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.     

Evolution of Phytophthora infestans populations in the two most important potato production areas of France during 1992–96

Phytophthora infestans isolates, collected in the two most important potato production regions of France (Brittany during 1992–96 and northern France during 1994–96), were analysed for mating type, Gpi and Pep allozyme genotypes, mitochondrial DNA haplotypes, response to metalaxyl and virulence patterns in order to determine the structure of populations. 'New' populations, similar to those observed in northern Europe and characterized by the presence of both mating types, Gpi 90/100 Pep 83/100, Gpi 90/100 Pep 100/100, Gpi 100/100 Pep 100/100 and Gpi 100/100 Pep 83/100 allozyme genotypes, Ia and IIa mitochondrial haplotypes and common races 1.3.4.7, 1.4.10.11, 1.3.4.7.11 and 1.3.4.7.10.11 were present in both areas. A new allozyme genotype ( Gpi 90/90 Pep 83/100), never previously detected in Europe, was discovered in Brittany in 1992, but disappeared in later years. Despite the appearance of the A2 mating type in Brittany in 1996 and in northern France since 1995, both populations were highly homogeneous and probably consisted of a few related clones. No evidence for sexual recombination was obtained. Although Brittany, isolated from other potato production areas, and northern France, situated close to north European production areas where the A2 mating type was discovered some years ago, are two contrasted regions, no geographic substructuring was apparent between the two regions. However, variations in genotypic frequencies and in race composition were observed from one year to the next at a single site, as well as between several local sites during the same year. The role of possible evolutionary mechanisms implicated in these evolutions, especially founder effects, is discussed.     

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.     

Studies on the inheritance of resistance to metalaxyl in Phytophthora infestans

Single-oospore progeny from matings between field isolates of Phytophthora infestans either highly resistant or sensitive to metalaxyl were analysed for fungicide sensitivity in vitro, mating type and, in some cases, allozyme variation at the locus for glucosephosphate isomerase ( GPI-1. ) In each cross the majority of first-generation (F₁) progeny showed intermediate sensitivity to metalaxyl. Frequency distributions were skewed towards sensitivity and a few progeny were either wholly sensitive or resistant phenotypes. Allozyme analysis of F₁ progeny from a cross between parents of Dutch and Mexican origin showed that c. 10% were selfs of both parents and of parental phenotype for metalaxyl sensitivity. The selfs of the A2 but not the Al parent segregated for mating type. Two backcross generations to the metalaxyl-resistant Dutch parent gave unexpected phenotypic frequency distributions and aberrant ratios for genotypes at the GPI-I locus.
Progeny of another backcross between an F₁ isolate of intermediate sensitivity and its sensitive Egyptian parent gave a 1:1 ratio for sensitive: intermediate phenotypes. F₂ progeny from a related sibmating between intermediate phenotypes segregated in a ratio close to 1:2:1 for sensitive: intermediate: resistant phenotypes. Segregation was also observed among sexual progeny of an intermediate self-fertile isolate from the backcross generation but not among progeny of a resistant self-fertile phenotype. Mating type segregated in both cases.
These data are consistent with resistance to metalaxyl in P. infestans being governed by a single nuclear locus exhibiting incomplete dominance. This hypothesis is discussed in relation to the incidence of resistance to metalaxyl in natural populations of P. infestans.     

Characterization of the Phytophthora infestans Population in the Columbia Basin of Oregon and Washington from 1992 to 1995

