (JWB), are common (3). Nested polymerase chain reactions (PCR) were performed on DNA samples extracted from leaves of six diseased trees using phytoplasma-universal 16S rDNA primers (R16mF2/R16mR1 and R16F2n/ R16R2) (1,2). Results revealed that all diseased trees examined were infected by phytoplasma, whereas PCR assays of leaf samples from two nearby symptomless Chinese wingnut trees were negative. Subsequent restriction fragment length polymorphism analysis of the PCR-amplified 16S rDNA indicated that all diseased trees contained the same phytoplasma and that the CWWB phytoplasma belongs to subgroup B of the "Candidatus Phytoplasma asteris" (AY) group (16SrI). Nucleotide sequence analysis of a 16S rRNA gene cloned from CWWB phytoplasma (GenBank Accession No. AY831966) suggested that this phytoplasma is closely related to, but distinct from, PaWB phytoplasma, another member of group16SrI. To our knowledge, this is the first report of Chinese wingnut witches'-broom disease and of its association with a phytoplasma. Further work is being undertaken to examine the ecological and evolutionary relationship between CWWB phytoplasma and other phytoplasmas in the region and to assess the impact of CWWB on walnut rootstock selection. References: (1) D. E. Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:144, 1996. (2) C. D. Smart et al. Appl. Environ. Microbiol. 62:2988, 1996. (3) S. Zhu et al. Acta Hortic. 472:701, 1998.
 
Article
Purple coneflower (Echinacea purpurea (L.) Moench) is a flowering perennial plant native to North America and is widely grown as an ornamental flower. It is also grown commercially to make herbal teas and extracts purported to help strengthen the immune system. Propagation is by seed or root cuttings. Aster yellows phytoplasmas (strains belonging to group 16SrI) have been reported to be associated with purple coneflower exhibiting virescence and phyllody symptoms in the northern United States and Canada. A subgroup 16SrI-A phytoplasma was identified to be associated with symptomatic purple coneflower in Wisconsin (2). During the summers of 1994 and 2007, purple coneflower plants in Maryland sporadically exhibited symptoms resembling those caused by phytoplasma infection. Symptoms included stunting, virescence, phyllody, and abnormal flower bud proliferation from the cone. Samples from four symptomatic and two asymptomatic purple coneflower plants were collected. Total nucleic acid was extracted from leaf ...
 
Article
http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS.2002.86.10.1177A Symptoms of phyllody of flowers and general plant yellowing indicating possible phytoplasma infection were observed in diseased plants of hairy willow-weed (Epilobium hirsutum L., family Onagraceae) growing in a meadow at Harku Village near Tallin, Estonia. DNA was extracted from diseased E. hirsutum using a Genomic DNA Purification Kit (Fermentas AB, Vilnius, Lithuania) and used as a template in nested polymerase chain reaction (PCR). Ribosomal (r) DNA was initially amplified in PCR primed by phytoplasma universal primer pair P1/P7 (4) and reamplified in PCR primed by nested primer pair 16SF2n/16SR2 (F2n/R2) (1) as previously described (2). Products of 1.8 kbp and 1.2 kbp were obtained in PCR primed P1/P7 and F2n/R2, respectively, from all four symptomatic plants examined. These data indicated that the diseased E. hirsutum plants were infected by a phytoplasma, termed epilobium phyllody (EpPh) phytoplasma. The 16S rDNA amplified in PCR primed by nested primer pair F2n/R2 was subjected to restriction fragment length polymorphism (RFLP) analysis using restriction endonucleases AluI, MseI, HpaI, HpaII, HhaI, RsaI, HinfI, and HaeIII (Fermentas AB). On the basis of the collective RFLP profiles, EpPh phytoplasma was classified in group 16SrI (aster yellows phytoplasma group), subgroup B (aster yellows phytoplasma subgroup), according to the phytoplasma classification scheme of Lee et al. (3). The 1.8-kbp rDNA product of P1/P7-primed PCR, which included 16S rDNA, 16S-23S intergenic spacer region, and the 5′ -end of 23S rDNA, was cloned in Escherichia coli using the TOPO TA Cloning Kit (Invitrogen, Carlsbad, Ca) according to manufacturer's instructions and sequenced. The sequence was deposited in the GenBank database as Accession No. AY101386. This nucleotide sequence shared 99.8% sequence similarity with a comparable rDNA sequence (GenBank Accession No. AF322644) of aster yellows phytoplasma AY1, a known subgroup 16SrI-B strain. The EpPh phytoplasma sequence was highly similar (99.9%) to operons rrnA (GenBank Accession No. AY102274) and rrnB (GenBank Accession No. AY102273) from Valeriana yellows (ValY) phytoplasma infecting Valeriana officinalis plants in Lithuania. ValY phytoplasma was found to exhibit rRNA interoperon sequence heterogeneity (D. Valiunas, unpublished data). To our knowledge, this is the first report to reveal E. hirsutum as a host of phytoplasma and to demonstrate the occurrence of a plant pathogenic mollicute in the northern Baltic region. References: (1) D. E. Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:144, 1996. (2) R. Jomantiene et al. HortScience 33:1069, 1998. (3) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (4) B. Schneider et al. Phlogenetic classification of plant pathogenic mycoplasma-like organisms or phytoplasmas. Page 369 in: Molecular and Diagnostic Procedures in Mycoplasmology, Vol 1, R. Razin, and J. G. Tully eds. Academic Press, San Diego, 1995. October 2002, Volume 86, Number 10 Page 1177 http://dx.doi.org/10.1094/PDIS.2002.86.10.1177A First Report of a Group 16SrI, Subgroup B, Phytoplasma in Diseased Epilobium hirsutum in the Region of Tallin, Estonia A. Alminaite, Plant Virus Laboratory, Institute of Botany, Zaliuju ezeru 49, Vilnius LT-2021, Lithuania; R. E. Davis, Molecular Plant Pathology Laboratory, USDA-Agricultural Research Service, Beltsville, MD 20705; and D. Valiunas and R. Jomantiene, Plant Virus Laboratory, Institute of Botany, Zaliuju ezeru 49, Vilnius LT-2021, Lithuania
 
Article
Potato purple top (PPT) is a devastating disease that occurs in the United States, Canada, Mexico, Russia, and elsewhere causing great economic loss to the potato industry through substantially reduced tuber yield and quality. Chips and fries processed from infected tubers often develop brown discoloration, greatly reducing their marketability. At least seven distinct phytoplasma strains belonging to five different phytoplasma groups (16SrI, 16SrII, 16SrVI, 16SrXII, and 16SrXVIII) have been reported to cause purple top and related symptoms in potato (3). During an unusual drought in 2007, a newly emerging potato disease with extensive yellowish or reddish purple discoloration of terminal shoots and leaves, similar to PPT symptoms, was observed in isolated potato fields in Montana where over 50% of plants exhibited symptoms. Shoot tissues were collected from three symptomatic plants and 17 tubers randomly collected from 17 other symptomatic plants. The tubers were cold treated to induce sprouting and then ...
 
Article
Broussonetia papyrifera, commonly known as paper mulberry, is an ornamental tree that is native to northeastern Asia. Because of its fast-growing nature and tolerance of dust, smoke, and high temperatures, paper mulberry is an important component of the biological diversity in natural ecosystems as well as a favorable shade tree in the region. In September of 2003, a disease characterized by pronounced witches'-broom symptoms was observed in paper mulberry trees growing near a jujube (Ziziphus jujuba) orchard and in home gardens located in Taian, Shandong, China. The diseased trees developed dense clusters of highly proliferating branches with shortened internodes. Leaves on the affected branches were chlorotic and greatly reduced in size. Phytoplasma infection was first suspected in this paper mulberry witches'-broom (PMWB) disease because the disease occurred in an area where other phytoplasmal diseases, including jujube witches'-broom (JWB) disease and paulownia witches'-broom (PaWB) disease, are common (4). Results from nested polymerase chain reactions (PCR), performed using phytoplasma-universal 16S rDNA primers (P1/P7 and R16F2n/R16R2) (1,2,3), revealed that all seven diseased trees tested contained phytoplasma, whereas PCR assay of comparable leaf samples from three nearby symptomless paper mulberry trees were negative. Subsequent restriction fragment length polymorphism (RFLP) analysis of the PCR-amplified 16S rDNA indicated that all diseased trees contained the same phytoplasma and that the PMWB phytoplasma belongs to the subgroup B of the elm yellows (EY) phytoplasma group (16SrV-B). Nucleotide sequence analysis of the cloned PMWB phytoplasma partial rRNA operon (GenBank Accession No. AY576685), spanning a near full-length 16S rRNA gene, a 16S-23S rRNA intergenic spacer, a tRNA-Ile gene, and a partial 23S rRNA gene, suggested that PMWB phytoplasma is most closely related to JWB phytoplasma, a member of the subgroup16SrV-B. To our knowledge, this is the first report of a paper mulberry witches'-broom disease and the first report of its association with a phytoplasma. Further work is underway to determine whether the PMWB phytoplasma is distinct from previously characterized phytoplasmas included in group 16SrV and to assess impacts of the phytoplasma on the ecosystems in the region.
 
