Characterization of pathotypes among isolates of Xanthomonas axonopodis pv. manihotis in Colombia

Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihotis ( Xam ) is a destructive disease occurring in most cassava growing-areas. Although Colombian isolates of Xam differ in DNA polymorphism and pathogenicity, no suitable host differentials have been identified to demonstrate physiological specialization. A set of 26 Xam isolates from three edaphoclimatic zones (ECZs) in Colombia was selected for inoculation on a set of 17 potential cassava differentials. Leaf inoculation and stem puncture were used in order to detect possible specific interactions between cultivars and isolates. Cultivar × isolate interaction was highly significant ( P  < 0·001) after stem inoculation, but not after leaf inoculation. The stem inoculation technique was selected as a method for resistance screening of cassava cultivars for bacterial blight resistance. A highly significant interaction was also detected when cultivar behaviour was rated as area under the disease progress curve (AUDPC) after stem inoculation. Different pathotypes were defined among the 26 isolates and differential cultivars were proposed to define the pathotypic composition of Xam populations in three ECZs in Colombia. The results should help to improve selection of sources of resistance to cassava bacterial blight.     

Measuring the Genetic Diversity of Xanthomonas axonopodis pv. manihotis Within Different Fields in Colombia

ABSTRACT Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihotis, is a widespread disease that affects cassava (Manihot esculenta). We collected 238 X. axonopodis pv. manihotis strains by intensively sampling single fields in four edaphoclimatic zones (ECZs) in Colombia. DNA polymorphism of different X. axonopodis pv. manihotis populations was assessed by restriction fragment length polymorphism (RFLP) analyses, repetitive sequence-based polymerase chain reaction (rep-PCR), and amplified fragment length polymorphism (AFLP) assays. Genetic diversity, phenetic relationships among strains, and the coefficient of genetic differentiation were determined. All strains were tested for aggressiveness on the susceptible cassava cv. MCOL 1522. Strains were also tested for virulence on cassava differentials adapted to the strains' respective ECZs. Our study showed that the Colombian X. axonopodis pv. manihotis population has a high degree of genetic diversity. The hierarchical analysis of diversity showed genotypic differentiation at all levels, among ECZs, among fields within ECZs, and among strains within fields planted to several cassava genotypes. New RFLP haplotypes were detected, leading to the characterization of a new pathotype. Dendrograms from AFLP were more robust than those from RFLP data. A close association between the strains' geographical origin and DNA polymorphism was obtained using RFLP and AFLP data. We suggest that the host played a role in causing pathogen differentiation.     

Bacterial blight of rice and its control in Nepal

Research on Xanthomonas oryzae pv. oryzae, the bacterial blight of rice pathogen, was initiated at the Institute of Agriculture and Animal Science (IAAS) with the main objective of assessing the population structure of X. o. pv. oryzae through the use of both conventional and molecular markers in combination with virulence typing. A high DNA polymorphism was detected in the pathogen populations using different DNA probes and rep-PCR primers. Most strains were avirulent to cultivars containing the bacterial blight resistance gene Xa-21, which suggested the strategy that targets gene deployment is feasible in Nepal.     

Phenotypic variation within a clonal lineage of Phytophthora infestans infecting both tomato and potato in Nicaragua

Late blight caused by Phytophthora infestans (Mont.) de Bary is a constraint to both potato and tomato crops in Nicaragua. The hypothesis that the Nicaraguan population of P. infestans is genotypically and phenotypically diverse and potentially subdivided based on host association was tested. A collection of isolates was analyzed using genotypic markers (microsatellites and mitochondrial DNA haplotype) and phenotypic markers (mating type, virulence, and fungicide sensitivity). The genotypic analysis revealed no polymorphism in 121 of 132 isolates of P. infestans tested. Only the Ia haplotype and the A2 mating type were detected. Most of the tested isolates were resistant to metalaxyl. The virulence testing showed variation among isolates of P. infestans. No evidence was found of population differentiation among potato and tomato isolates of P. infestans based on the genotypic and phenotypic analysis. We conclude that the Nicaraguan population of P. infestans consists of a single clonal lineage (NI-1) which belongs to the A2 mating type and the Ia mitochondrial DNA haplotype. Moreover, based on the markers used, this population of P. infestans does not resemble the population in countries from which potato seed is imported to Nicaragua or the population in neighboring countries. The data presented here indicate that the NI-1 clonal lineage is the primary pathogen on both potato and tomato, and its success on both host species is unique in a South American context.     