ABSTRACT Isolates of Phytophthora infestans collected from 1992 to 1995 from potato fields in the Columbia Basin of Oregon and Washington were analyzed for compatibility type, metalaxyl sensitivity, and glucose-6-phosphate isomerase (Gpi) genotype. In 1992, 30 of 31 isolates were of the US-1 multilocus genotype. A single metalaxyl-resistant isolate of the US-6 (A1 Gpi 86/100) genotype was found near the end of the growing season. In 1993, only 2 of the 59 isolates collected were A1 isolates with Gpi 86/100. Ten isolates were of the A2 compatibility type, seven with Gpi 100/111, two with Gpi 100/100, and one was undetermined. The remaining isolates were metalaxyl-resistant A1 compatibility types with either Gpi 100/100 or 100/111. The first A2 isolates in the Columbia Basin were found in 1993. In 1994, 10 of 18 isolates were of the US-1 genotype. The remaining isolates were US-6 and US-8 genotypes. In 1995, 97% of 268 isolates tested were of the US-8 genotype. Five isolates were A2 compatibility type with Gpi 100/122. One A2, metalaxyl-resistant isolate was Gpi 100/100/111, and two A1 isolates were Gpi 100/111/122. The population of P. infestans quickly changed between 1992 and 1995, from a population comprised almost exclusively of the US-1 genotype to a population represented by new or recombinant genotypes.     

The Northern Ireland Phytophthora infestans population 1998–2002 characterized by genotypic and phenotypic markers

A total of 204 isolates of Phytophthora infestans from Northern Ireland, almost all from commercial potato crops, were collected over 5 years (1998–2002). Phenotypic diversity was assessed using mating type and metalaxyl resistance; genotypic diversity was assessed using two allozyme loci (glucose-6-phosphate isomerase, Gpi, and peptidase, Pep ), mitochondrial DNA haplotype and the multilocus RFLP probe RG57. All isolates were A1 mating type and Gpi 100/100 . The majority were Pep 100/100 , but four Pep 83/100 and six Pep 96/100 isolates were identified. Three mtDNA haplotypes were detected; haplotype IIa was the most common, but each year up to 2001 its frequency declined, with a concomitant increase in Ia isolates. Three isolates had the rare haplotype IIb, but this was only detected in 1998. Metalaxyl resistance and mtDNA haplotype were markedly associated: most haplotype Ia isolates were metalaxyl-resistant, whereas haplotype IIa was more commonly associated with metalaxyl sensitivity. Analysis of a subsample of 91 isolates revealed nine RG57 genotypes, three associated exclusively with haplotype IIa and six exclusively with haplotype Ia. The most common RG57 genotype (51% of isolates) comprised both metalaxyl-resistant and -sensitive haplotype IIa isolates. However, of haplotype Ia isolates, all metalaxyl-resistant phenotypes belonged to one of four RG57 types, one of which was the second most frequent overall (29% of isolates), while all metalaxyl-sensitive isolates belonged to one of two other types. The P. infestans population in Northern Ireland appears to consist of a limited number of clones related to, but differentiated from, the populations in mainland Britain and elsewhere in Europe.     

Population Genetic Structure of Phytophthora infestans in Ecuador

ABSTRACT The population genetic structure of Phytophthora infestans in Ecuador was assessed from 101 isolates collected from 1990 to 1992 and 111 isolates collected in 1993. All isolates were analyzed for mating type and allozyme genotype. Both samples were dominated (>95%) by a clonal lineage (EC-1) defined from neutral markers: 90/100 genotype for glucose-6-phosphate isomerase, 96/100 genotype for peptidase, A1 mating type, and a previously unreported nuclear DNA fingerprint. The remaining isolates belonged to the US-1 clonal lineage, which has a worldwide distribution. Isolates in the 1993 sample were analyzed for virulence and metalaxyl sensitivity. All representatives of EC-1 had complex patho-types, with three pathotypes representing >60% of the collection. There was variation for metalaxyl sensitivity. There was no evidence for geographical substructuring on the basis of neutral markers, but there was evidence for limited substructuring based on metalaxyl sensitivity and specific virulence. We hypothesize that EC-1 has been recently introduced to Ecuador.     

