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Identification of Armillaria species in California
Abstract
Mating experiments were conducted with 30 California isolates of Armillaria species to determine their intersterility grouping. Basidiomes were collected from a variety of coniferous and hardwood hosts in urban, agricultural and forested regions in northern and central California. Monosporous cultures were obtained and paired with haploid tester strains representing 10 North American biological species. A majority of the California isolates were compatible with Armillaria mellea sensu stricto (= North American biological species VI). The exceptions were four isolates from a low elevation, coastal forest. Three of these were compatible with North American biological species IX, a still unidentified Armillaria species, and one was compatible with A. gallica (= North American biological species VII). These findings show that Armillaria species diversity exists in California, but that A. mellea predominates on several hosts and in diverse habitats.
... Two species of Heterobasidion are known from China but there is no information on their pathogenicity to P. radiata (Dai et aL 2002, 2003). (Jacobs et aL 1994;Mohammed et al. 1994) but is sensitive to low soil moisture potentials (Whiting et al. 1999) and thus unlikely to be a serious pathogen in pine plantations in the Min River valley. The pathogenicity to R radiata of the numerous other species of Armillaria known from China has yet to be determined. ...
Pinus radiata D. Don, native to a Californian coastal environment, has been introduced to many parts of the world as an exotic species for afforestation. It is now a major plantation species in the Southern Hemisphere. In 1990, it was introduced to the heavily degraded, dry Min river valley area in Aba prefecture of Sichuan Province, P. R. China. Survival and growth of young trees planted at several sites appear to be reasonable. This review is to serve as an introduction to the large body of literature on P. radiata for forest scientists in China. It covers the following aspects: P. radiata in its native environment and in ex situ plantations, provenance and genetic variations, environmental limitations and climate niche, diseases and pests, lessons from unsuccessful introductions, and the use of P radiata for ecological restoration. The early growth of P radiata planted in the dry river valley area is briefly described. Potential problems associated with the introduction of P. radiata in Aba and future research needs are also identified.
To determine the identity and distribution of Armillaria species in California, we collected 589 isolates from managed areas (urban landscapes, orchards, and vineyards) and 20 natural forest types. From our collection of isolates, we identified A. mellea, A. gallica, A. nabsnona, and North American biological species X (NABS X). State-wide ranges of each species were estimated based on distributions of forest types from which they were collected. Only A. mellea and A. gallica are widely-distributed in California; A. nabsnona and NABS X appear to have restricted ranges. Armillaria mellea is common in most regions of the state dominated by development, such as the Central Valley, the San Francisco Bay area, and Los Angeles. It was the only species, with the exception of a single isolate of NABS X, isolated from symptomatic hosts, including both forest trees and planted hosts. Armillaria mellea does cause mortality in California forests, but large disease centers are rare. The distribution of A. gallica in California includes the greatest variety of forest types of all Armillaria species identified from our collection. Although it shares many forest types in common with A. mellea, A. gallica is extremely rare in developed areas. The range of A. nabsnona in California appears to be restricted to northern redwood forests. NABS X was only identified in high elevation red and white fir forests of the southern Cascades.
A new method is described for the production of inoculum of Armillaria species. An Armillaria mellea isolate was obtained from an infected grapevine plant. Inocula were then obtained by the new method (in which the host-wood rods are incubated in benomyl-dichloran-streptomycin medium) and by three existing methods. Also, the efficacy of the inoculum produced by the new method was assessed by an experimental infection assay using healthy plants of six grapevine rootstocks (196–17 Castel, 110 Richter, 161–49 Couderc, 3309 Couderc, 1103 Paulsen, and 102). The new method produced inoculum within a very short period (15 d, versus 3 mo with the best of the existing methods). All rootstocks were infected by the A. mellea isolate, the most resistant being 161–49 Couderc. This method thus offers a significant reduction in the time necessary for pathogenicity testing or any research requiring Armillaria inoculum.