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.     

Population Analysis of Fusarium graminearum from Wheat Fields in Eastern China

ABSTRACT Wheat heads showing symptoms of Fusarium head blight were collected from four commercial fields in Zhejiang Province, China, an area where epidemics occur regularly. A total of 225 isolates were subjected to population-level analyses using restriction fragment length polymorphism (RFLP) as markers. Diagnostic RFLP markers established that all isolates belonged to Fusarium graminearum lineage 6. Nine polymorphic probes were hybridized to all isolates, resulting in 65 multilocus RFLP haplotypes (MRH). Probing with the telomeric clone pNla17, which reveals differences among isolates in the hypervariable subtelomeric region, differentiated the 65 MRH further into 144 clones. Mean gene diversity for the four field populations was similar, ranging from H = 0.306 - 0.364 over the nine RFLP loci for clone-corrected data. High levels of gene flow were inferred from a low level of population subdivision among all field populations, indicating that they were part of the same population. Pairwise linkage disequilibrium measures did not unequivocally support a random mating population, because one-third of locus pairs were significantly different from the null hypothesis of no-association between alleles. We speculate therefore that sexual recombination may not be frequent and that high levels of genotypic diversity may be maintained by relatively low selection pressure acting on a highly diverse population.     

Characterisation of <Emphasis Type="Italic">Phytophthora infestans</Emphasis> isolates collected from potato in Estonia during 2002–2003

A collection of 101 isolates of Phytophthora infestans, obtained from seven sampling sites representing central, east and south-east Estonia during 2002 and 2003 were assessed for several phenotypic and genotypic markers. All 101 isolates were assessed for virulence and resistance to metalaxyl. Virulence to each of the 11 classic resistance genes was found among the tested isolates. The mean number of virulences per isolate was 6.3, with a very low frequency of virulence against resistance genes R5 (5%) and R9 (14%). The most common pathotypes were 1.3.4.7.8.10.11 and 1.3.4.7.10.11, representing altogether 12% of the studied strains. In terms of metalaxyl resistance, 30 resistant, 52 intermediate and 19 sensitive isolates were found. A subgroup of 50 isolates was assessed for mating type, allozymes [glucose-6-phosphate isomerase (Gpi) and peptidase (Pep)], DNA fingerprints with probe RG57 and mtDNA haplotype. Of this subset, 30 were A1 and 20 were A2. Collections from three of the seven fields contained both mating types. Allozyme analysis did not reveal any polymorphism. However, 19 diverse RG57 fingerprints were detected, and two mitochondrial DNA haplotypes, Ia and IIa, were detected. By combining the mating type, mtDNA haplotype and RG57 fingerprint data, 26 multilocus genotypes were identified, of which 18 were detected only once. Genotypic diversity measured by the normalised Shannon diversity index was high (0.76). The large number of multilocus genotypes and the presence of both mating types in some fields indicate that sexual reproduction may take place in Estonian populations of P. infestans.     

Evidence of parasexual exchange of DNA in the rice blast fungus challenges its exclusive clonality

