Abstract: In 2002–03, 597 Phytophthora infestans isolates were collected from potato and tomato crops in nine provinces of Vietnam. A representative set of isolates were characterised for mating type, metalaxyl sensitivity, virulence, mitochondrial DNA (mtDNA) haplotype and RG57-fingerprint. All of the 294 tested isolates were of mating type A1. Metalaxyl-resistant isolates were found on both the potato and tomato hosts, and dominated the population in Lam Dong province (south of Vietnam). Nine known virulence genes were found among 27 tested isolates, although virulence to resistance gene R5 was not found. Among the 95 isolates tested for mtDNA haplotype, all were Ib type. Of these, all 34 isolates fingerprinted showed US-1-like genotype. Mating type, mtDNA haplotype and RG57-fingerprint data from this study indicate that the P. infestans population on tomato and potato from Vietnam still consist of the ‘old’ population.
Abstract: Dieback due to Phytophthora cinnamomi has significant impacts on susceptible native Australian vegetation communities and dependent fauna. Here we report the occurrence and distribution of P. cinnamomi at North Head and Middle Head, Sydney Harbour.
Abstract: The metapopulation structure of Phytophthora infestans sensu lato is genetically diverse in the highlands of Ecuador. Previous reports documented the diversity associated with four putative clonal lineages of the pathogen collected from various hosts in the genus Solanum. This paper simultaneously analyzes diversity of the complete collection of isolates, including a large number that had not yet been reported. This analysis confirmed the existence of three pathogen populations, which all appear to be clonal lineages, and that correspond to those previously reported as US-1, EC-1, and EC-3. No evidence was found from the analyses of recently collected isolates that would contradict earlier reports about these three lineages. In contrast, new data from a group of isolates from several similar hosts caused us to modify the previous description of clonal lineage EC-2 and its previously proposed hosts, S. brevifolium and S. tetrapetalum. Given the uncertainty associated with the identification of these hosts, which all belong to the section Anarrhichomenum, we refer to them as the Anarrhichomenum complex, pending further taxonomic clarification. New pathogen genotypes associated with the Anarrhichomenum complex were isolated recently that are A1 mating type and Ia mitochondrial DNA (mtDNA) haplotype, and therefore differ from the previously described EC-2 lineage, which is A2 and Ic, respectively. Because of uncertainty on host identification, we do not know if the new genotypes are limited to one host species and therefore represent yet another host-adapted clonal lineage. For now, we refer to the new genotypes and previously described EC-2 genotypes, together, as the pathogen group attacking the Anarrhichomenum complex. Two A2 isolates identical to the previously described EC-2 archetype were collected from severely infected plants of pear melon (S. muricatum). Pear melon is generally attacked by US-1, and this is the first clear case we have documented in which two distinct pathogen genotypes have caused severe epidemics on the same host. Based on presence of unique marker alleles (restriction fragment length polymorphism [RFLP] and mtDNA) and genetic similarity analysis using RFLP and amplified fragment length polymorphism data, EC-3 and isolates from the Anarrhichomenum complex are genetically distinct from all genotypes of P. infestans that have been reported previously. No current theory of historical migrations for this pathogen can adequately support a Mexican origin for EC-3 and genotypes of the Anarrhichomenum complex and they may, therefore, be palaeoendemic to the Andean highlands. To date, we have identified 15 hosts in the genus Solanum, in addition to the Anarrhichomenum complex, and some unidentified species of P. infestans sensu lato in Ecuador. Five of the Solanum hosts are cultivated. One isolate was collected from Brugmansia sanguinea, which represents the first report from Ecuador of a host of this pathogen that is not in the genus Solanum. However, P. infestans sensu lato was only found on flower petals of B. sanguinea. This study provides new insights into the population structure of highly specialized genotypes of P. infestans sensu lato in the Andean highlands. The results are discussed in light of previous hypotheses regarding the geographic origin of the pathogen.
Abstract:Phytophthora sojae causes root and stem rot, one of the most important diseases of soybean worldwide. Genetic diversity of 32 Phytophthora sojae isolates of different geographic origin from Argentina was evaluated with RAPD markers. The isolates were collected from diseased soybean plants and soil samples from Santa Fe, Buenos Aires, Córdoba and Entre Ríos provinces, in the Pampeana Region. DNA was amplified with 20 decanucleotides primers. Seven primers amplified 49 fragments, of which 35 were polymorphic, indicating high variability. RAPD analysis detected intraspecific variability even among isolates of the same geographic origin.
Abstract: Twelve isolates of Japanese Phytophthora infestans, which differed from the major genotypes US-1, JP-1, JP-2, and JP-3, were analyzed for RG57 fingerprints, mtDNA haplotypes, two allozyme genotypes, and mating types. Genotypes JP-1.1, JP-2.1, JP-2.2, JP-3.1, and JP-4 were newly defined. JP-1.1 and JP-2.1 were isolated discontinuously from potato fields in several years, and JP-1.1 was found in Hokkaido and Kagoshima. These results show that some minor genotypes can overwinter and disperse from their original site.