Increased incidence of Armillaria root disease on orchard and landscape trees in northern California may be related to changes in irrigation practices. Continuously moist soil from sprinkler irrigation may favor survival and growth of the fungus. The effects of water potential on radial colony growth were tested for two species of Armillaria. Isolates were tested on malt extract agar amended with either KCl or sucrose to control osmotic potential and produce six different water potentials from -0.5 to -8.5 MPa. Eleven Armillaria mellea isolates from orchard crops and hardwood forest trees and three isolates of A. gallica from hardwood forest trees in California grew well between -0.5 and -1.5 MPa water potential. Rates of mycelial growth were 7–10 mm/wk. Below -1.5 MPa mycelial growth of all isolates declined with decreasing water potential. In this study, A. mellea, and to a lesser extent A. gallica, were sensitive to changes in water potential in culture.
Armillaria nabsnona is characterized morphologically and described as a new species. It is compatible with tester strains of North American biological species (NABS) IX. Using restriction fragment length polymorphism (RFLP) of the polymerase chain reaction (PCR) products amplified from the intergenic spacer (IGS) region of the ribosomal DNA, we also demonstrate an association of cultures of specimens with tester strains of NABS IX. Armillaria nabsnona appears to be restricted to several species of hardwood trees in the states of Washington, Oregon, California, Idaho and Alaska, and in the Canadian province of British Columbia. Basidiomata have been found in both the fall and spring.
Incidence of Armillaria root disease and the population structure of associated Armillaria spp. was studied in California mixed-hardwood forests. Four 3,000-m(2) plots, consisting of Quercus kelloggii, Q. agrifolia, Lithocarpus densiflorus, Arbutus menziesii, Umbellularia californica, and Pseudotsuga menziesii, were intensively sampled. Root collars were examined for mycelial fans, decayed wood, and rhizomorphs. From 404 trees (333 living, 71 dead), Armillaria mellea and A. gallica were recovered and identified from 148 isolates. The most abundant tree species in each plot was the tree species most frequently infected by A. mellea or A. gallica. The majority of A. mellea isolates were recovered from mycelial fans on living trees. Recent mortality due to A. mellea was found only in one plot. A. gallica was mainly identified from epiphytic rhizomorphs. Only one to three somatic incompatibility groups (SIGs) for each species were found in each plot. Estimated sizes of SIGs varied from the extent of a single root system to the entire width of the plot. Based on our results, both A. mellea and A. gallica are common in mixed-hardwood forests, yet A. mellea appears to be a more aggressive pathogen. A. gallica is capable of attacking live hosts, but occurs on living roots more frequently as epiphytic rhizomorphs.
Pinus radiata was introduced to the summer rainfall environments of Sichuan Province, China in the 1990s as a part of an affore-station
program for soil and water conservation in the arid and semi-arid river valley area of Aba Prefecture. Within this region
a total area of 26 000 ha have been identified through climate matching as suitable and a further 63 000 ha potentially suitable
for environmental plantings of P. radiata. The plantations are being established in widely separated small patches on steep and degraded slopes along the dry river
valley. The newly introduced P. radiata are exposed to two kinds of forest health risks: they may be attacked by (a) indigenous pathogens and pests against which
they may not possess any resistance or (b) by inadvertently introduced foreign pests or pathogens. This paper presents a survey
of the potential damaging pests and a preliminary assessment of forest health risks facing the P. radiata plantations over a much longer timeframe than the initial phase of introduction and early plantation establishment. An empirical
approach was adopted to evaluate forest health risks by a combination of literature review, examination of historical records
of pest and disease outbreaks in the surrounding coniferous forests, field surveys and inspections, specimen collection and
identification, and most importantly, expert analysis of the likelihood of attack by specific pests and pathogens and the
subsequent impact of such attacks. The assessment identified some specific forest health risks to the long-term success of
P. radiata introduction in this area. These risks are closely associated with the indigenous pests and pathogens of the two native pine
species, P. tabulaeformis and P. armondii since these pests and pathogens are considered more likely to establish on P. radiata over time. Exotic pests and pathogens are of a quarantine concern at present. Based on the results of assessment, recommendations
are provided to improve forest vigour and to reduce the forest health risks posed by indigenous as well as exotic pests and
pathogens to the introduced P. radiata. Ways to increase the ability to manage the forest health risks once a particular pest infestation and disease eventuates
are also recommended. Although detrimental to the survival and growth of the introduced P. radiata, the impact of identified forest health risks are not considered to be fatal to the long term success of P. radiata in this area.