Article
Surveys of commercial soybean fields, disease nurseries, and trial plots of soybean were conducted throughout eastern Australia between 1979 and 1996, and 694 isolates of Phytophthora sojae were collected and classified into races. Fourteen races, 1, 2, 4, 10, 15, and 25, and eight new races, 46 to 53, were identified, but only races 1, 4, 15, 25, 46, and 53 were found in commercial fields. Races 1 and 15 were the only races found in commercial fields in the soybean-growing areas of Australia up until 1989, with race 1 being the dominant race. Race 4 was found in central New South Wales in 1989 on cultivars with the Rps1a gene, and it is now the dominant race in central and southern New South Wales. Races 46 and 53 have only been found once, in southern New South Wales, and race 25 was identified in the same region in 1994 on a cultivar with the Rps1k gene. Only races 1 and 15 have been found in the northern soybean-growing regions, with the latter dominating, which coincides with the widespread use of cultivars with the Rps2 gene. Changes in the race structure of the P. sojae population from commercial fields in Australia follow the deployment of specific resistance genes.
 
Article
Wheat stem rust caused negligible yield losses in 1997 and 1998. Overwintering sites were found in central and east-central Louisiana in 1997, and in northwestern Florida, northeastern Louisiana, and central Texas in 1998. Race Pgt-TPMK predominated in 1997 with 69% of 100 isolates with race RCRS next at 11%. In 1998, race RCRS predominated with 55% of 132 isolates, and TPMK occurred at 10%. Race QFCS occurred at 8% in 1997 and 31% in 1998. Races QCCS and QTHJ were found in 1997, and races QFBS, RKMQ, RKQQ, and RCMS were found in 1998. Race QCCJ, virulent to barley with the Rpg1 gene for stem rust resistance, was not found from wheat in 1997 or 1998. No virulence was found to wheat lines with Sr 13, 22, 24, 25, 26, 27, 29, 30, 31, 32, 37, Gt, or Wld-1. Oat stem rust was found earlier in 1997 than 1998, but was more widespread in 1998. Race NA27, virulent to Pg-1, -2, -3, -4, and -8, was the predominant race in the United States in 1997 (79% of 116 isolates) and again in 1998 (79% of 116 isolates). NA16, virulent to Pg-1, -3, and -8, was found in the south (1997 and 1998), and NA5, virulent to Pg-2 and -15, and NA10, virulent to Pg-2, -3, and -15, were found in the west (1997).
 
Article
Frequent epidemics of leaf rust in Egypt have been attributed to the appearance of new races virulent on commonly grown wheat cultivars. In 1998, 1999, and 2000, 726 isolates of Puccinia triticina collected in Egypt were tested on a set of 20 single Lr gene differential wheat lines, and '160 races were identified. Three races, MBDLQ, MCDLQ, and TCDMQ, were found in Egypt in all 3 years. Race MCDLQ occurred at >20% frequency each year. Virulences to wheat lines with Lr1, 3, 10, 14b, 15, 17, 23, and 26 occurred at >45% each year. Seven races found in Egypt also were found in either Israel, Sudan, Turkey, or Romania in 1998 or 1999, although the one race common to Sudan and Egypt was rare in Egypt (only 1 year, <1%). Four races found in Israel also were found in Egypt, and the similarity of virulence frequencies in Israel and Egypt indicate at least some exchange of inoculum. Romania and Turkey did not appear to be major sources of inoculum for leaf rust epidemics in Egypt. The level of genetic diversity in leaf rust collections in Egypt in 1998 to 2000 was similar to that of collections from the Southern and Central Plains of the United States in 1998 to 2000. The high diversity of races and the recurrence of common races in each year in Egypt as in the Southern and Central Plains of the United States is consistent with oversummer survival of P. triticina within Egypt or in a neighboring country. The buildup of races virulent on cultivars with the most commonly used Lr genes for resistance in Egypt also is consistent with year-round survival within Egypt or cyclical exchange of inoculum between Egypt and a neighboring country.
 
Article
Collections of Puccinia triticina were made from rust-infected wheat leaves in Georgia, South Carolina, North Carolina, and Virginia in 1999 to examine if these states can be considered as a single epidemiological unit for virulence phenotypes of the wheat leaf rust pathogen. Single-uredinial isolates derived from the leaf rust collections were processed for identification of virulence phenotypes on seedling plants in greenhouse tests. Twenty-one virulence phenotypes from 253 isolates were described based on infection type to 16 Thatcher wheat lines near-isogenic for leaf rust resistance genes. Virulence phenotype MBRK (virulent to leaf rust resistance genes Lr1, Lr3, Lr3ka, Lr11, Lr30, Lr10, Lr14a, and Lr18) was the most common phenotype in the region, at 38.7% of all isolates. Phenotype TLGF (virulent to Lr1, Lr2a, Lr2c, Lr3, Lr9, Lr11, Lr14a, and Lr18) was the second most common phenotype overall, at 33.8% of isolates. Twenty-nine isolates selected on the basis of seedling virulence phenotypes also were tested for virulence to adult wheat plants with the resistance genes Lr12, Lr13, Lr22b, and Lr34. In all, 23 isolates were avirulent to Lr12 and 26 isolates were virulent to Lr13. All isolates had fewer and smaller uredinia on the Thatcher line with Lr34 compared with Thatcher. The widespread occurrence of the predominant P. triticina virulence phenotypes throughout the region indicated that the South Atlantic states should be considered as a single epidemiological area for wheat leaf rust. Some virulence phenotypes which occurred at lower frequencies were found primarily in the Coastal Plain and mountains of North Carolina or in breeding plots in southern Georgia. Localized populations of P. triticina may develop in the South Atlantic region due to overwintering of leaf rust infections or specific selection by leaf rust resistance genes in wheat cultivars.
 
An agarose gel with individual fingerprint profiles of Penicillium digitatum isolates from random amplified polymorphism DNA analysis using primer omt1. Lanes 1 and 18 each contain a 100-bp marker, lane 11 is a negative control, and lanes 2 to 10 and 12 to 17 are profiles from P. digitatum isolates.
Origin of isolates collected from California packinghouses and citrus trees and the history of thiabendazole (TBZ) use
Article
Thiabendazole (TBZ) is commonly applied to harvested citrus fruit in packinghouses to control citrus green mold, caused by Penicillium digitatum. Although TBZ is not used before harvest, another benzimidazole, thiophanate methyl, is commonly used in Florida and may be introduced soon in California to control postharvest decay of citrus fruit. Isolates from infected lemons and oranges were collected from many geographically diverse locations in California. Thirty-five isolates collected from commercial groves and residential trees were sensitive to TBZ, while 19 of 74 isolates collected from 10 packinghouses were resistant to TBZ. Random amplified polymorphic DNA analysis indicated that the isolates were genetically distinct and differed from each other. Nineteen TBZ-resistant isolates and a known TBZ-resistant isolate displayed a point mutation in the beta-tubulin gene sequence corresponding to amino acid codon position 200. Thymine was replaced by adenine (TTC leads to TAC), which changed the phenylalanine (F) to tyrosine (Y). In contrast, for 49 TBZ-sensitive isolates that were sequenced, no mutations at this or any other codon positions were found. All of the isolates of P. digitatum resistant to TBZ collected from a geographically diverse sample of California packinghouses appeared to have the same point mutation conferring thiabendazole resistance.
 
