Characterization of isolates of Phytophthora infestans collected in northwestern France from 1988 to 1992

Isolates of the potato late-blight pathogen Phytophthora infestans , collected in several French regions from 1988 to 1992, were characterized in order to determine the structure and pathogenicity of French populations and to compare them with other European samples. All 254 French isolates tested were of the Al mating type, as were two Sicilian, five Portuguese and one Spanish isolate. Metalaxyl sensitivity of 157 isolates was investigated. The frequency of resistance to this chemical decreased in western France from 1989 to 1992. Two glucose phosphate isomerase phenotypes (90/100 and 90/90) were detected among the 46 isolates tested; the former largerly predominated (75–80%) in the samples investigated. Both phenotypes corresponded to the population recently introduced into Europe. A third genotype (86/100), typical of the former European population, was found in one isolate from Picardy, originating from an artificially infected field trial. Five isolates from Portugal and Italy were of the 100/100 phenotype. Race patterns among 26 isolates from western France were very similar, and could not be explained by local selection. The unusual composition of the Brittany population, consisting of 'new' genotypes but all belonging to the Al mating type, is possibly related to the metapopulation type of structure of P. infestans in Europe.     

In vitro leaf inoculation studies as an indication of tree foliage susceptibility to Phytophthora ramorum in the UK

Leaves of 11 coniferous and 23 broad-leaved tree species important to UK forestry were tested for their susceptibility to the quarantine pathogen Phytophthora ramorum using a detached leaf assay. Two European and two USA isolates were used. Wounded and unwounded leaves were dipped in zoospore suspensions during summer; conifers were also tested in winter. Successful infection of tissue and amount of necrosis were assessed. Highly susceptible broad-leaved hosts included Aesculus hippocastanum , Fraxinus excelsior , Quercus ilex , Ulmus procera and, to a lesser extent, Castanea sativa , Q. cerris and Q. petraea , together with Umbellularia californica and rhododendrons. Acer pseudoplatanus , Alnus glutinosa , Carpinus betulus , Corylus avellana , Fagus sylvatica , Prunus avium , Q. robur , Q. rubra and Q. suber showed consistently low susceptibility. Conifer species including Abies procera , Picea abies , P. sitchensis , Pseudotsuga menziesii , Sequoia sempervirens and Tsuga heterophylla were also susceptible. Pseudotsuga menziesii and A. procera were severely affected. Pinus contorta , P. nigra var. maritima and P. sylvestris were virtually resistant, while Taxus baccata was only slightly affected. Increased necrosis was apparent on leaves that were wounded prior to inoculation. These results extend the known range of trees that P. ramorum is able to attack and confirm its relative host-nonspecificity.     

Susceptibility of Iberian trees to Phytophthora ramorum and P. cinnamomi

The capacity of Phytophthora ramorum to colonize the inner bark of 18 native and two exotic tree species from the Iberian Peninsula was tested. Living logs were wound-inoculated in a growth chamber with three isolates belonging to the EU1 and two to the NA1 clonal lineages of P. ramorum . Most of the Quercus species ranked as highly susceptible in experiments carried out in summer, with mean lesion areas over 100 cm² in Q. pubescens , Q. pyrenaica , Q. faginea and Q. suber and as large as 273 cm² in Q. canariensis , ca . 40 days after inoculation. Quercus ilex ranked as moderately susceptible to P. ramorum , forming lesions up to 133 cm² (average 17·2 cm²). Pinus halepensis and P. pinea were highly susceptible, exhibiting long, narrow lesions; but three other pine species, P. pinaster , P. nigra and P. sylvestris , were resistant to slightly susceptible. No significant difference in aggressiveness was found between the isolates of P. ramorum . In addition, there was evidence of genetic variation in susceptibility within host populations, and of significant seasonal variation in host susceptibility in some Quercus species. The results suggest a high risk of some Iberian oaks to P. ramorum , especially in forest ecosystems in southwestern Spain, where relict populations of Q. canariensis grow amongst susceptible understory species such as Rhododendron ponticum and Viburnum tinus . One isolate of P. cinnamomi used as positive control in all the inoculations was also highly aggressive to Iberian oaks and Eucalyptus dalrympleana .     