Pinus radiata D. Don, native to a California coastal environment, has been introduced to many parts of the world as an exotic species for
afforestation. It is now a major plantation species in the Southern Hemisphere. In 1990, it was introduced to the heavily
degraded, dry Min river valley area in Aba prefecture of Sichuan Prevince, P. R. China. Survival and growth of young trees
planted st several sites appear to be reasonable. This review is to serve as an introduction to the large body of literature
onP. radiata for forest scientists in Chine. It convers the following aspects:P. radiata in its native environment and in ex situ plantations, provenance and genetic variations, environmental limitations and climate
niche, diseases and pests, lessons from unsuccessful introductions, and the use ofP. radiata for ecological restoration. The early growth ofP. radiata planted in the dry river valley area is briefly described. Potential problems associated with the introduction ofP. radiata in Aba and future research needs are also identified.
Pears have traditionally been considered to be highly resistant to Armillaria root disease (causal agent: Armillaria mellea). In recent years, however, the incidence of Armillaria root disease in pears has increased in California. To determine the spatial distribution of Armillaria root disease in the field, a total of 156 isolates of Armillaria were collected from dead and dying pear trees located within two orchards in Lake County. All isolates from these two orchards, as well as from an additional 10 pear orchards, were identified as Armillaria mellea sensu stricto. Based on pairings among 102 Armillaria isolates, four somatic incompatibility groups (SIGs) were identified at orchard 1. Three of the four SIGs at this site were over 100 m in length; the largest SIG was at least 200 m in length. Pairings among 54 isolates identified five SIGs at orchard 2. The SIGs at orchard 2 were generally smaller than those detected at orchard 1 and ranged from 20 to 60 m in length. The size of the SIGs points toward long-term establishment of the fungus on the two sites, most likely predating the establishment of the pear orchards. Extensive root excavations of 19 trees indicated that the primary means of secondary spread of Armillaria was via rhizomorphs, as opposed to root-to-root contact.
Root collar excavation for control of Armillaria root disease of grapevine was investigated in two California vineyards (vineyard K1 and vineyard N1) from 2002 to 2004. The hypothesis tested was that root collar excavation, when timed in early stages of root collar infection, may cause mycelial fans of the pathogen Armillaria mellea to recede from the root collar before severe disease results from vascular tissue decay. In vineyard N1, excavation significantly increased yield and cluster weight of symptomatic grapevines; symptomatic-excavated grapevines had the same high mean cluster weight as healthy grapevines, and there were no significant effects of excavation on yield or pruning weight of healthy grapevines. In vineyard K1, where excavated root collars frequently refilled with soil, excavation had no significant effects on yield or pruning weight of symptomatic grapevines, and significantly reduced pruning weight and shoot weight of healthy grapevines. Reexamination in March 2004 revealed that mycelial fans had receded from root collars of symptomatic-excavated grapevines, but remained on root collars of symptomatic-nonexcavated grapevines. Root collar excavation appears to be a promising cultural approach for control of Armillaria root disease, as long as excavated root collars are kept clear of soil.