ABSTRACT We applied DNA markers to determine whether parasexual recombination may contribute to the extreme genetic diversity and variability observed in Magnaporthe grisea, the causal agent of rice blast disease. Dispersed repetitive elements and mapped, low-copy restriction fragment length polymorphism (RFLP) probes were used to detect transfers of DNA between cultured isolates of M. grisea. Low-copy RFLP probes also were used to detect putative recombinants among isolates from well-characterized field populations of the pathogen. Microscopic examination of tufted mycelium between cocultured isolates revealed frequent hyphal fusions. Hyphal tips and conidia were recovered without selection from tufted zones in two separate vegetative pairings involving isolates with dissimilar haplotypes, based on the repetitive element MGR586. Haplotypic changes were observed at a higher frequency in tuft derivatives than in subcultures of each isolate alone. From 136 tuft derivatives analyzed, 5 putative recombinant haplotypes were identified. Introgression was demonstrated with two independent repetitive elements, fosbury and MGR586, as probes on DNA digested with several restriction enzymes. Introgressions were characterized by addition of 1 to 10 MGR586 bands, and 1 to 3 fosbury bands from one parent into the background of the other. Polymorphic single-copy probes were used to analyze putative recombinants. One probe detected an introgression event as predicted by analysis with MGR586. To assess the possible role of parasexual recombination in field populations of the pathogen, isolates in the Philippines previously grouped based on DNA fingerprinting were analyzed with low-copy RFLP markers. Polymorphism in single-copy loci typically was seen between, but not within, putative pathogen lineages. One lineage (designated lineage 4), however, was polymorphic for several probes. For some isolates, alleles at these loci comigrated with alleles characteristic of other lineages, suggesting the transfer of DNA fragments between lineages. One isolate was apparently a merodiploid, carrying an allele typical of lineage 4 plus another allele characteristic of a different lineage. In a survey of isolates from the Indian Himalayas, a merodiploid also was found with single- or low-copy probes. Examination of MGR586 profiles of the putative recombinant and its putative donor strains showed the expected introgression of MGR586 bands. The detection of parasexual DNA exchanges in wild-type strains under unselected conditions and the existence of merodiploids in nature suggest that parasexual recombination occurs in field populations of M. grisea. This raises questions concerning exclusive clonality in the blast fungus.     

Genetic Diversity of phlD from 2,4-Diacetylphloroglucinol-Producing Fluorescent Pseudomonas spp

ABSTRACT Fluorescent Pseudomonas spp. that produce 2,4-diacetylphloroglucinol (2,4-DAPG) have biocontrol activity against damping-off, root rot, and wilt diseases caused by soilborne fungal pathogens, and play a key role in the natural suppression of Gaeumannomyces graminis var. tritici, known as take-all decline. Diversity within phlD, an essential gene in the biosynthesis of 2,4-DAPG, was studied by restriction fragment length polymorphism (RFLP) analysis of 123 2,4-DAPG-producing isolates from six states in the United States and six other locations worldwide. Clusters defined by RFLP analysis of phlD correlated closely with clusters defined previously by BOX-polymerase chain reaction (PCR) genomic fingerprinting, indicating the usefulness of phlD as a marker of genetic diversity and population structure among 2,4-DAPG producers. Genotypes defined by RFLP analysis of phlD were conserved among isolates from the same site and cropping history. Random amplified polymorphic DNA analyses of genomic DNA revealed a higher degree of polymorphism than RFLP and BOX-PCR analyses. Genotypic diversity in a subset of 30 strains representing all the phlD RFLP groups did not correlate with production in vitro of monoacetylphloroglucinol, 2,4-DAPG, or total phloroglucinol compounds. Twenty-seven of the 30 representative strains lacked pyrrolnitrin and pyoluteorin biosynthetic genes as determined by the use of specific primers and probes.     

Genetic Diversity in South American Colletotrichum gloeosporioides Isolates from Stylosanthes guianensis, a Tropical Forage Legume

The degree of genetic diversity of 127 Colletotrichum gloeosporioides isolates from Stylosanthes guianensis genotypes in South America was measured at the molecular level by random amplified polymorphic DNA (RAPD) with nine arbitrary primers of 10 bases, and by restriction fragment length polymorphism (RFLP) with a non-LTR (long terminal repeats) retrotransposon DNA sequence. The RAPD products revealed scorable polymorphism among the isolates, and a total of 80 band positions were scored. Sixty-three of the 127 isolates were clustered into 13 distinct lineages usually correlating with geographic origin. Where isolates from various regions were clustered together, most had identical host genotype origin. The pathogen population sampled from Carimagua, Colombia, a long-time Stylosanthes breeding and selection site, with a savanna ecosystem, was highly diverse. A set of 12 S. guianensis genotype differentials was used to characterize pathogenic variability of 104 isolates and their virulence patterns were grouped into 57 pathotypes. However, when they were tested on four Australian differentials, they grouped into 11 pathotypes. As shown in previous studies, no strict correlations existed between genetic diversity measured by RAPD or RFLP, and pathotype defined by pathogenicity pattern on the differentials. Southern blot analysis of the 127 isolates revealed 23 hybridizing fragments, resulting in 41 fingerprint patterns among the 127 isolates. Relationships between RFLP and RAPD variables were examined using Spearman's Rank Correlation Coefficient, which showed that the two measures of genotypic variation are in agreement.     