Development of a One-Step Real-Time Polymerase Chain Reaction Assay for Diagnosis of Phytophthora ramorum

ABSTRACT Phytophthora ramorum is a recently described pathogen causing bleeding cankers, dieback, and leaf blight on trees and shrubs in parts of Europe and North America, where the disease is commonly known as sudden oak death. This article describes the development of a single-round real-time polymerase chain reaction (PCR) assay based on TaqMan chemistry, designed within the internal transcribed spacer 1 region of the nuclear ribosomal (nr)RNA gene for detection of P. ramorum in plant material. Unlike previously described methods for the molecular detection of P. ramorum, this assay involves no post amplification steps or multiple rounds of PCR. The assay was found to have a limit of detection of 10 pg of P. ramorum DNA, and could detect P. ramorum in plant material containing 1% infected material by weight within 36 cycles of PCR. The assay also was used to test DNA from 28 other Phytophthora spp. to establish its specificity for P. ramorum. A quick and simple method was used to extract DNA directly from host plant material, and detection of P. ramorum was carried out in multiplex with an assay for a gene from the host plant in order to demonstrate whether amplifiable DNA had been extracted. Amplifiable DNA was extracted from 84.4% of samples, as demonstrated by amplification of host plant DNA. The real-time protocol was used to test 320 plant samples (from 19 different plant species) from which DNA extraction had been successful, and was shown to give results comparable with a traditional isolation technique for diagnosis of P. ramorum in plant material from common U.K. hosts.     

The occurrence of the A2 mating type of Phytophthora infestans in the Netherlands; significance and consequences

Phytophthora infestans (Mont.) de Bary, the causal agent of potato late blight, was first discovered in Europe in 1845. Until 1980, only A1 mating type isolates were known to occur in Europe. The absence of A2 mating type isolates restrained the fungus from sexual reproduction. In the early 1980s, A2 mating type isolates were discovered in Europe. Presumably, a new introduction ofP. infestans isolates originating from Mexico had taken place. In this paper, the significance of the presence of A1 and A2 mating type isolates in the Netherlands is reviewed. Now that both mating types are present, sexual reproduction can occur and its consequences for the control of potato late blight are discussed.     

Genetic differentiation at microsatellite loci among populations of Mycosphaerella graminicola from California, Indiana, Kansas, and North Dakota

Mycosphaerella graminicola causes Septoria tritici blotch (STB) in wheat (Triticum aestivum) and is considered one of the most devastating pathogens of that crop in the United States. Although the genetic structures of M. graminicola populations from different countries have been analyzed using various molecular markers, relatively little is known about M. graminicola populations from geographically distinct areas of the United States and, in particular, of those from spring versus winter wheat. These are exposed to great differences in environmental conditions, length and season of host-free periods, and resistance sources used in geographically separated wheat breeding programs. Thus, there is more likely to be genetic differentiation between populations from spring versus winter wheat than there is among those within each region. To test this hypothesis, 330 single-spore isolates of M. graminicola representing 11 populations (1 from facultative winter wheat in California, 2 from spring wheat in North Dakota, and 8 from winter wheat in Indiana and Kansas) were analyzed for mating type frequency and for genetic variation at 17 microsatellite or simple-sequence repeat (SSR) loci. Analysis of clone-corrected data revealed an equal distribution of both mating types in the populations from Kansas, Indiana, and North Dakota, but a deviation from a 1:1 ratio in the California population. In total, 306 haplotypes were detected, almost all of which were unique in all 11 populations. High levels of gene diversity (H = 0.31 to 0.56) were observed within the 11 populations. Significant (P ≤ 0.05) gametic disequilibrium, as measured by the index of association (rBarD), was observed in California, one Indiana population (IN1), and three populations (KS1, KS2, and KS3) in Kansas that could not be explained by linkage. Corrected standardized fixation index (G″(ST)) values were 0.000 to 0.621 between the 11 populations and the majority of pairwise comparisons were statistically significant (P ≤ 0.001), suggesting some differentiation between populations. Analysis of molecular variance showed that there was a small but statistically significant level of genetic differentiation between populations from spring versus winter wheat. However, most of the total genetic variation (>98%) occurred within spring and winter wheat regions while <2% was due to genetic differentiation between these regions. Taken together, these results provide evidence that sexual recombination occurs frequently in the M. graminicola populations sampled and that most populations are genetically differentiated over the major spring- and winter-wheat-growing regions of the United States.     

