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.
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.
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.
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.
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...
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
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
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...
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.
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 ...
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 ...
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.
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.
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).
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.
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.
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 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.
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.
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.
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.
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.
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.
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.