Phytophthora infestans Populations from Tomato and Potato in North Carolina Differ in Genetic Diversity and Structure

ABSTRACT Phytophthora infestans causes a destructive disease on tomato and potato. In North Carolina (NC) potatoes are mostly grown in the east, whereas tomatoes are grown in the mountainous areas in the western part of the state. Five genotypes of P. infestans were identified from 93 and 157 isolates collected from tomato and potato over a 5 year period between 1993 and 1998. All isolates collected from potato in eastern NC were the US-8 genotype, whereas only a single isolate was the US-1 genotype. Tuber blight was found on immature daughter tubers in a single field in 1997, however infection on mature tubers was not observed. Within potato fields, a range of sensitivity to metalaxyl was observed among isolates but all were either intermediate or highly resistant to the fungicide. In contrast, isolates from tomatoes included previously reported US-7 and US-8 genotypes and two new genotypes called US-18 and US-19 (A2 mating type, allozyme genotype Gpi 100/100 and Pep 92/100). These genotypes had unique restriction fragment length polymorphism banding patterns, were sensitive to metalaxyl, and have not been reported elsewhere. All genotypes, with the exception of the US-1, were the Ia mitochondrial haplotype. Thus, isolates of P. infestans from tomato were more genetically diverse over time in NC than those from potato and include two new genotypes that are sensitive to metalaxyl.     

云南省马铃薯晚疫病菌交配型及生物学特性研究

 作者对1998~2000年间采自云南省13个县、23个地点的马铃薯晚疫病菌的交配型、菌落形态、燕麦培养基上生长情况、生长速度和产孢量进行了测定。结果显示,采自云南13个县、23个地点的共157个菌株全部为A1交配型,表明云南马铃薯主产区的晚疫病菌以A1交配型为主,同时,被测的代表菌株在生长速度和产孢量上存在显著差异,表明这一地区的晚疫病菌种群内存在丰富的遗传多样性。此外,结果还显示,晚疫病菌菌株在燕麦培养基上的生长情况与其菌落形态和交配型不相关。     

福建省致病疫霉交配型、甲霜灵敏感性及生理小种组成分析

为明确福建省致病疫霉的群体结构及其变化特点,对2001—2007年分离自11个市(县)的番茄和马铃薯产区的致病疫霉菌株进行交配型、甲霜灵敏感性及生理小种测定。交配型测定的187个菌株中,181株为A1交配型,1株为A2交配型,1株为A1,A2交配型,1株为自育交配型,其余3株交配型未知。对187个菌株的甲霜灵敏感性测定结果表明,高抗、中抗和敏感菌株分别占97.3%、2.1%、0.5%。生理小种测定结果表明,供试的11个抗性基因均可被克服,其毒性频率在3.3%～100%之间,51个菌株存在30个生理小种,其中小种3.7出现频率最高,为13.7%,是福建省优势生理小种类型;其次为小种3.4.7,发生频率为9.8%。     

Aggressiveness and Competitive Fitness of Phytophthora infestans Isolates Collected from Potato and Tomato in France

ABSTRACT To test the hypothesis that host-related differences in the genotypic composition of populations of the late blight pathogen Phytophthora infestans can be explained by differential pathogenicity, the aggressiveness of isolates of the pathogen collected in France from potato and tomato was measured on detached leaflets of potato (cv. Bintje) and tomato (cv. Marmande). A preliminary trial with four isolates (two each from potato and tomato) showed that lesion appearance and development were similar for each isolate in detached leaflets and in whole plant tests in growth cabinets. Isolates collected from tomato were more pathogenic to tomato than isolates collected from potato. This was particularly the case for isolates belonging to the A2 mating type. Isolates originating from potato had a higher infection efficiency and a higher sporulation capacity on this host, but they induced lesions that generally spread more slowly than those caused by isolates from tomato. Extensive variation for components of aggressiveness on potato, and to a lesser extent on tomato, was observed in collections of isolates from each of the two hosts. Competition experiments between one potato isolate and one tomato isolate in field plots of the susceptible potato cv. Bintje clearly demonstrated the higher competitive fitness of the potato isolate on its host of origin. Therefore, differential pathogenicity to potato and tomato certainly contributes to the differentiation between P. infestans populations present on potato and tomato in France; however, additional factors, possibly related to survival ability or random genetic drift, are probably also involved and may explain the persistence of weakly pathogenic isolates in these populations.     