Contrasting genetic structure between Magnaporthe grisea populations associated with the golf course turfgrasses Lolium perenne (perennial ryegrass) and Pennisetum clandestinum (kikuyugrass)

Gray leaf spot (GLS) disease of perennial ryegrass (Lolium perenne) and kikuyugrass (Pennisetum clandestinum) in golf courses in California was first noted in 2001 and 2003, respectively, and within 5 years had become well established. The causal agent of the disease is the fungus Magnaporthe grisea, which is known to consist primarily of clonal lineages that are highly host specific. Therefore, our objective was to investigate host specificity and population dynamics among isolates associated primarily from perennial ryegrass and kikuyugrass since the disease emerged at similar times in California. We also obtained isolates from additional hosts (tall fescue, St. Augustinegrass, weeping lovegrass, and rice) and from the eastern United States for comparative purposes. A total of 38 polymorphic amplified fragment length polymorphism makers were scored from 450 isolates which clustered by host with high bootstrap support (71 to 100%). Genetic structure between kikuyugrass and perennial ryegrass isolates differed significantly. Isolates from kikuyugrass were genotypically diverse (n = 34), possessed both mating types, and some tests for random mating could not be rejected, whereas isolates from perennial ryegrass were less genotypically diverse (n = 10) and only consisted of a single mating type. Low genotypic diversity was also found among the other host specific isolates which also only consisted of a single mating type. This is the first study to document evidence for the potential of sexual reproduction to occur in M. grisea isolates not associated with rice (Oryza sativa). Moreover, given the significant host specificity and contrasting genetic structures between turfgrass-associated isolates, the recent emergence of GLS on various grass hosts in California suggests that potential cultural practices or environmental changes have become conducive for the disease and that the primary inoculum may have already been present in the state, despite the fact that two genotypes associated with perennial ryegrass and St. Augustinegrass in California were the same as isolates collected from the eastern United States.     

Characterization of Pyricularia grisea in the United States Using Independent Genetic and Molecular Markers

ABSTRACT A total of 540 isolates of Pyricularia grisea from rice in the United States were examined for vegetative compatibility, MGR586 DNA fingerprint diversity, and mating type based on hybridization with the mat1-1 and mat1-2 sexual mating type alleles. The collections contained both archived and contemporary field isolates representative of the known MGR586 lineages and races that occur throughout the United States. Complementary nitrate nonutilizing (nit) or sulfate nonutilizing (sul) mutants were used to assess vegetative compatibility in P. grisea. There was a complete correspondence between vegetative compatibility groups (VCGs), MGR586 lineage, and mating type among 527 contemporary isolates (collected between 1991 and 1997) from Arkansas, Louisiana, Missouri, Mississippi, and Texas; all isolates in MGR586 lineages A, B, C, and D belonged to VCGs US-01, US-02, US-03, and US-04, respectively. In addition, all isolates tested in VCGs US-01 and US-04 had the mat1-1 mating type allele whereas those in VCGs US-02 and US-03 had the mat1-2 allele. The strict association of independent markers during this sample period was consistent with a strictly asexual mode of reproduction. However, examination of archived isolates collected in the 1970s and 1980s and contemporary isolates revealed an incongruent relationship between the independent markers. MGR586 C and E isolates were vegetatively compatible which indicated that multiple robust MGR586 delineated lineages could be nested within certain VCGs. Although isolates in lineages C and E were vegetatively compatible, they were of opposite mating type. Several hypotheses, including recombination, could account for the incongruence between the various markers. Among the eight MGR586 lineages (A through H) that occur in the United States, all isolates in lineages A, D, E, G, and H had the mat1-1 allele, whereas isolates in lineages B, C, and F had the mat1-2 allele. Nit mutants can be recovered relatively easy from P. grisea and should allow large numbers of individuals within a population to be assessed for vegetative compatibility. VCGs may prove to be an effective multilocus marker in P. grisea. Thus, VCGs should be a useful means for characterizing genetic structure in populations of the rice blast fungus worldwide, provide a useful genetic framework to assist in interpreting molecular population data, and may provide insight into potential sexual or asexual recombination events.     