Wheat cultivars used to differentiate races of Puccinia striiformis f. sp. tritici Differential Cultivar 
Estimated yield losses of wheat due to stripe rust (Puccinia striiformis f. sp. tritici) in 2000 
Number of collections of wheat stripe rust and PST races of Puccinia striiformis f. sp. tritici identified in the United States in 2000 
Article
Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is most destructive in the western United States and has become increasingly important in the south-central states. The disease has been monitored by collaborators through field surveys and in disease nurseries throughout the United States. In the year 2000, stripe rust occurred in more than 20 states throughout the country, which was the most widespread occurrence in recorded history. Although fungicide applications in many states reduced yield losses, the disease caused multimillion dollar losses in the United States, especially in Arkansas and California. One of the prevalent cultivars, RSI 5, had a yield loss of about 50% in the Sacramento-San Joaquin Delta region of California. In the Pacific Northwest, wheat losses due to stripe rust were minimal because cultivars with durable resistance were widely grown and the weather in May 2000 was not favorable for the disease. To identify races of the pathogen, stripe rust collections from 20 states across the United States were analyzed on 20 wheat differential cultivars, including Clement (Yr9, YrCle), Compair (Yr8, Yr19), and the Yr8 and Yr9 near-isogenic lines. In 2000, 21 previously identified races and 21 new races were identified. Of the 21 new races, 8 were pathotypes with combinations of virulences previously known to exist in the United States, and 13 had virulences to one or more of the lines Yr8, Yr9, Clement, or Compair. This is the first report of virulence to Yr8 and Yr9 in the United States. Most of the new races were also virulent on Express. Races that are virulent on Express have been identified in California since 1998. The races virulent on Yr8, Yr9, and Express were widely distributed in California and states east of the Rocky Mountains in 2000. The epidemic in 2000 demonstrates that increased efforts to breed for stripe rust resistance are needed in California, the south-central states, and some other states in the Great Plains. Diversification of resistance genes and use of durable resistance should prevent large-scale and severe epidemics.
 
Number of isolates per race of Puccinia coronata f. sp. avenae from single-uredinial isolates collected in the U.S. winter and spring oat regions from 2001 through 2005. Races were determined using the differential cultivars and nomenclature of Chong et al. (4). 
Virulence frequencies (%) to 28 oat Pc genes in populations of Puccinia coronata f. sp. avenae collected in the winter and spring oat regions of the United States from 2001 through 2005. Asterisks *, **, and *** indicate that the difference in virulence frequency in the two oat-production regions of the United States to a given Pc gene is significant at the 0.05, 0.01, and 0.001 level of probability, respectively, as determined by Fisher's exact test. 
Significant changes in virulence fre- quencies in the oat crown rust (Puccinia coro- nata f. sp. avenae) populations from the winter and spring oat regions of the United States from 2001 through 2005 a 
Virulence frequencies (%) to 28 oat Pc genes in populations of Puccinia coronata f. sp. avenae collected from wild oat (Avena fatua) and domesticated oat (A. sativa) in the A, spring and B, winter oatproduction areas of the United States from 2001 through 2005. Asterisks *, **, and *** indicate that the difference in virulence frequency in collections from the two sources to a given Pc gene is significant at the 0.05, 0.01, and 0.001 level of probability, respectively, as determined by Fisher's exact test. 
Article
In all, 680 single-pustule isolates of the oat crown rust pathogen, Puccinia coronata f. sp. avenae, were collected from cultivated and wild oat (Avena sativa and A. fatua, respectively) in the major oat-production areas of the United States from 2001 through 2005. They were tested for virulence on seedlings of differential oat lines in the greenhouse. In all, 171 races were found among the 357 isolates from the winter oat region of the United States, whereas 212 races were found among 323 isolates from the spring oat region. The crown rust population derived from winter oat in the southern United States was distinct from the spring oat population in the upper Midwest, although there was no virulence unique to either population. Virulence to Pc48 and Pc52 increased significantly in both regions during 2001 to 2005. Virulence to Pc59 increased and virulence to Pc53 decreased in the winter oat region during the same period. Many of the virulence associations previously reported in the U.S. oat crown rust population in the early 1990s also were found in both regions in this survey. Associations between virulence to the Pc genes were predominately positive in both regions; however, both positive and negative associations occurred more frequently in the winter oat region. Much of the virulence diversity in the oat crown rust population in the United States can be related to the deployment of resistance genes in commercial oat cultivars and virulence associations existing in the oat crown rust population. The mean virulence of the U.S. populations of crown rust continued to increase from 2001 to 2005. Genes for crown rust resistance derived from A. sterilis appear to be rapidly defeated, as has happened to Pc genes from A. sativa.
 
Efficacy of fungicide treatments. Photo by Wilfrido Morel.  
Expansion of rust in 2002. Fig. 4. Expansion of rust in 2003.  
Expansion of rust in 2002. Fig. 4. Expansion of rust in 2003.  
Race–2003 sporulating on cotyledons.  
Article
In 5 March 2001, a severe rust outbreak was recorded at Pitapó, Paraguay, and the causal organism was determined to be Phakopsora pachyrhizi using polymerase chain reaction (PCR) and DNA sequence analysis. In May, rust surveys showed spread throughout most of Paraguay and into western and northern Parana, Brazil. In the 2001-02 season, rust was widespread in Paraguay, but losses were reduced due to severe drought; however, in Brazil it spread to more than 60% of the soybean acreage, causing field losses estimated at 0.1 million metric tons (MMT). In 2003, the disease was observed in more than 90% of the fields in Brazil, and the projected losses in Mato Grosso and Bahia alone are 2.2 MMT (US$487.3 million). Approximately 80% of the soybean acreage in Brazil was sprayed twice with fungicides at the cost of US$544 million. Differences in efficacy have been observed among the commercial strobilurin and triazol fungicides.
 
Distribution of the wheat stripe rust epidemic (shaded area) in China in the 2001–2002 growing season. Provinces that are mentioned in the text are marked.  
Wheat genotypes used to differentiate races of Puccinia striiformis f. sp. tritici in China 
Frequencies of races of Puccinia striiformis f. sp. tritici from 1997 to 2002. A, Predominant races occurring at frequencies from 2.5 to 35%, B, races at frequencies from 1.5 to 2.3%, C, races at frequencies from 0.9 to 1.4%, and D, races at frequencies from 0.5 to 0.8% in 2002. Races that had frequencies less than 0.5% are not shown.  
Infection types (IT) produced by race CYR32 and some 2002 representative isolates of Puc- cinia striiformis f. sp. tritici collected from Henan, Sichuan, and Yunnan provinces on the world, European, and supplemental differential genotypes IT produced by isolates of P. striiformis f. sp. tritici b
Stripe rust severities and infection types of wheat cultivars in trap nurseries in various locations in 2002 Stripe rust severity (%) and infection type (in parentheses) of wheat culitvars at various locations b 
Article
In China, wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases of wheat and can cause severe yield losses when susceptible cultivars are grown and weather conditions are favorable for the disease. Wheat stripe rust most frequently affects the winter wheat growing areas in Northwest, Southwest, and North China, and the spring wheat growing areas in Northwest China. In the 2001-2002 growing season, a widespread stripe rust epidemic affected about 6.6 million hectares of wheat in 11 provinces: Sichuan, Chongqing, eastern Gansu, southern and western Shaanxi, southern and central Ningxia, Yunnan, Guizhou, Hubei, Henan, southern and central Hebei, and Shandong. The epidemic could be attributed to relatively warm weather from November 2001 to March 2002, high frequencies of stripe rust races CYR31 and CYR32, and widely grown susceptible cultivars. Race CYR31 was virulent on the Chinese differential cultivars Trigo Eureka, Fulhard, Lutescens 128, Mentana, Virgilio, Abbondanza, Early Premium, Funo, Danish 1, Fengchan 3, Lovrin 13, Shuiyuan 11, Lovrin 10, and Hybrid 46. Race CYR32 had all the virulence factors of CYR31, plus virulences on Chinese differential cultivars Jubilejina 2 and Kangyin 655, i.e., CYR32 was virulent on all differential cultivars, except Zhong 4. When tested on the world and European differential and some other resistant genotypes, CYR32 was virulent on Chinese 166 (Yr1), Heines VII (Yr2, Yr25, and YrHVII), Vilmorin 23 (Yr3a and Yr4a), Heines Kolben (Yr6 and YrHK), Lee (Yr7, Yr22, and Yr23), Clement (Yr9, Yr25, YrCle), VPM1 (Yr17), Selkirk (Yr27), Anza (YrA), Carstens V (YrCV1, YrCV2, and YrCV3), Gaby (YrG), Strubes Dickkopf (Yr25), and Suwon 92/Omar (YrSO). Resistance genes in Triticum spelta album (Yr5), Zhong 4, and Moro (Yr10 and YrMor) were effective against all races identified.
 