Genetic Uniformity Among Isolates of Peronospora tabacina, the Tobacco Blue Mold Pathogen

ABSTRACT As a first step toward analysis of genetic variation and population structure in Peronospora tabacina, we used a collection of random genomic DNA fragments to survey for restriction fragment length polymorphisms (RFLPs) in DNA from a collection of isolates from Kentucky and other tobacco-growing regions of the United States. Also included in the study were isolates from the wild tobacco species, Nicotiana repanda, and from ornamental tobacco, N. alata. In a preliminary survey using DNA from 10 pathogen isolates, no polymorphisms were detected at six single-copy DNA loci using 22 probe-enzyme combinations. Moderately repetitive and highly repetitive regions of the genome were also remarkably similar between isolates, with only 6 of 15 different probes identifying genetic differences. Some of the polymorphic probes were then used to analyze a larger collection of isolates, most of which were from Kentucky. This resulted in the identification of very few additional polymorphisms, indicating that the population of P. tabacina that infects the Kentucky tobacco crop is genetically very homogeneous. The low level of polymorphism detected in this study overall, suggests that genetic variability may be lacking in P. tabacina populations throughout the United States. Two of the RFLP markers gave hybridization patterns that were consistent with P. tabacina being diploid. Frequencies of alleles at these loci and linkage disequilibrium between different marker loci indicated that genetic recombination does not occur frequently in the pathogen population. DNA polymorphisms that were identified in this study enabled us to differentiate the pathogen population into at least 10 haplotypes. One isolate was analyzed in detail and was shown to be genetically stable through several rounds of single-spore isolation and through several pathogenic cycles.     

Population genetics of Phytophthora infestans in Denmark reveals dominantly clonal populations and specific alleles linked to metalaxyl‐M resistance

Control of the potato late blight pathogen Phytophthora infestans relies heavily on chemicals. The fungicide metalaxyl‐M (Mefenoxam) has played an important role in controlling the disease, but insensitivity to the fungicide in certain isolates is now of major concern. A genetic basis for resistance to metalaxyl suggests the possibility for linking resistance phenotypes to specific population genetic markers, but in order to do this, the population genetic structure and mode of reproduction in a population must first be well described. The dynamics of metalaxyl‐M resistance in the Danish population of P. infestans was characterized over the course of the 2013 growing season, as was the population genetic structure, using simple sequence repeat (SSR) genotypes and single nucleotide polymorphism (SNP)‐based mitochondrial haplotyping of over 80 isolates. Both mating types A1 and A2 were present in most fields, but tests for recombination showed that clonal reproduction dominates in Danish populations. Genotype was not linked to haplotype and no differentiation was observed at the haplotype level, but rather between fields. Resistance phenotypes were linked to specific SSR alleles, demonstrating the potential for a more precise SNP‐based marker system for predicting resistance to metalaxyl‐M.     

Single Cystosorus Isolate Production and Restriction Fragment Length Polymorphism Characterization of the Obligate Biotroph Spongospora subterranea f. sp. subterranea

ABSTRACT Spongospora subterranea f. sp. subterranea causes powdery scab in potatoes and is distributed worldwide. Genetic studies of this pathogen have been hampered due, in part, to its obligate parasitism and the lack of molecular markers for this pathogen. In this investigation, a single cystosorus inoculation technique was developed to produce large amounts of S. subterranea f. sp. subterranea plasmodia or zoosporangia in eastern black nightshade (Solanum ptycanthum) roots from which DNA was extracted. Cryopreservation of zoosporangia was used for long-term storage of the isolates. S. subterranea f. sp. subterranea-specific restriction fragment length polymorphism (RFLP) markers were developed from randomly amplified polymorphic DNA (RAPD) fragments. Cystosori of S. subterranea f. sp. subterranea were used for RAPD assays and putative pathogen-specific RAPD fragments were cloned and sequenced. The fragments were screened for specificity by Southern hybridization and subsequent DNA sequence BLAST search. Four polymorphic S. subterranea f. sp. subterranea-specific probes containing repetitive elements, and one containing single copy DNA were identified. These RFLP probes were then used to analyze 24 single cystosorus isolates derived from eight geographic locations in the United States and Canada. Genetic variation was recorded among, but not within, geographic locations. Cluster analysis separated the isolates into two major groups: group I included isolates originating from western North America, with the exception of those from Colorado, and group II included isolates originating from eastern North America and from Colorado. The techniques developed in this study, i.e., production of single cystosorus isolates of S. subterranea f. sp. subterranea and development of RFLP markers for this pathogen, provide methods to further study the genetic structure of S. subterranea f. sp. subterranea.     