Efficacy of phosphonic acid, metalaxyl-M and copper hydroxide against Phytophthora ramorum in vitro and in planta

The ability of metalaxyl-M, phosphonic acid in the form of phosphonate, and copper hydroxide to inhibit different stages in the life cycle of Phytophthora ramorum , the causal agent of sudden oak death (SOD), was tested in vitro using 12 isolates from the North American forest lineage. In addition, experiments were conducted in planta to study the ability of phosphonic acid injections and metalaxyl-M drenches to control pathogen growth on saplings of California coast live oak ( Quercus agrifolia ), and of copper hydroxide foliar sprays to control infection of California bay laurel ( Umbellularia californica ) leaves. Phytophthora ramorum was only moderately sensitive to phosphonic acid in vitro , but was highly sensitive to copper hydroxide. In planta experiments indicated the broad efficacy of phosphonic acid injections and of copper hydroxide sprays in preventing growth of P. ramorum in oaks and bay laurels, respectively. Finally, although metalaxyl-M was effective in vitro , drenches of potted oak trees using this active ingredient were largely ineffective in reducing the growth rate of the pathogen in planta .     

Characterization and distribution of mating type genes in the dothistroma needle blight pathogens

ABSTRACT Dothistroma septosporum and D. pini are the two causal agents of Dothistroma needle blight of Pinus spp. in natural forests and plantations. Degenerate primers amplified portions of mating type genes (MAT1-1-1 and MAT1-2) and chromosome walking was applied to obtain the full-length genes in both species. The mating-type-specific primers designed in this study could distinguish between the morphologically similar D. pini and D. septosporum and between the different mating types of these species. Screening of isolates from global collections of D. septosporum showed that only MAT2 isolates are present in Australian and New Zealand collections, where only the asexual form of the fungus has been found. In contrast, both mating types of D. septosporum were present in collections from Canada and Europe, where the sexual state is known. Intriguingly, collections from South Africa and the United Kingdom, where the sexual state of the fungus is unknown, included both mating types. In D. pini, for which no teleomorph is known, both mating types were present in collections from the United States. These results provided new insights into the biology and global distribution of two of the world's most important pine pathogens and should facilitate management of the diseases caused by these fungi.     

Molecular Evidence of Distinct Introductions of the European Race of Gremmeniella abietina into North America

ABSTRACT The presence of the European (EU) race of Gremmeniella abietina var. abietina, the causal agent of Scleroderris canker of conifers, was first reported in North America in 1975 in the northeastern United States and subsequently in southern Quebec and Newfoundland during the late 1970s, where it quickly became established. We analyzed DNA profiles in samples from a historic collection of G. abietina var. abietina that included some of the first isolates of the EU race reported in the United States to test hypotheses concerning the G. abietina var. abietina epidemic in North America. Genetic diversity was partitioned by an analysis of molecular variance with haplotype frequencies and distances. Genetic differentiation was high between populations in continental North America and Newfoundland (between region differentiation, Phi(ct) = 0.665, P < 0.001). This result was not consistent with the hypothesis of a single introduction of the pathogen into the northeastern United States followed by secondary spread into northeastern Canada. In contrast, small levels of genetic differentiation were observed among continental North American populations (Phi(ct) = 0.047, P = 0.079), suggesting gene flow among these populations. A single haplotype of G. abietina var. abietina dominated the continental populations (80% of the isolates) but was absent from Newfoundland and Europe. Five haplotypes were found in the New-foundland population, all of which were either absent or very rare on the continent. Populations from continental North America clustered together and were distinct from a second cluster composed of European and Newfoundland populations. A phylogenetic analysis of the haplotypes indicated that some of the rare haplotypes may have derived from somatic mutations, whereas others probably occurred as the result of new introductions. The results are consistent with a scenario of distinct primary introductions of this pathogen into Newfoundland and continental eastern North America followed by secondary asexual propagation.     