Article
Collections of Puccinia triticina were obtained from rust-infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, Southeast, California, and the Pacific Northwest, in order to determine the virulence of the wheat leaf rust fungus in 2002. Single uredinial isolates (785 in total) were derived from the wheat leaf rust collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17, Lr30, LrB, Lr10, Lr14a, and Lr18. In the United States in 2002, 52 virulence phenotypes of P. triticina were found. Virulence phenotype MBDS, which is virulent to resistance gene Lr17, was the most common phenotype in the United States. MBDS was found in the Southeast, Great Plains, and the Ohio Valley regions, and also in California. Phenotype MCDS, virulent to Lr17 and Lr26, was the second most common phenotype and occurred in the same regions as MBDS. Virulence phenotype THBJ, which is virulent to Lr16 and Lr26, was the third most common phenotype, and was found in the southern and northern central Great Plains region. Phenotype TLGJ, with virulence to Lr2a, Lr9, and Lr11, was the fourth most common phenotype and was found primarily in the Southeast and Ohio Valley regions. The Southeast and Ohio Valley regions differed from the Great Plains regions for predominant virulence phenotypes, which indicate that populations of P. triticina in those areas are not closely connected. The northern and southern areas of the Great Plains were similar for frequencies of predominant phenotypes, indicating a strong south to north migration of urediniospores.
 
Article
Collections of Puccinia triticina were obtained from rust infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, Southeast, California, and the Pacific Northwest, in order to determine the virulence of the wheat leaf rust fungus in 2003. Single uredinial isolates (580 in total) were derived from the wheat leaf rust collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17, Lr30, LrB, Lr10, Lr14a, and Lr18. In the United States in 2003, 52 virulence phenotypes of P. triticina were found. Virulence phenotype MBDS, which has been selected by virulence to resistance gene Lr17, was the most common phenotype in the United States. MBDS was found in the Southeast, Great Plains, the Ohio Valley, and California. Virulence phenotype THBJ, which has been selected by virulence to genes Lr16 and Lr26, was the second most common phenotype, and was found in the southern and northern central Great Plains region. Phenotype MCDS, which has been selected by virulence to genes Lr17 and Lr26, was the third most common phenotype and occurred in the same regions as MBDS. The use of wheat cultivars with leaf rust seedling resistance genes has selected leaf rust phenotypes with virulence to genes Lr9, Lr16, Lr17, Lr24, and Lr26. The population of P. triticina in the United States is highly diverse for virulence phenotypes, which will continue to present a challenge for the development of wheat cultivars with effective durable resistance.
 
Article
Stem rust of small grain cereals, caused by Puccinia graminis, is a major disease of wheat, barley, and oat. In order to effectively utilize stem rust resistance in the improvement of small grain cereals, it is necessary to monitor the virulence composition and dynamics in the stem rust population. Races of P. graminis from barberry, wheat, barley, and oat were surveyed across the United States during 2003. Aecial infections on barberry were primarily due to P. graminis f. sp. secalis, as inoculations using aeciospores failed to produce infection on wheat and oat. Race QFCS of P. graminis f. sp. tritici was the most common race identified from wheat and barley. Race QFCS has virulence on stem rust resistance genes Sr5, 8a, 9a, 9d, 9g, 10, 17, and 21 that are used for race identification. Race TTTT was identified in 2003. This race possesses virulence to all 16 stem rust resistance genes present in the wheat stem rust differentials and should be targeted in breeding for stem rust resistance. Race QFCN appeared to be a new race in the U.S. stem rust population. Races QCCJ and MCCF were identified, but at low frequencies. Seven races of P. graminis f. sp. avenae were identified from oat, and races NA-27, NA-29, and NA-67 were the predominant races. Race NA-76 was identified for the first time in the United States.
 
Article
Collections of Puccinia triticina were obtained from rust-infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, southeast, California, and Pacific Northwest, in order to determine the virulence of the wheat leaf rust population in 2004. Single uredinial isolates (757 in total) were derived from the collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3a, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17a, Lr30, LrB, Lr10, Lr14a, Lr18, Lr21, and Lr28, and winter wheat lines with genes Lr41 and Lr42. In the United States in 2004, 52 virulence phenotypes of P. triticina were found. Virulence phenotype MCDSB, selected by virulence to resistance genes Lr17a and Lr26, was the most common phenotype in the United States and was found in all wheat growing areas. Virulence phenotype TBBGG, with virulence to Lr2a, was the second most common phenotype and was found primarily in the spring wheat region of the north-central states. Virulence phenotype MBDSB, which has virulence to Lr17a, was the third most common phenotype and was found in all wheat growing areas except California. Phenotype TNRJJ, with virulence to genes Lr9, Lr24, and Lr41, was the fourth most common phenotype and occurred in the southeastern states and throughout the Great Plains region. Virulence phenotypes avirulent to a second gene in the Thatcher differential line with Lr1 increased in frequency in the United States in 2004. The highly diverse population of P. triticina in the United States will continue to present a challenge for the development of wheat cultivars with effective durable resistance.
 
Article
The soybean cyst nematode, Heterodera glycines, is a serious economic threat to soybean producers in Missouri. Periodic monitoring for the presence, population densities, and virulence phenotypes of H. glycines is essential for determining crop losses and devising management strategies implementing the use of resistant cultivars. A survey using area-frame sampling was conducted to determine the distribution and virulence phenotypes of H. glycines in Missouri during 2005. Two samples from each of 125 fields representing eight geographical regions of Missouri were collected; 243 samples were processed for extraction of cysts and eggs. In all, 49% of samples had detectable cyst nematode populations, which ranged from 138 to 85,250 eggs per 250 cm3 of soil. Race and H. glycines type tests were conducted on populations from 45 samples. Nearly 80% of the populations that were tested, irrespective of the region, were virulent on the indicator line plant introduction (PI) 88788, which is the source of resistance for most H. glycines-resistant cultivars. More than 70% of populations could reproduce on the indicator lines PI 88788, PI 209332, and PI 548316 (Cloud), indicating that soybean cultivars with resistance derived from these sources need to be carefully monitored and used only in rotation with nonhost crops and soybean cultivars with resistance from other sources. Approximately one-third of the populations, primarily in the southern regions of Missouri, could reproduce on PI 548402 (Peking), another common source of resistance. Fewer than 10% of the populations could reproduce on PI 90763, PI 437654, PI 89772, or PI 438489B, suggesting that these sources of resistance should be used in soybean breeding programs to develop H. glycines-resistant soybean cultivars.
 
Article
Collections of Puccinia triticina were obtained from rust infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, Gulf Coast, and Atlantic Coast States in order to determine the virulence of the wheat leaf rust fungus in 2000. Single uredinial isolates (1,120 in total) were derived from the wheat leaf rust collections and tested for virulence phenotype on 16 lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes. In the United States in 2000. 54 virulence phenotypes of P. triticina were found. Virulence phenotypes MBDS and MCDS, which are virulent to resistance gene Lr17, were the first and third most common phenotypes in the United States and were found in the Great Plains and the Ohio Valley regions. MCRK, which is virulent to Lr26, was the second most common phenotype and was found primarily in the Southeast, Ohio Valley, and Northeast regions. In the northern area of the Great Plains, phenotypes with virulence to Lr16 increased in frequency from 1998 and 1999. The Southeast and Great Plains regions had different predominant virulence phenotypes, which indicates that populations of P. triticina in those areas are not closely connected. The northern and southern areas of the Great Plains region had the same predominant virulence phenotypes, indicating movement of virulence phenotypes of P. triticina within this region.
 