Phenotypic and Genotypic Characterisation of Northern Ireland Isolates Of Phytophthora Infestans

Genetic and phenotypic diversity of the population of the late blight pathogen Phytophthora infestans in Northern Ireland was assessed from 223 single-lesion isolates collected from 29 sites in 1995 and 1996. The proportion of metalaxyl-resistant isolates was approximately 14%. The A2 mating type was not detected. Allozyme analyses revealed that the Northern Ireland isolates were monomorphic and homozygous at the loci coding for allozymes of glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100). The majority of isolates tested (156 out of 162) were mitochondrial DNA haplotype IIa. Among the remaining isolates, four were haplotype Ia, two were the rare IIb haplotype and none was of the old Ib type. RAPD-PCR analysis of selected isolates revealed relatively little diversity among the Northern Ireland isolates, which clustered separately from isolates from GB and the European mainland.     

High Genetic Similarity Among Populations of Phaeosphaeria nodorum Across Wheat Cultivars and Regions in Switzerland

ABSTRACT Phaeosphaeria nodorum was sampled from nine wheat fields across a 30-km transect representing three geographical regions in Switzerland to determine the scale of genetic differentiation among subpopulations. Three different wheat cultivars were sampled three times to determine whether differences in host genotype correlated with differences among corresponding pathogen populations. Seven restriction fragment length polymorphism (RFLP) loci and one DNA fingerprint were assayed for each of the 432 isolates in the collection. DNA fingerprints differentiated 426 unique genotypes. Though absolute differences were small, five RFLP loci exhibited significant differences in allele frequencies across the nine sub-populations. Gene diversity within all subpopulations was high (H(T) = 0.51), but only 3% of the total genetic variation was distributed among the nine subpopulations. When subpopulations were grouped according to geographical region or host cultivar, less than 1% of the genetic variation was distributed among groups, suggesting widespread gene flow and the absence of pathogen adaptation to specific wheat cultivars. Tests for gametic equilibrium within subpopulations and across the entire Swiss population supported the hypothesis of random mating.     

Development of Contamination-Free Restriction Fragment Length Polymorphism Probes for the Obligate Biotroph Peronospora tabacina, an Oomycete Causing Blue Mold of Tobacco

ABSTRACT Peronospora tabacina is an obligately parasitic oomycete that causes blue mold, a devastating disease of tobacco. Genetic studies of this pathogen have been hampered by the lack of molecular markers. We generated a set of molecular markers for P. tabacina by collecting sporangiospores from infected tobacco leaves, extracting spore DNA, and cloning it in a plasmid vector. The resulting clones were then used to probe DNA from a collection of P. tabacina isolates to survey for polymorphisms. Most probes gave unexpected hybridization patterns with signal intensities that varied significantly from one DNA sample to another or between different DNA preparations of the same isolate. These results indicated that certain DNA preparations contained DNA from a source other than P. tabacina, which in turn suggested that some probes might have been derived from contaminating organisms present in the spore suspensions. Therefore, we characterized the inserts of several recombinant plasmids to determine their origins. Sequence analysis revealed that several of the inserts encoded peptides with similarity to bacterial proteins, suggesting that they were derived from bacterial contaminants. Of the remaining clones, five exhibited similarity to retroelements, one resembled eukaryotic helicase genes, and nine had no similarity to sequences in the databases. These were postulated to be true P. tabacina DNA clones. Verification of the origin of each probe was achieved by filtering a spore suspension, extracting DNA from the retentate and filtrate, and probing Southern blots of these DNA samples. These experiments confirmed the probe origins predicted by sequence analysis, resulting in the generation of 20 different restriction fragment length polymorphism probes that are specific for P. tabacina DNA. These probes should enable identification of reliable genetic markers for population studies of the blue mold organism.     