Genetic Change Within Populations of Phytophthora infestans in the United States and Canada During 1994 to 1996: Role of Migration and Recombination

ABSTRACT Dramatic changes occurred within populations of Phytophthora infestans in the United States and Canada from 1994 through 1996. Occurrence of the US-8 genotype, detected rarely during 1992 and 1993, increased rapidly and predominated in most regions during 1994 through 1996. US-7, which infected both potato and tomato and made up almost 50% of the sample during 1993, was detected only rarely among 330 isolates from the United States analyzed during 1994. It was not detected at all in more limited samples from 1996. Thus, ability to infect both potato and tomato apparently did not increase the fitness of this genotype relative to US-8, as predicted previously. US-1, the previously dominant genotype throughout the United States and Canada, made up 8% or less of the samples analyzed during 1994 through 1996. A few additional genotypes were detected, which could indicate the beginnings of sexual reproduction of P. infestans within the United States and Canada. However, clonal reproduction still predominated in all locations sampled; opportunities for sexual reproduction probably were limited, because the A1 and A2 mating types usually were separated geographically. The high sensitivity of the US-1 genotype to the fungicide metalaxyl also could have reduced opportunities for contact between the mating types in fields where this compound was applied. The previous correlation between metalaxyl sensitivity and genotype was confirmed and extended to a new genotype, US-17: all US-1 isolates tested were sensitive; all isolates of the US-7, US-8, and US-17 genotypes tested to date have been resistant. Isolates of P. capsici and P. erythroseptica, two other species often found on tomato and potato, could be easily distinguished from each other and from P. infestans using a simple allozyme assay for the enzyme glucose-6-phosphate isomerase. This technique could be useful for rapid identification of species, in addition to genotype of P. infestans. It generally was not possible to predict which genotypes would be present in a location from 1 year to the next. Long-distance movement of US-8 in seed tubers was documented, and this was probably the primary means for the rapid spread of this genotype from 1993 through 1996.     

Genotypic and phenotypic characterization of the European A2 isolates of <Emphasis Type="Italic">Phytophthora ramorum</Emphasis>

Phytophthora ramorum, a recently described North American and European pathogen, has three clonal lineages. The NA1 and NA2 lineages are found in North American forests and nurseries, while the EU1 lineage appears mainly in European nurseries. P. ramorum is heterothallic, having two mating types A1 and A2. All NA1 and NA2 isolates are of A2 mating type. When first collected, all EU1 isolates were of A1 mating type, with the exception of one A2 isolate collected in Belgium in 2002. Screening 410 other Belgian isolates for mating type revealed two additional EU1-A2 isolates collected in 2002 and 2003. PCR-RFLP, AFLP and SSR markers were used to determine the nature of the mating type change. The three isolates show no indications of sexual recombination or mitotic crossing over, indicating that mutation or mitotic gene conversion is the most likely explanation for the mating type change. We compared the pathogenicity and sporulation characteristics of the EU1-A2 isolates to those of EU1-A1 and NA1-A2 isolates on four host plants. Despite small differences in pathogenicity on some hosts, the EU1-A2 isolates were similarly aggressive to each other and to the EU1-A1 isolates and more aggressive than the NA1-A2 isolates. Sporulation characteristics were also comparable among EU1-A2 isolates and between EU1-A1 and EU1-A2 isolates, except for EU1-A2 isolate BBA 26/02. The limited genotypic and phenotypic differences between EU1-A2 isolates probably evolved after the mating type change, which may have occurred several years before the isolates were detected. There are strong indications that the EU1-A2 population has been eradicated from Belgium.     

French isolates of Phytophthora infestans from potato and tomato differ in phenotype and genotype

Prior to 1996, the A2 mating type of Phytophthora infestans was not detected on potato in France, but was found at one site on tomato in 1995. This finding lead to the question of the extent of differences and relationships existing between the populations of P. infestans present on each host. A collection of 76 isolates collected in France, mainly in 1996, from potato and tomato was characterised for mating type, allozyme genotype at the Gpi and Pep loci, and mitochondrial DNA haplotype; 74 of these isolates were also characterised for multilocus RFLP fingerprint, and 62 for virulence. All isolates except four showed allozyme genotypes (Gpi 90/100 or 100/100, Pep 83/100 or 100/100) and mtDNA haplotypes (Ia or IIa) characteristic of the populations introduced into Europe in the late 1970s. The four exceptions were isolates collected from tomato in Southern France in 1988-1991, which showed some characteristics of the former European populations (Gpi 86/100, Pep 92/100, mtDNA Ib). Both mating types were present among the collections from both hosts, but isolates with the A2 mating type were found on potato only in one garden crop, adjacent to tomato. Nine different RG57 fingerprints were observed, with a greater diversity among tomato isolates. Furthermore, tomato and potato collections differed markedly in the frequencies of genotypes present. Finally, tomato isolates generally had a lower virulence complexity than potato isolates. These data suggest that P. infestans populations on tomato and potato are largely separated, despite the occurrence of limited gene flow.     