A statewide investigation was conducted to identify the species of Armillaria and determine their geographic distributions and host/substrate relationships in New York. At 273 sites, samples of rhizomorphs, wood, and/or basidiomata were collected from 414 trees, snags, stumps, or logs that were colonized by Armillaria. Field isolates were identified to species by mating the unknown isolates with previously identified haploid isolates. Six species were identified in New York forests. A. calvescens, the most common species, was widely distributed and had a broad host/substrate range. It occurred mainly on hardwoods (86% of collections), commonly causing butt rot. A. gallica, the second most common species, also was found primarily on hardwoods (86% of collections), especially oaks, and often caused butt rot. It was widespread but was not found in the Adirondack region. A. ostoyae, the third most common species, was found predominantly in the Adirondack region. It was the only species found principally on conifers (89% of collections). A. gemina was scattered throughout the state and was found almost always on hardwoods (96% of occurrences). A. sinapina, like A. ostoyae, was found mainly in the Adirondack region but was found mostly on hardwoods (60% of occurrences). A. mellea sensu stricto was found only once on Long Island on Quercus alba and once in central New York on an unidentified stump. All species except A. mellea were found infecting cambial tissue of living hosts. There was no significant difference among Armillaria species in the frequencies of cambial infections.
The root rot fungus Armillaria mellea in the broad sense represents a complex of different morphological forms. Two new and previously undescribed forms were found to belong to two biological species using mating tests with standard voucher strains. Monosporous cultures from basidiocarps harvested in Québec were compatible with strains of groups II and III corresponding to two new species: Armillaria gemina Bérubé & Dessureault and Armillaria calvescens Bérubé & Dessureault, respectively. They are described and their occurrence and ecology documented. Morphological characteristics of basidiocarps and vegetative isolates can be used to differentiate biological species I, II, III, V, VI and VII found in eastern North America. A key to the A. mellea complex in North America is also presented.
Armillaria mellea consists of at least ten reproductively isolated groups, the equivalent of “biological species.” Each biological species possesses bifactorial heterothallism with compatibility discernible by the gross mycelial morphology of paired monosporous isolates rather than the presence or absence of clamp connections and dikaryotic cells. Monosporous testers were obtained from 97 fruiting bodies in North America. Pairings of testers from different fruiting bodies indicated that each isolate belongs to one and only one intersterile group, i.e., intersterility between groups is complete. Nutritional selection applied to confronted auxotrophic strains from two of the biological species revealed no prototrophy (genetic complementation) between the strains of these groups, whereas prototrophy was revealed by the same method within groups. Members of several of the biological species are distributed widely in North America. Isolates may be collected from a broad range of host species or also as saprophytes. The 10 biological species are not clearly distinguishable by unique geographical ranges or substrate specificities. Armillaria mellea is considered to be a complex of morphologically distinct species. This study shows that the taxon is divided into genetically isolated distinct biological species.
Diploid-haploid pairings to identify field isolates of Armillaria to species generally rely on morphological differences between diploid and haploid colonies (flat and flufly colonies, respectively) and morphological changes in haploid tester colonies. Presumably, the change in morphology of the haploid tester is due to diploidization. Isozyme markers, aconitase and glucosephosphate isomerase, were used to follow nuclear migration from putatively diploid isolates into a haploid tester strain of A. ostoyae. After pairing diploid field isolates with a haploid tester for 3 or 4 weeks, subcultures were taken from the hap!oid tester at 2 and 10 mm from the confrontation zone. Subcultures from 2 mm were mostly flat; subcultures from 10 mm were flufly, flat, or flufly with flat sectors. Hyphal tip cultures from the subcultures were analyzed for the isozyme markers. All flufly hyphal tip cultures had the isozyme phenotype of the haploid tester. Flat hyphal tip cultures either had the isozyme phenotype of the diploid parent or had a hybrid isozyme phenotype with markers of both the diploid parent and the haploid tester. The data suggest that in conspecific diploid-haploid pairings, the diploid nucleus usually migrates into the haploid mycelium and eventually displaces the haploid nucleus.