Article
Sunflower rust caused by Puccinia helianthi (Schw.) is widespread in North America and occurs annually on cultivated sunflower (Helianthus annuus L.) and wild annual and perennial Helianthus spp., although severity on the U.S. sunflower crop has been increasing in recent years (2). P. helianthi is a autoecious, macrocyclic rust, but the aecial stage is rarely observed in the field (1,3,4). In most years, the earliest appearance of sunflower rust in North Dakota (ND) and surrounding states usually occurs in early August as the uredinial stage. Initial inoculum can result from urediniospores that overwinter in the Northern Great Plains, urediniospores blown in from areas south of North Dakota, or basidiospores completing the life cycle. However, aecia have been noted very infrequently and never widespread, indicating initial inoculum is usually urediniospores. Aecia of P. helianthi were first observed on 24 June 2008 in a commercial sunflower field (confection hybrid CHS 3126) near Mohall, ND. Aecia cups measuring 0.2 to 0.3 mm in diameter were observed in clusters that were 1 to 7 mm wide in diameter and containing as many as 150 cups. Aecia cups were bright orange but turned brown-black as they senesced. As many as 15 aecial clusters occurred on individual leaves or cotyledons. Aeciospores were ellipsoid, hyaline, and measured approximately 20 μm in diameter. On 4 July 2008, uredinia were first observed in the same Mohall, ND field. At that time, uredinia, aecia, and senesced aecia could all be found on the same leaves. In a non-fungicide-treated strip of the field, pustule coverage on the lower leaves was 10 to 20% by mid-July, pustule coverage on the upper four leaves of plants in the untreated strip was 56% by 27 August, and yield at harvest was less than 200 kg/ha. The rest of the field was sprayed twice with fungicides and yielded 1,571 kg/ha, which is similar to the statewide yield average of 1,573 kg/ha in 2008. To determine the prevalence of aecia in the primary growing regions of ND and Minnesota (MN), surveys were conducted in 75 sunflower fields in 18 counties between 22 and 24 July in ND and 34 fields in 8 counties between 17 and 31 July in MN. Incidence of aecia and uredinia were determined by visual observation of a minimum of 20 plants scouted in a 'W' pattern in the field. Rust was found in 31 and 53% of fields in ND and MN, respectively. In fields where rust was found, both aecia and uredinia were observed in 37% of the fields, aecia only in 29% of the fields, and uredinia only in 34% of the fields. Although it is uncertain why aecia were widespread in 2008, night temperatures in Mohall, ND, where aecia were first observed, reached the dew point temperature on 51 of 61 days in June and July, suggesting that dew or fog likely formed. Thus, favorable conditions for germination and infection early in the growing season indicate widespread occurrence of rust was likely a result of local inoculum sources. The early appearance of aecia is cause for concern for two reasons: significant yield loss can occur when rust appears early in the growing season; and the presence of aecia suggest that the pathogen completed its sexual cycle. When P. helianthi completes its life cycle it is likely that a greater diversity of races will result (4).
 
Article
Leaf rust, caused by the fungus Puccinia triticina, is an important disease of soft red winter wheat cultivars that are grown in the southern and eastern United States. The objectives of this study were to identify the leaf rust resistance genes in two soft red winter wheat cultivars, Coker 9663 and Pioneer 26R61, that have been widely grown and were initially highly resistant to leaf rust. Both cultivars were crossed with the leaf-rust-susceptible spring wheat cv. Thatcher and the F1 plants were crossed to Thatcher to obtain backcross (BC1) F2 families. In seedlings, the Thatcher/Coker 9663 BC1F2 families segregated for three independent seedling resistance genes when tested with different leaf rust isolates. The leaf rust infection types of selected BC1F3 lines, when tested with different leaf rust isolates, indicated that seedling resistance genes Lr9, Lr10, and Lr14a were present. In field plot tests, BC1F4 lines that were seedling susceptible had some adult plant resistance to leaf rust. Seedlings of the Thatcher/Pioneer 26R61 BC1F2 families segregated for two independent resistance genes. Infection types of selected BC1F3 lines indicated the presence of Lr14b and Lr26. The adult plant gene Lr13 was determined to be present in selected BC1F4 lines that were tested with different leaf rust isolates in greenhouse tests.
 
Article
Inoculation of an industry standard light sphagnum peat potting mix with Trichoderma hamatum 382 (T382) significantly (P = 0.05) reduced the severity of Botrytis blight, caused by Botrytis cinerea, on begonia plants grown in a greenhouse. In data combined from three experiments, the degree of control provided by T382 did not differ significantly (P = 0.05) from that provided by weekly topical sprays with chlorothalonil. In addition, T382 significantly (P = 0.05) increased shoot dry weight and salability of flowering plants. Incorporation of composted cow manure (5%, vol/vol) into the light peat mix also significantly (P = 0.05) decreased blight severity while shoot dry weight and salability were increased. Blight severity on plants in this compost mix did not differ significantly (P = 0.05) from that on those in the light peat mix inoculated with T382. Finally, T382 and chlorothalonil did not significantly (P = 0.05) affect blight severity, shoot dry weight, or salability of plants grown in the compost mix. Spatial separation was maintained in begonias between the biocontrol agent T382 and the pathogen. It was concluded, therefore, that the decrease in disease severity provided by inoculation of the peat mix with T382 most likely was due to systemic resistance induced in begonia against Botrytis blight. The suppressive effect of the compost mix against Botrytis blight was unusual because composts typically do not provide such effects unless inoculated with a biocontrol agent capable of inducing systemic resistance in plants to disease.
 
Article
In a study of the Phytophthora infestans population in Taiwan, samples with symptoms typical of late blight were collected from field crops in an important potato- (Solanum tuberosum) and tomato-(Lycopersicon esculentum) production area in the central highlands region. Isolates were obtained by surface disinfecting leaf sections and plating them onto antibiotic-amended rye A agar (1). After subculturing, the pathogen was confirmed as P. infestans on the basis of morphological characters (2). Mating type was determined by co-inoculating unamended rye agar plates with mycelial plugs of the test isolate and a reference P. infestans isolate of either the A1 or A2 mating type (four plates per test isolate, two with different A1, and two with different A2 reference isolates). After incubation (15°C darkness, 7 to 14 days), plates were examined microscopically for the presence of oospores where the colonies interacted. In 2004, one isolate of 200 tested, and in 2006, one isolate of 102 tested, produced oospores ...
 
Gel electrophoretogram of polymerase chain reaction (PCR) products generated using primer pair CLi.po.F/O12c from symptomatic potato tuber (lane 1), roots (lanes 2, 3, 9, and 10), midplant leaf midrib (lane 5), midplant petiole (lane 12), and plant apex (lane 11). Lanes 4 and 8 are from root and leaf midrib, respectively, of healthy plant. Lanes L a and L b denote 10-bp and 100-bp DNA ladders, respectively.
Experimental transmission of zebra chip (ZC) disease by psyllids collected from ZC-infected field plants in Pearsall, TX to potato and tomato in greenhouse cages
Transmission electron micrograph of bacteria-like organisms in phloem cells in the stem of A, a field-grown potato plant cv. Atlantic from 2001, and B, a graft-inoculated potato plant cv. Atlantic from 2008 expressing symptoms of the zebra chip disease.
Sample identity and 16s rRNA gene sequence of 'Candidatus Liberibacter solanacearum' subclades amplified by polymerase chain reaction (PCR) from field-infected potato tissue, psyllid-fed greenhouse-grown tomato plants, or psyllids collected from multiple locations during 2005 to 2008
Article
A new disease of potatoes, tentatively named zebra chip (ZC) because of the intermittent dark and light symptom pattern in affected tubers which is enhanced by frying, was first found in Mexico in 1994 and in the southwestern United States in 2000. The disease can cause severe economic losses in all market classes of potatoes. The cause of ZC has been elusive, and only recently has been associated with 'Candidatus Liberibacter' sp. Field samples of potato plants were collected from several locations in the United States, Mexico, and Guatemala to determine transmission to potato and tomato by grafting of ZC-infected scions and psyllid feeding. The disease was successfully transmitted, through up to three generations, by sequential top- and side-grafting ZC-infection scions to several potato cultivars and to tomato. The disease was also successfully transmitted to potato and tomato plants in greenhouse experiments by potato psyllids collected from potato plants naturally affected with ZC. Transmission electron microscopic observation of ZC-affected tissues revealed the presence of bacteria-like organisms (BLOs) in the phloem of potato and tomato plants inoculated by grafting and psyllid feeding. The BLOs were morphologically similar in appearance to BLOs associated with other plant diseases. Polymerase chain reaction (PCR) amplified 16S rDNA sequences from samples representing different geographic areas, including the United States, Mexico, and Guatemala, were almost identical to the 16S rDNA of 'Ca. L. solanacearum' previously reported from solanaceous plants in New Zealand and the United States. Two subclades were identified that differed in two single base-pair substitutions. New specific primers along with an innovative rapid PCR were developed. This test allows the detection of the bacteria in less than 90 min. These data confirm the association of 'Ca. L. solanacearum' with potatoes affected by ZC in the United States, Mexico, and Guatemala.
 