Oospores in cultures of Phytophthora infestans resulting from selfing induced by the presence of P. drechsleri isolated from blighted potato foliage

A Phytophthora species isolated from blighted potato foliage was identified as P. drechsleri , mating type A2. The isolate was found to be weakly pathogenic on a range of hosts. Pairings in vitro of P. drechsleri with British isolates of P. infestans mating type A1 when separated by a polycarbonate membrane resulted in selfed oospores of both species. The possibility that A1 P. infestans is induced to self in the field and the genetic consequences of such selfing are discussed.     

Host Specificity of Phytophthora infestans on Tomato and Potato in Ecuador

ABSTRACT Sixty Ecuadorian isolates of Phytophthora infestans from potato and 60 isolates from tomato were compared for dilocus allozyme genotype, mitochondrial DNA haplotype, mating type, and specific virulence on 11 potato R-gene differential plants and four tomato cultivars, two of which contained different Ph genes. Restriction fragment length polymorphism (RFLP) fingerprints of subsamples of isolates from each host were compared by using RG57 as the probe. All potato isolates had the allozyme genotype, haplotype, and mating type of the clonal lineage EC-1, which had been previously described in Ecuador. With the same markers, only one isolate from tomato was classified as EC-1; all others belonged to the globally distributed US-1 clonal lineage. RFLP fingerprints of isolate subsets corroborated this clonal lineage classification. Specific virulence on potato differentials was broadest among potato isolates, while specific virulence on tomato cultivars was broadest among tomato isolates. Some tomato isolates infected all tomato differentials but no potato differentials, indicating that specific virulence for the two hosts is probably controlled by different avirulence genes in P. infestans. In two separate experiments, the diameters of lesions caused by nine isolates from potato and 10 from tomato were compared on three tomato and three potato cultivars. All isolates produced larger lesions on the host from which they were isolated. No isolates were found that were highly aggressive on both tomato and potato. We conclude that there are two different populations of P. infestans in Ecuador and that they are separated by host.     

A Novel Population of Phytophthora, Similar to P. infestans, Attacks Wild Solanum Species in Ecuador

ABSTRACT Twenty-six isolates of a Phytophthora population from two wild solanaceous species, Solanum tetrapetalum (n 11) and S. brevifolium (n = 15), were characterized morphologically, with genetic and phenotypic markers, and for pathogenicity on potato and tomato. Based on morphology, ribosomal internal transcribed spacer region 2 (ITS2) sequence, and pathogenicity, all isolates closely resembled P. infestans and were tentatively placed in that species. Nonetheless, this population of Phytophthora is novel. Its primary host is neither potato nor tomato, and all isolates had three restriction fragment length polymorphism (RFLP) bands (probe RG57) and a mitochondrial DNA haplotype that have not been reported for P. infestans. All the isolates were the A2 mating type when tested with a P. infestans A1 isolate. The A2 mating type has not been found among isolates of P. infestans from potato or tomato in Ecuador. Geographical substructing of the Ecuadorian A2 population was detected. The three isolates from the village of Nono, identical to the others in all other aspects, differed by three RFLP bands; those from Nono lacked bands 10 and 16, but possessed band 19. Most of the Ecuadorian A2 isolates were nonpathogenic on potato and tomato, but a few caused very small lesions with sparse sporulation on necrotic tissue. Cluster analysis of multilocus genotypes (RFLP, mating type, and two allozymes) dissociated this A2 population from genotypes representing clonally propagated populations of P. infestans worldwide. The current hypotheses for the historical global movements of P. infestans do not satisfactorily explain the origin or possible time of introduction into Ecuador of this A2 population. Assuming the population is P. infestans, its presence in Ecuador suggests either a hitherto unreported migration of the pathogen or an indigenous population that had not previously been detected.     