Diploid-haploid pairings to identify field isolates of Armillaria to species generally rely on morphological differences between diploid and haploid colonies (flat and fluffy colonies, respectively) and morphological changes in haploid tester colonies. Presumably, the change in morphology of the haploid tester is due to diploidization. Isozyme markers, aconitase and glucosephosphate isomerase, were used to follow nuclear migration from putatively diploid isolates into a haploid tester strain of A. ostoyae. After pairing diploid field isolates with a haploid tester for 3 or 4 weeks, subcultures were taken from the haploid tester at 2 and 10 mm from the confrontation zone. Subcultures from 2 mm were mostly flat; subcultures from 10 mm were fluffy, flat, or fluffy with flat sectors. Hyphal tip cultures from the subcultures were analyzed for the isozyme markers. All fluffy hyphal tip cultures had the isozyme phenotype of the haploid tester. Flat hyphal tip cultures either had the isozyme phenotype of the diploid parent or had a hybrid isozyme phenotype with markers of both the diploid parent and the haploid tester. The data suggest that in conspecific diploid-haploid pairings, the diploid nucleus usually migrates into the haploid mycelium and eventually displaces the haploid nucleus.
The systematics of the Armillaria mellea complex. Phytopathological consequences . The five biolo‐gical species of the Armillaria complex as defined by Korhonen coincide with species already defined by other authors on the basis of their morphology and have been given latin namenelature.
The main characteristies of the five Eurmpean species are deseribed: Fruitbody morphology, myce‐lium morephology in loboratory culture, rhizomorphs grown in a rhizotron and the ease with which fruithodies are produced in the laboratory. A descriptiom is given of the geographical distribution ecological specialisation and pathogenicity of these 5species.
In the infection cycle of Armillaria spore dispersal is followed by colonization of stumps, which is not uncommon on the time-scale of plantation forestry. After a period of exploitation, further suitable woody tissues nearby may be colonized by means of rhizomorphs or direct mycelial transfer. The cycle is illustrated by four species present in southern England, which vary in pathogenicity, capacity to invade tissues deelining in resistance, and ability to form rhizomorphs. Various host responses, such as production of oleoresin by conifers and secondary meristematic activity, are briefly discussed.
The root rot fungus Armillaria mellea in the broad sense represents a complex of different morphological forms. Two new and previously undescribed forms were found to belong to two biological species using mating tests with standard voucher strains. Monosporous cultures from basidiocarps harvested in Québec were compatible with strains of groups II and III corresponding to two new species: Armillaria gemina Bérubé & Dessureault and Armillaria calvescens Bérubé & Dessureault, respectively. They are described and their occurrence and ecology documented. Morphological characteristics of basidiocarps and vegetative isolates can be used to differentiate biological species I, II, III, V, VI and VII found in eastern North America. A key to the A. mellea complex in North America is also presented.
Armillaria mellea consists of at least ten reproductively isolated groups, the equivalent of "biological species." Each biological species possesses bifactorial heterothallism with compatibility discernible by the gross mycelial morphology of paired monosporous isolates rather than the presence or absence of clamp connections and dikaryotic cells. Monosporous testers were obtained from 97 fruiting bodies in North America. Pairings of testers from different fruiting bodies indicated that each isolate belongs to one and only one intersterile group, i.e., intersterility between groups is complete. Nutritional selection applied to confronted auxotrophic strains from two of the biological species revealed no prototrophy (genetic complementation) between the strains of these groups, whereas prototrophy was revealed by the same method within groups. Members of several of the biological species are distributed widely in North America. Isolates may be collected from a broad range of host species or also as saprophytes. The 10 biological species are not clearly distinguishable by unique geographical ranges or substrate specificities. Armillaria mellea is considered to be a complex of morphologically distinct species. This study shows that the taxon is divided into genetically isolated distinct biological species.
Early references to Armillaria root rot in California
- M W Gardner
- R D Raabe
- Bérubé J. A.
The systematics of the Armillaria mellea complex, phytopathological consequences
- J J Guillaumin
- C Berthelay
- B Lung
- H Romagnesi
- H Marxmuller
- O Lemoure
- G Durrieu
- S Bertheley
- C Mohammed
Diseases of temperate zone tree fruit and nut crops
- J M Ogawa
- H English
Armillaria root disease
- C G Shaw
- G A Iii
- Kile