Cultural characteristics, pathogenicity and genetic diversity of Fusarium oxysporum isolates from tobacco fields in Spain

Several formae speciales of Fusarium oxysporum are capable to produce disease in tobacco plants. Different authors have classified those isolates as a forma specialis or a race within on the basis of the severity of disease and host specificity. Fusarium wilt of tobacco plant in Extremadura (central Spain) tobacco fields have been recorded in the last years and F. oxysporum was isolated from symptomatic plants. The aim of our study was to characterize these F. oxysporum populations. For this purpose, the in vitro spore production and growth and the virulence (severity of disease) have been tested. Although all isolates behaved as pathogen, the virulence of isolates was different. The differences in growth could not be correlated with other characteristics but the two isolates with scarce spore production have also behaved as the weakest pathogen. We have analyzed intergenic spacer (IGS) region polymorphism of ribosomal DNA and random amplified polymorphic DNA (RAPD) markers to assess the genetic diversity within F. oxysporum isolates. These molecular analyses showed two major groups with different physiological capabilities that could reflect two different lineages. One group was characterized by medium–high sporulation, high virulence and the same IGS-RFLP pattern. The other group was more heterogeneous featuring low–medium sporulation and variable virulence and growth. This first experimental approach to pathogen population could be a good starting point for further studies including non-pathogenic isolates and a larger number of pathogen that could clarify if there are two or more genetic lineages.     

Population changes in Phytophthora infestans in Taiwan associated with the appearance of resistance to metalaxyl

In recent years, late blight, caused by Phytophthora infestans (Mont) De Bary, has increased in severity in many parts of the world, and this has been associated with migrations which have introduced new, arguably more aggressive, populations of the pathogen. In Taiwan, late blight has been endemic on outdoor tomato crops grown in the highlands since the early 1900s, but recent epidemics have been more damaging. To ascertain the present status of the Taiwanese population of P infestans, 139 isolates of the pathogen collected and maintained by the Asian Vegetable Research and Development Center (AVRDC) were characterized using mating type, metalaxyl sensitivity, allozyme genotype, mitochondrial haplotype and RFLP fingerprinting. Up to 1997, all isolates were found to belong to the old clonal lineage of P infestans (US-1 and variants), but in isolates from 1998 a new genotype appeared, and by 2000 this had apparently completely displaced the old population. This new genotype was an A1 mating type and has the dilocus allozyme genotype 100/100/111, 100/100 for the loci coding for glucose-6-phosphate isomerase and peptidase, respectively. These characters, together with RG57 fingerprinting, indicated that these isolates belonged to the US-11 clonal lineage, a minority (11%) being a previously unreported variant of US-11. Whereas metalaxyl-resistant isolates were not detected in the old population, 96% of the new genotypes proved resistant, with the remainder being intermediate in sensitivity. It may be inferred from this sudden, marked change in the characteristics of the Taiwanese P infestans that a new population of the pathogen was introduced around 1997-98 and that this may well have already been metalaxyl-resistant when it arrived, although a role for in situ selection cannot be excluded.     

Genetic Structure of Peruvian Populations of Phytophthora infestans

ABSTRACT Isolates of the late blight pathogen Phytophthora infestans (n = 327) from the central to southern Peruvian Andes were systematically collected in 1997 to 1999 and analyzed to determine the pathogen's population structure at its host's center of diversity. No isolates of the A2 mating type were detected. Cluster analysis of DNA fingerprinting data indicated that the collection consisted of five major groups that were interpreted to be clonal lineages. Two of the lineages (US-1 and EC-1) have been previously described, and three (PE-3, 5, and 6) are described here for the first time. Collections from three areas in the central Peruvian Andes, including two key sites used in an international potato breeding program, consisted of isolates of the EC-1 lineage, which has been reported to dominate the pathogen population in Andean countries to the north of Peru. The collections from Cusco and Puno were more diverse. More than one lineage was detected in 10 of the 20 fields sampled in Cusco. Data on virulence, metalaxyl sensitivity, and band data for allozymes, mitochondrial DNA, and ipiB1 suggested that PE-3 may have been produced through recombination events between US-1 and EC-1. Restriction fragment length polymorphism and amplified fragment length polymorphism marker data were not consistent with this hypothesis.