Article
Citrus canker is caused by Xanthomonas axonopodis pv. citri. All grapefruit (Citrus paradisi Macf.) cultivars are highly susceptible to the A-group strains of Xanthomonas axonopodis pv. citri. A type of grapefruit (Citrus sp.) called Dalan Dan (DD) has shown high resistance in the field in Misiones, Argentina where it is widely planted. The DD is Accession No. NC678 in the Campinas Citrus Germplasm Collection (Centro APTA Citrus Sylvio Moreira, Cordeiropolis, SP, Brazil) and Accession No. CCC-618 Dalan Dan in the Argentina Citrus Germplasm Collection at INTA (Concordia, Entre Ríos). Molecular studies completed in Brazil have shown high similarities among grapefruit varieties and DD (1). We report here the characterization of quantitative resistance to citrus canker in DD. Quantitative resistance in citrus is expressed as lower number of lesions per leaf area (3). Lesions per square centimeter were calculated after 3 to 4 weeks of two sets of inoculation of three inoculum levels (5 × 10 ² , 2.5 × 10 ³ , and 5 × 10 ³ cells per ml) in leaves of developing leaf flushes. The period of greatest susceptibility in DD was 15 to 23 days for the three inoculum levels; 1.35 ± 0.34 to 0.01 ± 0.01, 4.98 ± 0.91 to 0.91 ± 0.48, and 9.85 ± 2.17 to 1.91 ± 1.37 lesions per cm ² , respectively. By contrast, the period of greatest susceptibility in grapefruit cv. Duncan was 15 to 29 days for the same three inoculum levels; 2.78 ± 0.46 to 1.22 ± 0.39, 17.29 ± 2.79 to 5.55 ± 0.93, and 22.87 ± 3.27 to 11.95 ± 3.55 lesions per cm ² , respectively. The results shown here suggest that the DD citrus accession is susceptible to citrus canker for a very short time during the development of leaf flushes, which results in very high resistance in the field. This citrus germplasm material could be valuable as a source of resistance to improve other varieties or as a replacement of grapefruit due to the similar fruit and juice qualities. References: (1) M. J. Corazza-Nunes et al. Euphytica 126:169, 2002. (2) M. J. Corazza-Nunes et al. Summa Phytopathol. 32:322, 2006. (3) R. E. Stall et al. Phytopathology 72:1097, 1982.
 
Article
Oryza spp., wild relatives of cultivated rice (Oryza sativa), may contain novel resistance genes for sheath blight, caused by Rhizoctonia solani, that could be used to enhance resistance to this important disease in commercial rice. To identify resistant sources for sheath blight disease, 73 Oryza genotypes were evaluated with three different methods conducted in the greenhouse, growth chamber, or laboratory because there are significant limitations to screening wild Oryza spp. under field conditions. For the microchamber method, 4-week-old seedlings were inoculated with a potato dextrose agar plug containing mycelia, covered with a 2-liter soft drink bottle, and rated 1 week after inoculation. A detached-leaf method involved placing a potato dextrose agar plug containing mycelia on the abaxial surface of a leaf section that was cut from a 5-week-old plant and placed on moist filter paper in a petri dish under constant light, then evaluated after 72 h. For the toothpick inoculation method, toothpicks colonized with mycelia were placed in the leaf collar region of plants at the panicle initiation stage, plants were placed in a growth chamber, and disease symptoms were evaluated after 7 days. The microchamber method gave a more uniform, reproducible response, and was easier to use under greenhouse conditions. Seven Oryza spp. accessions were identified as moderately resistant with three accessions classified as O. nivara (IRGC104705, IRGC100898, and IRGC104443) and one each as O. barthii (IRGC100223), O. meridionalis (IRGC105306), O. nivara/O. sativa (IRGC100943), and O. officinalis (IRGC105979).
 
Distribution of Avena barbata accessions with broad-spectrum resistance to Puccinia coronata in the world.  
Article
The use of race-specific seedling genes for resistance has been the primary means of controlling crown rust of oat (Puccinia coronata). As resistance genes from hexaploid cultivated oat, Avena sativa and, later, the wild hexaploid animated oat, A. sterilis, were deployed in oat cultivars, corresponding virulence in the crown rust population increased rapidly, such that the effective lifespan of a resistant cultivar in the United States is now 5 years or less. Introgression of resistance genes from diploid and tetraploid Avena spp. into hexaploid oat has been difficult due to differences in ploidy levels and the lack of homology of chromosomes between the two species. The wild tetraploid slender oat, A. barbata, has been a source of powdery mildew and stem rust resistance in cultivated oat but has largely been unexploited for crown rust resistance. In total, 359 accessions of A. barbata from the National Small Grains Collection were evaluated in seedling greenhouse tests. Of these accessions, 39% were at least moderately resistant when inoculated with a crown rust race with low virulence (DBBC). When tested further with a highly diverse bulk inoculum from the 2006 and 2007 St. Paul buckthorn nursery, 48 accessions (approximately 13%) were resistant. Many of these accessions were heterogeneous in reaction, but two accessions (PI320588 from Israel and PI337893 from Italy) were highly resistant (immune) and two others (PI337886 from Italy and PI367293 from Spain) consistently produced resistant reactions (chlorotic flecks) in all tests. Resistant accessions were found from throughout much of the natural range of A. barbata. Crosses of some of the better accessions have been made to cultivated oat.
 
Article
Pepino mosaic virus (PepMV) is an emerging disease on greenhouse tomato. This highly contagious disease is difficult to control. The best disease management strategy is likely through the use of disease resistance. A major tomato germplasm core collection was evaluated for its resistance against PepMV. These accessions included 23 Solanum lycopersicum, 8 S. pimpinellifolium, 33 S. peruvianum, 18 S. chilense, and 27 S. habrochaites. The results showed that all plants in the accessions corresponding to S. lycopersicum and S. pimpinellifolium were susceptible to PepMV-US infection. On the other hand, two accessions of S. peruvianum (LA107 and LA1305) and S. chilense (LA1971 and LA2748) appeared to have some levels of moderate resistance. However, the most promising resistance segregated in three S. habrochaites accessions (LA1731, LA2156, and LA2167). Resistant plants from these three S. habrochaites accessions were saved for selection. A secondary screening was carried out with progenies generated from the selected plants. These tests showed the segregation of broad-spectrum resistance from the selected S. habrochaites plants against PepMV, especially in LA1731. The segregated LA1731 plants were resistant to not only the U.S. type isolate but also the European type isolate.
 
Adult oat leaves showing (from left to right) highly resistant, “blotchy” resistance (leaves 2 and 3), chlorotic resistant, and necrotic reactions to oat crown rust, Puccinia coronata f. sp. avenae . 
Article
Crown rust (Puccinia coronata f. sp. avenae) is considered the most damaging disease of oat and the use of race-specific seedling (Pc) genes for resistance has been the primary means of control. As these resistance genes from cultivated oat, Avena sativa, and the wild hexaploid animated oat, A. sterilis, were deployed in oat cultivars, corresponding virulence in the U.S. crown rust population increased rapidly, such that the effective lifespan of a resistant cultivar in the United States is now 5 years or less. Introgression of resistance from diploid and tetraploid Avena spp. into hexaploid oat has been difficult due to the difference in ploidy levels and the lack of pairing of homeologous chromosomes between species. The wild tetraploid slender oat, A. barbata, has been a source of powdery mildew and stem rust resistance in cultivated oat but has largely been unexploited for crown rust resistance. A relatively high percentage of A. barbata accessions from the United States Department of Agriculture (USDA) National Small Grains Collection were resistant to a highly diverse crown rust population in recent tests. Tests of 1,099 A. barbata accessions from the Canadian Plant Gene Resources Center not represented in the USDA collection revealed that a similar percentage (11.4%) were at least moderately resistant at the seedling and adult plant stage when tested with a highly diverse bulk inoculum derived from the St. Paul buckthorn nursery. Eighteen accessions were rated as highly resistant or a mix of highly resistant and resistant plants in both seedling and adult plant tests. Three accessions (CN21531 from Italy and CN26271 and CN26305 from Spain) displayed a unique “blotchy” resistant reaction as adult plants. Resistant accessions were found from throughout much of the natural range of A. barbata but the Western Mediterranean and Lebanon had the highest frequency of accessions with broad-spectrum resistance.
 