Molecular comparisons amongst wheat bymovirus isolates from Asia, North America and Europe

To study the variation between wheat bymovirus isolates and to resolve uncertainties about the identity of the virus in some countries, leaves of infected plants were obtained from nine sites in China and from one each in Italy, Germany, USA and Canada. The German isolate was obtained from rye and the Canadian isolate was the type strain of wheat spindle streak mosaic virus (WSSMV). In RT-PCR, using primers designed from a partial sequence of a French isolate (tentatively described as WSSMV), genome fragments were obtained from the Italian and the French isolates but not from the Chinese ones. Conversely, products were consistently obtained from the Chinese isolates, but not from the Italian or French ones, when primers were designed from the sequence of a Japanese isolate of wheat yellow mosaic virus (WYMV). Nucleotide sequences were obtained from regions at or near the 3'-terminus of RNA1 of six Chinese isolates and the four from Europe and North America, usually including the coat protein. Nucleotide and amino acid sequence comparisons demonstrated that the European and North American isolates were extremely similar and were therefore WSSMV, while the Chinese isolates were close to the Japanese isolate and were thus WYMV.     

Variability in Virulence Among Asexual Progenies of Phytophthora infestans

ABSTRACT One hundred two single zoospore isolates of Phytophthora infestans, derived asexually from four parental isolates of US-8 genotype and one isolate of US-1 genotype, were characterized for their virulence phenotypes to determine changes in virulence during asexual reproduction. Potato differentials, each containing a major gene for resistance to P. infestans (R1 to R11), were used to characterize the virulence patterns. Isolates were also characterized for mating type, glucose-6-phosphate isomerase (Gpi) banding pattern, and DNA fingerprints using probe RG57 to determine any genotypic changes in the single zoospore isolates. A subset of these single zoospore isolates was tested for response to mefenoxam to determine any shifts in sensitivity. Results showed that single zoospore isolates derived from parent PI-1 (US-8, 11 isolates) were identical to their parental virulence. Isolates derived from parent PI-191 (US-8, 29 isolates) showed some differences in virulence, mainly toward R8 and R9. Isolates derived from parent PI-126 (US-8, 14 isolates) demonstrated a higher level of virulence diversity. Isolates derived from parents PI-52 (US-1, 28 isolates) and PI-105 (US-8, 20 isolates) showed the highest level of virulence variability among the single zoospore isolates. Mating type, Gpi banding pattern, and DNA fingerprints for the single zoospore isolates were, in most cases, identical to the parental isolates. Single zoospore isolates showed different levels of sensitivity to mefenoxam. Virulence and other genetic changes during asexual reproduction are likely to play a major role in changing the race structure of P. infestans populations. This continuous change in the race structure is a serious problem and now poses a new challenge for utilization of race-specific resistance to manage late blight of potato.     

Phenotypes of Phytophthora infestans collected in England and Wales from 1985 to 1988: mating type, response to metalaxyl and isoenzyme analysis

In a survey of 1864 isolates derived from samples of Phytophthora infestans obtained from potato and tomato collected in England and Wales between 1985 and 1988, the A2 mating type was identified in approximately 10% of samples. There was no evidence for an overall increase in the frequency of the A2 mating type over this period. In agar culture, A2 isolates exhibited a distinct lumpy-colony morphology. Lumps were made up of intensely dichotomously branched hyphae. The incidence of resistance to metalaxyl among samples increased from 41 to 60% between 1986 and 1988. Some of the A2 isolates were resistant to metalaxyl and like other A2 isolates gave rise to progeny from in vitro matings with Al isolates. Except in one case, Al and A2 isolates were uniformly homozygous at an isoenzyme locus for glucosephosphate isomerase but isolates exhibited some variation at a peptidase locus. The source of the A2 isolates could not be established from these data.     

Effect of Sexual Recombination on Pathotype Frequencies in Barley Powdery Mildew Populations of Artificially Inoculated Field Plots