RAPD Analysis Provides Insight into the Biology and Epidemiology of Uncinula necator

ABSTRACT Ninety isolates of grape powdery mildew (Uncinula necator) from Europe (sixty-two) and India (twenty-eight) were collected. Ten of the sixty-two European isolates originated from mycelium overwintering in dormant buds ("flagshoots"). Mating types were determined, and genetic variation was assessed by random amplified polymorphic DNA (RAPD). Forty-one European isolates, including all "flagshoot" isolates, were mating type +, and twenty-one were mating type -. All Indian isolates were mating type -. Phenetic analysis based on 414 amplicons revealed three main groups. Most European isolates (53) clustered together. Nine flagshoot isolates clustered in a second distinct group. These isolates, which coexisted with other isolates in the field, may represent a genetically isolated biotype of U. necator. Indian isolates clustered into two groups. The first group (15 isolates) was a subgroup of the group containing European nonflagshoot isolates. The second group (12 isolates) was distinct from the other groups. These two groups of Indian isolates may represent genetically isolated populations with different climatic tolerances. A polymerase chain reaction primer pair, derived from a RAPD fragment specific to the Indian isolates, proved to be suitable for field studies.     

Genetic structure of Phaeosphaeria nodorum populations in the north-central and midwestern United States

Stagonospora nodorum blotch, caused by Phaeosphaeria nodorum, is considered one of the most destructive foliar diseases of wheat in the United States. However, relatively little is known about the population biology of this fungus in the major wheat-growing regions of the central United States. To rectify this situation, 308 single-spore isolates of P. nodorum were analyzed from 12 populations, five from hard red spring wheat cultivars in Minnesota and North Dakota and seven from soft red winter wheat in Indiana and Ohio. The genetic structure of the sampled populations was determined by analyzing polymorphisms at five microsatellite or simple-sequence repeat (SSR) loci and the mating type locus. Although a few clones were identified, most P. nodorum populations had high levels of gene (H(S) = 0.175 to 0.519) and genotype (D = 0.600 to 0.972) diversity. Gene diversity was higher among isolates collected from spring wheat cultivars in North Dakota and Minnesota (mean H(S) = 0.503) than in those from winter wheat cultivars in Indiana and Ohio (H(S) = 0.269). Analyses of clone-corrected data sets showed equal frequencies of both mating types in both regional and local populations, indicating that sexual recombination may occur regularly. However, significant gametic disequilibrium occurred in three of the four populations from North Dakota, and there was genetic differentiation both within and among locations. Genetic differentiation between the hard red spring and soft red winter wheat production regions was moderate (F(ST) = 0.168), but whether this is due to differences in wheat production or to geographical variation cannot be determined. These results suggest that sexual reproduction occurs in P. nodorum populations in the major wheat-growing regions of the central United States, and that geographically separated populations can be genetically differentiated, reflecting either restrictions on gene flow or selection.     

Genetic structure and pathogenicity of populations of Phytophthora infestans from organic potato crops in France, Norway, Switzerland and the United Kingdom

Genetic variation and pathogenicity of Phytophthora infestans isolates collected from organic potato crops of the susceptible cv. Bintje and the moderately resistant cv. Santé were assessed in France, Norway, and the United Kingdom in 2001 and in Switzerland in 2001 and 2002. Population structures differed considerably between the four P. infestans populations. Those from France, Switzerland and the UK were mainly clonal populations showing restricted levels of genetic diversity, whilst those from Norway were mixed A1 and A2 mating type populations with high levels of genetic diversity, suggesting periodical sexual reproduction. Isolates collected from cv. Bintje were on average more aggressive than or comparable to isolates from cv. Santé. Race complexity varied considerably between the regional P. infestans populations, with isolates from France and Switzerland showing the highest number of virulence factors. In all pathogen samples but the French, isolates collected from cv. Santé were more complex than isolates collected from cv. Bintje. No directional selection towards increased aggressiveness towards the more resistant cultivar Santé was observed. This suggests that there is no shift towards increased levels of pathogenicity in P. infestans populations following the large-scale introduction of more resistant potato varieties in organic production systems in Europe.     