Mean number of uredinia per lesion and mean uredinia diameters for isolates Alabama 04-1, Brazil 01-1, and Louisiana 04-1 of Phakopsora pachyrhizi following inoculation of kudzu or soybean plants a
Article
Soybean rust, caused by Phakopsora pachyrhizi, was first discovered in the continental United States in the fall of 2004. The potential for economic loss in the United States hinges largely on whether or not the pathogen can survive winters in the absence of soybean. Kudzu (Pueraria lobata) is known to be a host for P. pachyrhizi in Asia and South America and is widely distributed in the southern United States. This study examined reactions of kudzu collected from several areas of the southeastern United States to three isolates of P. pachyrhizi, one each from Alabama, Louisiana, and Brazil. Susceptible tan (TAN) lesions, resistant reddish-brown (RB) lesions, and immune (IM) response, previously described on soybean, were produced on kudzu based on the evaluation of 125 plants. However, in contrast to soybean, the RB response on kudzu was common, with approximately 50% frequency. IM responses to at least one isolate were observed on five individual plants, and two plants were immune to all three pathogen isolates used in the test. TAN lesions averaged 3.2 uredinia per lesion with an average diameter per uredinium of 121 μm. In contrast, RB lesions had an average of 0.3 uredinia per lesion with an average uredinial diameter of 77 μm. In 25 of 39 (64%) instances in which multiple plants were tested from a site, each reacted the same to the individual pathogen isolates. This suggested a tendency for plants at specific sites to be genetically identical with respect to rust reaction. Only 19 of 125 (15%) individual plants produced a different reaction to one isolate than to the other two isolates. When four kudzu plants previously shown to produce only TAN lesions to P. pachyrhizi isolates Alabama 04-1, Brazil 01-1, and Louisiana 04-1 were inoculated with eight additional isolates from several areas of the world, all 11 isolates produced only TAN lesions. Likewise, when five other plants previously shown to produce only RB lesions when inoculated with the three isolates were inoculated with the 11 isolates, all produced only RB lesions. These results suggest that susceptibility or resistance to P. pachyrhizi in individual kudzu plants often is broad, extending over a wide range of P. pachyrhizi isolates.
 
Article
Five hundred thirty soybean accessions from maturity groups (MG) III through IX were evaluated for resistance to Phakopsora pachyrhizi in a replicated field trial at Centro Regional de Investigación Agrícola in Capitán Miranda, Itapúa, Paraguay during the 2005-06 season. Soybean rust severities of individual accessions ranged from 0% (resistant) to 30.0% (susceptible). In MG III and IV, the most resistant accessions were PI 506863, PI 567341, and PI 567351B, with severities less than 1.2%. In MG V, the most resistant accessions were PI 181456, PI 398288, PI 404134B, and PI 507305, with severities less than 0.3%. In MG VI, the most resistant accessions were PI 587886, PI 587880A, and PI 587880B, with severities less than 0.3%. In MG VII and VIII, the most resistant were PI 587905 and PI 605779E, with severities less than 1.0%. In MG IX, the most resistant accessions were PI 594754, PI 605833, PI 576102B, and PI 567104B, with severities less than 1.0%. The resistance in 10 selected accessions from MG VI, VII, VIII, and XI was confirmed in subsequent greenhouse and field experiments where severities of 0.4% or less and reddish-brown lesions with sporulation levels less than 3.0 were observed. These accessions, with low severities in the adult plant field evaluation, may be new sources of resistance to P. pachyrhizi.
 
Comparison of disease severity scores (Table 1) among four sugar beet cultivars currently available in California, Beta 4430, Beta 4776, Beta 4300, and Phoenix, with the moderately resistant cultivar USH11 and the average of 27 cultivars and experimental lines. Twenty-seven individual cultivars were included in the trial. Plots were planted as single rows, 8 m long, with at least 6.6 plants per meter and three replications per entry, resulting in at least 160 plants per variety. Scores were recorded monthly for 3 months and just prior to harvest (30 August). Each plot was scored as a single value representative of the average rating of all plants in the plot. The trial was conducted at the University of California Westside Research and Extension Center near Five Points, CA, in summer 2001, a year with severe curly top infection in the region.  
Curly top disease severity rating scale for sugar beet z
Median curly top severity scores by cultivar in sugar beet plants inoculated when plants had two, four, and six true leaves x
Article
Resistance to curly top disease caused by Beet curly top virus (BCTV) and related curtoviruses has been important to sustainable sugar beet (Beta vulgaris) production in the western United States for most of the last century. Recent advances in sugar beet genetics have led to the development of high-yielding cultivars, but these cultivars have little resistance to curly top disease. These cultivars are highly effective when disease management practices or environmental factors minimize curly top incidence, but can result in significant losses in years with early infection or abundant curly top. A greenhouse assay has been developed to rapidly test cultivars for a broad array of factors affecting performance in the presence of curly top. Previous studies have shown that sugar beet plants were more susceptible and losses more severe when seedlings were infected by BCTV, but less severe when plants were larger at the time of infection. To evaluate more precisely the relationship between age at infection, disease severity, virus accumulation, and yield loss in modern cultivars that were not bred for curly top resistance, individual sugar beet plants varying in degree of resistance and susceptibility to curly top were inoculated by viruliferous beet leafhoppers (Circulifer tenellus) when plants had two, four, or six true leaves, and maintained in a greenhouse for 6 weeks. When plants were inoculated at the two-leaf stage, all cultivars became severely stunted, with high disease ratings and similar rates of symptom development, regardless of resistance or susceptibility of the cultivar. Plants inoculated at four-and six-leaf stages exhibited increasing separation between resistant and susceptible phenotypes, with highly resistant cultivars performing well with low disease ratings and increased plant weights relative to susceptible cultivars. High-yielding cultivars performed only slightly better than the susceptible control cultivar. Results from greenhouse trials matched those from field trials conducted under heavy curly top pressure. Importantly, low virus concentration was directly correlated with lower disease ratings and higher plant weight, while elevated virus concentrations corresponded to higher disease ratings and lower weights. This demonstrates that a rapid greenhouse assay involving multiple traits can provide a rapid and effective means of selecting cultivars with improved curly top control, and could lead to more rapid incorporation of resistance into high-yielding sugar beet.
 
Article
Fumonisins produced by Fusarium verticillioides (syn = F. moniliforme) and F. proliferatum have been associated with potentially serious toxicoses of animals and humans. Thus, hybrids with low fumonisin accumulation in grain will be valuable for the production of corn-based human food products. We evaluated 68 food-grade dent corn hybrids for severity of Fusarium ear rot and fumonisin accumulation in grain in inoculated trials in Urbana, IL in 2000 and 2001. Our inoculation technique was successful in initiating fumonisin accumulation that allowed discrimination among hybrids. We identified several hybrids that could have acceptable levels (<4 micrograms/g) of fumonisin accumulation in Illinois in most years. Twenty-six hybrids with low or high fumonisin accumulation in 2000 were reevaluated in noninoculated trials at three locations in Illinois in 2001. Fumonisin concentration in grain at all three locations was relatively low; thus, separation of hybrids was poor. At two locations, those hybrids with the highest fumonisin concentration in grain also had high concentrations following inoculation. However, one hybrid that had relatively low fumonisin concentration following inoculation had unacceptable levels of fumonisin (5 micrograms/g) in natural conditions. Therefore, hybrids need to be evaluated by inoculation and further evaluated at locations where the environment favors fumonisin accumulation.
 
Article
Fusarium head blight (FHB), caused by Fusarium graminearum Schwabe, is an important disease of wheat worldwide. Production of deoxynivalenol (DON) in infected wheat grain by F. graminearum is a major safety concern when considering use of the grain as feed for livestock or for human consumption. Determining chromosome locations of FHB-related genes may facilitate enhancement of wheat resistance to FHB and DON accumulation. In this study, a set of 30 ditelosomic lines derived from Chinese Spring, a moderately FHB-resistant landrace from China, were evaluated for proportion of scabbed spikelets per inoculated spike in the greenhouse and for DON contamination in harvested grain over 2 years. Significant variation in the proportion of scabbed spikelets was observed among ditelosomic lines, ranging from 13 to 95%. Seven ditelosomic lines exhibited a greater proportion of scabbed spikelets and three of these also had greater DON content than Chinese Spring (P = 0.01), suggesting that those missing chromosome arms may carry genes that contribute to resistance to FHB. Six ditelosomic lines had a reduction in proportion of scabbed spikelets, suggesting that susceptibility factors or resistance suppressors may be on these missing chromosomal arms. Selection for low proportion of scabbed spikelets in general will select for low DON content.
 