Changes in pathotype frequencies due to sexual reproduction during summer were assessed in barley plots inoculated at the start of the growing season with powdery mildew isolates of the pathotype GL1 (Va22Vh) which belonged to a clonal lineage frequent in Northern France in 1992–1996 and was absent or rare in the local population at the time of the study. The capacity for recombination among isolates belonging to the pathotype GL1 was confirmed by performing crosses in the greenhouse. The field experiment was repeated over two years, with two field plots inoculated with either a single isolate (one mating type) or four isolates (two mating types). Following artificial inoculation, the frequency of GL1 increased to between 40% and 80% of the total conidia population at the end of the asexual epidemics before summer. When only one isolate (one mating type) of the pathotype GL1 was present, the frequency of the GL1 pathotype decreased to 4–5% of the ascospore population, following sexual recombination between the inoculated isolate and the local natural population. When isolates of both mating types of GL1 were inoculated, however, the GL1 pathotype remained dominant (28%) in the ascospore population. A pathotype dominant at the end of the summer might possibly have over-summered through asexual reproduction, or alternatively it may originate from sexual reproduction. The observed GL1 frequencies and numbers of virulences per isolate (complexity) observed on the field plots were compared to those calculated with a model assuming random or non-random mating.     

Commercial Fungicide Formulations Induce In Vitro Oospore Formation and Phenotypic Change in Mating Type in Phytophthora infestans

ABSTRACT A wide range of commercially formulated fungicides cause in vitro effects on mating behavior in specific isolates of Phytophthora infestans, the causal agent of late blight of potato and tomato. Four isolates of P. infestans representing each of the four common US genotypes, US-1, US-6, US-7, and US-8 and varying in their sensitivity to metalaxyl, were exposed to a variety of fungicides used to control late blight in petri dish assays at concentrations ranging from 1 to 100 mug a.i./ml. Exposure of each of these normally heterothallic single mating type isolates of P. infestans to 9 of the 11 commercial fungicide formulations tested resulted in the formation of oospores after 2 to 4 weeks. The highest numbers of oospores were formed on media amended with Ridomil 2E (metalaxyl) and Ridomil Gold EC (mefenoxam) at 0.1 to 10 mug a.i./ml, averaging as many as 471 and 450 oospores per petri dish, respectively. Several other fungicides including Maneb, Manzate (Mancozeb), Curzate (cymoxanil + mancozeb), and Acrobat MZ (dimethomorph + mancozeb) also induced oospore formation, producing from 0 to 200 oospores per plate at fungicide concentrations from 0.1 to 10 mug a.i./ml. The metalaxyl resistant isolates formed oospores in response to the fungicides more often than the metalaxyl sensitive isolates. No oospores were formed on media amended with Bravo (chlorothalonil) or Tattoo C (chlorothalonil + propamocarb HCl) and these compounds completely suppressed growth of the isolates at 0.1 and 1 mug a.i./ml. Three metalaxyl resistant A2 isolates mated with both A1 and A2 isolates after exposure to the fungicides Ridomil 2E and Ridomil Gold EC. Alterations in mating type expression were also observed in a metalaxyl sensitive A1 isolate after exposure to Benlate (benomyl). Copious amounts of chemicals are applied annually to potato and tomato production areas to control late blight. Our results indicate that a wide range of chemically diverse fungicides can induce normally heterothallic metalaxyl resistant isolates of P. infestans to form oospores in vitro after short exposures to the fungicides.     

Pathogenic characteristics of isolates of Aphanomyces euteiches from pea in France

Aphanomyces root rot ( Aphanomyces euteiches ) has become a very destructive disease in French pea crops since 1993. The host specificity of the French pea-infecting populations of this pathogen was investigated by inoculating pea, common vetch, alfalfa, broad bean and green bean with 91 pea-infecting A. euteiches isolates, originating from the main areas of infestation in France. These isolates were compared to 13 isolates from various countries and hosts (pea, green bean, alfalfa). Virulence phenotypes were defined according to the pathogenicity data on the different hosts: all isolates from France infected two to five legume species, with most infecting pea, vetch, alfalfa and broad bean. Four pathotypes were characterized within the French isolates: one type corresponded to broad host range isolates, the second was composed of isolates preferentially agressive on pea/vetch/alfalfa and weakly aggressive on broad bean, and two others corresponding to more specialized isolates that preferentially infected pea/vetch or pea/vetch/alfalfa. Most isolates from France were preferentially pathogenic on pea, like the pea-infecting isolates from other countries, but were less specialized than the alfalfa- and green bean-infecting isolates from other countries. These results suggest that A. euteiches isolates may be maintained on wild or cultivated legumes other than pea in France.