Abstract: Since 1987, Phytophthora root and stem rot of soybean [Glycine max (L.) Merr. cv. Tanbakuro], caused by Phytophthora sojae Kaufman and Gerdemann, has been increasing in the Sasayama, Nishiwaki, and Kasai regions in Hyogo, the most famous soybean (cv. Tanbakuro)-producing areas in Japan. In 2002 to 2004, 51 isolates (one from each field) of P. sojae were recovered from 51 fields in Hyogo. These isolates were tested for virulence on six Japanese differential soybean cultivars used for race determination in Japan, and three additional ones containing four Rps genes used in Indiana, USA. Race E was the most prevalent from 2002 to 2004, followed by races A, C, D, and four new races (proposed as races K, L, M, and N). Interestingly, none of the new races had high virulence on the Japanese differential cultivars, compared with other races in each area. One (race N) was avirulent on all six soybean differentials. There was a difference in race distribution on each of three individual areas; race E seemed to be a major component of the P. sojae population in Sasayama, whereas race A and the new race M were the most prevalent in Nishiwaki and Kasai, respectively. Rps6 (cv. Altona) and Rps1a + Rps7 (cv. Harosoy 63) were infected by 90.2% and 33.3% of all isolates, respectively. However, Rps1d (cv. PI103091) was not susceptible to any of the 51 isolates, nor was cv. Gedenshirazu-1. These two soybean cultivars were considered to be potential sources of resistance to breed new resistant cultivars with the desirable characteristics of cv. Tanbakuro for this region.
Abstract: 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.
Abstract: The phylogeny and taxonomy of Phytophthora cryptogea and Phytophthora drechsleri has long been a matter of controversy. To re-evaluate this, a worldwide collection of 117 isolates assigned to either P. cryptogea, P. drechsleri or their sister taxon, Phytophthora erythroseptica were assessed for morphological, physiological (pathological, cultural, temperature relations, mating) and molecular traits. Multiple gene phylogenetic analysis was performed on DNA sequences of nuclear (internal transcribed spacers (ITS), ß-tubulin, translation elongation factor 1α, elicitin) and mitochondrial (cytochrome c oxidase subunit I) genes. Congruence was observed between the different phylogenetic data sets and established that P. drechsleri and P. cryptogea are distinct species. Isolates of P. drechsleri form a monophyletic grouping with low levels of intraspecific diversity whereas P. cryptogea is more variable. Three distinct phylogenetic groups were noted within P. cryptogea with an intermediate group providing strong evidence for introgression of previously isolated lineages. This evidence suggests that P. cryptogea is an operational taxonomic unit and should remain a single species. Of all the morphological and physiological traits only growth rate at higher temperatures reliably discriminated isolates of P. drechsleri and P. cryptogea. As a homothallic taxon, P. erythroseptica, considered the cause of potato pink rot, is clearly different in mating behaviour from the other two species. Pathogenicity, however, was not a reliable characteristic as all isolates of the three species formed pink rot in potato tubers. The phylogenetic evidence suggests P. erythroseptica has evolved from ,i>P. cryptogea more recently than the split from the most recent common ancestor of all three species. However, more data and more isolates of authentic P. erythroseptica are needed to fully evaluate the taxonomic position of this species.
Abstract: Sequence analysis of the internal transcribed spacer (ITS) regions I and II of ribosomal DNA were used to deduce evolutionary relationships among 15 Phytophthora species. Analysis of papillate, semi-papillate and non-papillate species showed that sporangium papillation has phylogenetic significance, with the three morphological groups each forming separate clusters. Within the P. megasperma species complex, separate evolutionary lines were identified for P. medicaginis, P. trifolii, and P. sojae formerly regarded as formae spéciales of P. megasperma, confirming their recent reclassification as biological species. rDNA sequence analysis was able to distinguish P. cryptogea and P. drechsleri isolates indicating a valid basis for speciation. P. macrochlamydospora from soybean, which has only been observed in Australia was closely related to P. sojae, indicating possible common ancestory.
Abstract: Late blight, caused by Phytophthora infestans, is an ongoing threat to potato and tomato crop production worldwide and considerable fundamental and applied research is conducted with the long-term aim of improved disease control. Understanding the mechanisms, processes and rates of P. infestans evolution is an important factor in predicting the effectiveness and durability of new management practices. A range of phenotypic and genotypic tests has been applied to achieve this goal, but each has limitations and new methods are sought. Recent progress in P. infestans genomics is providing the raw data for such methods and new high-throughput codominant biomolecular markers are currently being developed that have tremendous potential in the study of P. infestans population biology, epidemiology, ecology, genetics and evolution. This paper reviews some key applications, recommends some changes in approach and reports on the status and potential of new and existing methods for probing P. infestans genetic diversity.
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