Plant material used in this study and their known R genes to Phytophthora infestans Clone Known R gene Identity 
Avirulence profiling of Phytophthora infestans isolates GER8601, 99018, 91001, and 89148-27 with both in vitro (I) and detached leaf (D) assays
Reaction of the SH × RH F1 population segregating at the R3 locus to inoculation with Phy- tophthora infestans isolates IPO-0 and H30P04 P. infestans isolate
Article
Research and breeding for resistance to Phytophthora infestans, the causal agent of the devastating late blight disease in potato (Solanum tuberosum), requires extensive disease assessments. Especially for large-scale experiments such as recombinant screenings for genetic mapping of novel resistance genes, a quick and space-effective method is required. Potato breeding clones and collections of wild Solanum germ plasm are often maintained in vitro, and this environment is extremely favorable for late blight disease development. In this paper, we describe the development of a high-throughput in vitro disease testing assay. We compared the new method with the well-established detached-leaf assay and proved that the in vitro assay is accurate and reliable, and can routinely be used for investigating the qualitative interaction between potato and P. infestans
 
Growth on potato dextrose agar (supplemented with ampicillin sodium salt and streptomycin sulfate, each at 100 mg/liter) 5 days after seeding dishes with a wheat extract treated 30 min with acidic electrolyzed water (AEW), 30 min with 0.05 M pH 2.5 glycine-HCl buffer, 2 min with 0.4% NaOCl, or no treatment to serve as a control. Note that no growth occurred with the AEW and 0.4% NaOCl treatments, and that low pH by itself reduced growth when compared with no treatment.  
Numbers of fungi and bacteria CFU present in soil extracts after treatment with acidic electrolyzed water (AEW) or sodium hypochlorite (NaOCl) and spread onto antibiotic water agar (AWA) a 
Article
Acidic electrolyzed water (AEW) is a germicidal product of electrolysis of a dilute solution (e.g., 0.4% vol/vol) of sodium chloride. This solution can be used to disinfest wheat seed or soil samples being tested for teliospores of Tilletia indica, causal agent of Karnal bunt, without risk of damaging the teliospores. The AEW used in this study had a pH of 2.5 to 2.8 and oxidation- reduction potential of approximately 1,130 mV. In simulations of routine extractions of wheat seed to detect teliospores of T. indica, the effectiveness of a 30-min AEW treatment was compared with a 2-min 0.4% sodium hypochlorite (NaOCl) treatment to eradicate bacteria and nonsmut fungi. Each treatment reduced bacterial and fungal populations in wheat seed extracts by 6 to 7 log10 units when determined on 2% water agar with antibiotics. Reductions of 5 log10 units or more were observed on other media. NaOCl and AEW also were very effective at eliminating bacteria and fungi from soil extracts. In studies to detect and quantitate T. indica teliospores in soil, AEW was nearly 100% effective at eliminating all nonsmut organisms. Free chlorine levels in AEW were very low, suggesting that compounds other than those with chlorine play a significant role in sanitation by AEW. The low pH of AEW was shown to contribute substantially to the effectiveness of AEW to reduce microorganisms. A standardized protocol is described for a 30-min AEW treatment of wheat seed washes or soil extracts to eliminate contaminating microorganisms. A significant advantage of the use of AEW over NaOCl is that, with AEW, teliospore germination is not reduced and usually is stimulated, whereas teliospore germination declines after contact with NaOCl. The protocol facilitates detection and enumeration of viable teliospores of T. indica in wheat seed or soil and the isolation of pure cultures for identification by polymerase chain reaction. The germicidal effects of AEW, as demonstrated in this study, illustrate the potential of AEW as an alternative to presently used seed disinfestants.
 
Article
The feeding periods required by corn flea beetles to acquire and transmit Pantoea stewartii were investigated in the Stewart's disease of corn pathosystem. To quantify the effect of acquisition feeding period on percentage of acquisition, field-collected corn beetles were allowed to feed for 6, 12, 24 36, 48, and 72 h on corn seedlings previously inoculated with a rifampicin- and nalidixic acid-restraint strain of P. stewartii. Acquisition of P. stewartii by corn flea beetles was considered positive if the rifampicin- and nalidixic acid-marked strain was recovered on selective media. To quantity the effect of transmission feeding period on percent transmission of P. stewartii by corn flea beetles, P. stewartii-infested corn flea beetles were allowed to feed on healthy corn seedlings for periods of 3, 6, 12, 24, 36, 48, and 72 h. After the appropriate transmission feeding period, leaf tissues surrounding the sites of feeding scars were cultured for the presence of the P. stewartii-marked strain. Transmission of P. stewartii was considered positive if the marked strain was recovered on selective media. Acquisition of P. stewartii occurred within 6 h and the percentage of corn flea beetles that had acquired P. stewartii after 72 h ranged from 68 to 94%. The change in P. stewartii acquisition by corn flea beetles (Y) with respect to acquisition feeding period (X) was best described by the Gompertz model, with R2 values ranging from 91 to 99%. The mean time for acquisition by 50% of the corn flea beetles was 36.5 +/- 11.6 h. The minimum transmission feeding time required for corn flea beetles to transmit P. stewartii following a 48-h acquisition feeding period was less than 3 h. The percent transmission of P. stewartii by corn flea beetles was nearly 100% after a 48-h transmission feeding period and was 100% by 72 h. Among population growth models evaluated, the monomolecular model best described the relationship between percent transmission (Y) and transmission feeding periods (X), with R2 values of up to 84%. However, a nonlinear form of the monomolecular model better quantified the relationship between percent transmission and transmission feeding period, because pseudo-R2 values ranged between 98.1 and 99.5%. The predicted transmission feeding time required for 50% of P. stewartii-infested corn flea beetles to transmit the pathogen was 7.6 +/- 0.87 h. These results suggest that the corn flea beetle is a highly efficient vector that can quickly acquire and transmit P. stewartii, thereby requiring insecticide seed treatments and foliar insecticides that act quickly to prevent corn flea beetles from acquiring and transmitting P. stewartii to corn plants.
 
Tomato yellow leaf curl virus (TYLCV) detection by triple-antibody sandwich enzyme-linked immunosorbent assay in different parts of A, 17 vine tomato fruit of two infected bunches of cv. Durinta and B, 9 fruit of two infected bunches of cv. Cencara. Positive and negative controls were from tomato leaves collected on TYLCV-infected and noninfected plants, respectively, of susceptible tomato cv. Farmer. The standard deviation is indicated for each mean absorbance. Fruit parts with the same letter indicate that their mean absorbance values were not significantly different according to the Tukey test (a = 0.05). The controls were not included in the analysis of variance analyses and in the mean comparison test.
Detection of Tomato yellow leaf curl virus (TYLCV) in 27 batches of tomato fruit imported into Réunion during the first quarter of 2002 a
Article
The whitefly Bemisia tabaci is an insect pest causing worldwide economic losses, especially as a vector of geminiviruses such as Tomato yellow leaf curl virus (TYLCV). Currently, imported and exported tomato fruit are not monitored for TYLCV infection because they are not considered to represent a potential risk as a virus source for whiteflies. A survey of tomato fruit imported into Reunion Island indicated that more than 50% of the fruit contained TYLCV as determined by DNA blot analysis. Moreover, we showed that TYLCV was present at a high titer in tomato fruit, and demonstrated that it can be acquired by whiteflies and subsequently transmitted to healthy tomato plants. Potential risk of the spread of TYLCV by tomato fruit in natural conditions needs to be further assessed.
 
Article
Acroptilon repens (L.) DC. (Russian knapweed) is a long-lived perennial weed from central Asia that is widely distributed in the western United States (U.S.). Recently, accessions of a rust disease were collected from Colorado (CO), Montana (MT), and Wyoming (WY) for comparison with Eurasian isolates. U.S. accessions had two-celled teliospores with slight constrictions in the middle and urediniospores with three germ pores ± equatorial in location. Urediniospores were (state, width × length, [n = 100]): CO, 16.4 to 25.7 × 19.2 to 27.0 μm; MT, 18.4 to 23.1 × 17.4 to 24.6 μm; and WY, 18.0 to 26.2 × 20.2 to 26.7 μm. These were similar to those of 16.6 to 25.7 × 21.2 to 28.0 μm from two New Mexican (NM) herbarium specimens (BPI Nos. 1107952 and 1110177) (1). Teliospores measured 19.9 to 27.7 × 29.8 to 47.4 μm, 17.4 to 26.0 × 32.4 to 44.2 μm, 16.5 to 27.5 × 29.4 to 45.7 μm, and 18.7 to 27.6 × 31.0 to 46.4 μm for CO, MT, WY, and NM accessions, respectively. These rust isolates have been identified as Puccinia acroptili Syd. on the basis of host plant record and spore morphology (2). To our knowledge, this is the first record of P. acroptili in CO, MT, and WY. Besides NM, P. acroptili has been reported in North America from California, British Columbia, and Saskatchewan. References: (1) M. E. Palm and S. G. Vesper. Plant Dis. 75:1075, 1991. (2) D. B. O. Savile. Can. J. Bot. 48:1567, 1970.
 
Top-cited authors
Ravi Singh
  • International Maize and Wheat Improvement Center
Walter Gubler
  • University of California, Davis
Jeff Jones
  • University of Florida
Joseph Smilanick
  • United States Department of Agriculture
Zacharias A. Pretorius
  • University of the Free State