Diversity Among Potato virus Y Isolates Obtained from Potatoes Grown in the United States

ABSTRACT Potato field isolates (Solanum tuberosum) of Potato virus Y (PVY) collected from the midwestern and western United States were characterized using serological, molecular, and biological assays. PVY field isolates were grouped into the previously defined categories: PVY(O), European PVY(NTN), North American PVY(NTN), and PVY(N:O) recombinant and four previously undefined groups. Studies reported here agree with published reports from Europe and elsewhere in North America as PVY isolates capable of causing veinal necrosis in tobacco indicator plants appear in high frequency. In contrast to European experiences, PVY tuber necrosis isolates have a PVY(O) coat protein rather than that of PVY(N). Several PVY(N:O) recombinant isolates induced potato tuber necrotic ringspot disease (PTNRD) in the highly susceptible potato cv. Yukon Gold. The PTNRD symptoms produced by these PVY(N:O) recombinants were atypical compared with lesions found on the same cultivar infected with either the European or North American PVY(NTN) isolates. These PVY(N:O) isolates produced a roughly circular, sunken necrotic lesion on the surface of the tuber instead of the typical external sunken ring pattern displayed by PVY(NTN) isolates. This study establishes the complex nature of PVY populations within the U.S. potato industry and clearly demonstrates the diverse nature of PVY in the United States.     

Distinguishing Galactomyces citri-aurantii from G. geotrichum and characterizing population structure of the two postharvest sour rot pathogens of fruit crops in California

A growth assay in lemon juice and polymerase chain reaction amplifications using newly designed species-specific primers from endopolygalacturonase and β-tubulin genes rapidly differentiated isolates of the morphologically similar fruit sour rot pathogens Galactomyces citriaurantii and G. geotrichum. Isolates of both species were collected from agricultural soils and decaying fruit at locations within and outside California, including worldwide locations, and were used in population genetic studies based on amplified fragment length polymorphic (AFLP) DNA markers. For all four geographically defined subpopulations (three counties of California and locations outside California) among 97 isolates of G. citri-aurantii and for the two subpopulations (origin within or outside California) among 35 isolates of G. geotrichum, the proportion of polymorphic loci and haplotypic diversity was high. In total, 82 unique haplotypes were identified for G. citri-aurantii for the four subpopulations and, of these, 80 haplotypes were unique among subpopulations. For G. geotrichum, 25 unique haplotypes were identified among the two subpopulations and no haplotype was shared. Indices of genetic differences (F(ST)) between subpopulations within each species were all low (e.g., 0.038 for G. geotrichum and 0.085 to 0.226 for G. citriaurantii), indicating a low level of genetic differentiation. Following clone correction, mating type segregation ratios for G. citri-aurantii did not significantly (P > 0.1) deviate from a 1:1 ratio for all four subpopulations or the entire population. Tests of the index of association (I(A)) and parsimony tree-length permutation tests (PTLPT) supported a random mating structure for clone-corrected data for the Kern, Tulare, and Ventura County subpopulations and the null hypothesis of random mating could not be rejected. Additionally, PTLPT also supported random mating for the "outside of California" population. For G. geotrichum, random mating was only tested using I(A) and PTLPT and the null hypothesis of random mating was not rejected (P > 0.05) using clone-corrected data. Further evidence that sexual recombination likely occurs in both species of Galactomyces was the lack of grouping consistency in the unweighted pair-group method with arithmetic mean clustering of AFLP data. A high confidence based on bootstrap values was obtained for only a few of the nodes in each of the two trees. A mixed reproduction system with an out-crossing sexual mating system and a prolific asexual phase is proposed for both species.     

Sudden oak death: geographic risk estimates and predictions of origins

Ecological niche modelling techniques were applied to address the questions of the origins and potential geographic extent of sudden oak death, caused by the pathogen Phytophthora ramorum . Based on an ecological niche model derived from the phytopathogen's California distribution and distributions of potential host species, it was determined that the disease has high potential to colonize the southeastern United States, and that its likely source area is eastern Asia.