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Taro (Colocasia esculenta) is an important food crop, and taro leaf blight caused by Phytophthora colocasiaecan significantly affect production. Our objectives were to develop single nucleotide polymorphism (SNP) markers for P. colocasiaeand characterize populations in Hawaii (HI), Vietnam (VN) and Hainan Island, China (HIC). In total, 379 isolates were analyzed for mating type and multilocus SNP profiles including 214 from HI, 97 from VN and 68 from HIC. A total of 1152 single nucleotide variant (SNV) sites were identified via restriction site-associated DNA (RAD) sequencing of two field isolates. Genotyping with 27 SNPs revealed 41 multilocus SNP genotypes grouped into seven clonal lineages containing 2–232 members. Three clonal lineages were shared among countries. In addition, five SNP markers had a low incidence of loss of heterozygosity (LOH) during asexual laboratory growth. For HI and VN, >95% of isolates were the A2 mating type. On HIC, isolates within single clonal lineages had A1, A2 and A0 (neuter) isolates. The implications for the wide dispersal of clonal lineages are discussed.

Phytophthora capsiciis a soilborne oomycete plant pathogen that limits pepper production worldwide. The population structure varies significantly depending on the location (e.g. Peru vs. USA) and little is known about the diversity of P. capsiciin Argentina. Our objective was to assess the diversity of P. capsiciin Argentina at key pepper production areas. Forty isolates were recovered 2006–2009 from pepper and one isolate from pumpkin at 11 locations. Isolates were assessed for mating type, mefenoxam sensitivity and multilocus single nucleotide polymorphism (SNP) genotype profiles. Ten isolates with identical SNP profiles also were genotyped with amplified fragment length polymorphism (AFLP) markers. All 41 isolates had the A1 mating type and were sensitive to mefenoxam. Genotypic analysis using eight polymorphic SNP markers indicated 87% of the isolates had the same multilocus genotype, which is fixed for heterozygosity at seven of the eight SNP sites. AFLP analyses confirmed these findings, and overall it appears that clonal reproduction drives the population structure of P. capsiciin Argentina. The implications for breeding resistant peppers and overall disease management are discussed.

In 2008 statewide surveys of symptomatic foliage of nursery plants from Tennessee resulted in isolation of 43 isolates of Phytophthoraspp. This sample set includes four described species (P. citrophthora, P. citricola, P. nicotianae, P. syringae), and a provisional species of Phytophthora(‘P. hydropathica’). At the same time a stream-baiting survey was initiated to recover Phytophthorafrom eight watersheds in eastern Tennessee, some of which are near plant nurseries. Baiting was accomplished by submerging healthy Rhododendronleaves approximately 1 wk and isolation onto selective media. Six baiting periods were completed, and in total 98 Phytophthoraisolates and 45 isolates of Pythiumspp. were recovered. Three described species (P. citrophthora, P. citricolaand P. irrigata) and the provisional species ‘P. hydropathica’ were obtained as well as three undescribed Phytophthorataxa and Pythium litorale. Isolates from both surveys were identified to species with morphology and the internal transcribed spacer (ITS) sequence. Isolates from species co-occurring in streams and nurseries (P. citricola, P. citrophthoraand ‘P. hydropathica’) were characterized further with amplified fragment length polymorphism (AFLP) analyses and mefenoxam tolerance assays. Isolates representing a putative clonal genotype of P. citricolawere obtained from both environmental and nursery sample sets.

Loss of heterozygosity (LOH) occurs in a variety of diploid organisms after chemical mutagenesis and was observed in the vegetable pathogen Phytophthora capsiciafter N-ethyl-nitrosourea (ENU) mutagenesis at three loci during reverse genetic screening. Our objectives were to determine (i) the frequency of LOH among mutants, (ii) the directionality of the LOH events and (iii) the length of the genomic tracts exhibiting LOH. Of the 1152 ENU mutants screened, LOH was most frequent at locus 3 (99 ENU mutants), with locus 1 (10 ENU mutants) and locus 2 (9 ENU mutants) undergoing LOH at similar frequencies. LOH was bidirectional for all three loci, with locus 3 mutants biased toward one haplotype. Analysis of upstream and downstream heterozygosity indicates that the LOH events spanned up to at least 4.6 kb. The implications of mitotic recombination and LOH for reverse genetics and natural variation in Phytophthoraare discussed.

Natural Phytophthorahybrids (P. nicotianae× P. cactorum) infecting loquat in Peru and Taiwan were characterized with AFLP (amplified fragment length polymorphism) markers, the internal transcribed spacer (ITS) region and the phenol acid carboxylase gene (Pheca) and inheritance of the mitochondrial cytochrome oxidase I gene (coxI). AFLP profiles of two Taiwanese isolates recovered in 1995 were polymorphic in ~50% of the fragments whereas five Peruvian isolates, recovered 2002–2003 and 2007, showed no genotypic variation. Sequencing analysis of the cloned ITS region resulted in the identification of sequences with high homology to either P. nicotianae(99%) or P. cactorum(97%). Direct sequence analysis of the Phecagene revealed 13 heterozygous sites suggesting the presence of both P. nicotianaeand P. cactorumgenes in P.hybrids isolates. Melting analyses of coxI suggested that all seven Phytophthorahybrids inherited the mitochondrial DNA from P. nicotianae. Our results suggest that Phytophthorahybrids from Peru might have originated from a single hybridization event and that the two isolates from Taiwan might have originated through different hybridization events. The Peruvian hybrids appear to have persisted at least 3 y at three locations. Possible factors influencing the population structure of Phytophthorahybrids infecting loquat are discussed.

Phytophthora capsiciand the closely related Phytophthora tropicalisinfect different hosts that have documented overlapping geographical distributions. Phytophthora capsiciattacks annual vegetable hosts whereas P. tropicalishas been recovered from woody perennial hosts. Our objective was to test whether interspecific hybridization is possible and to characterize the resulting progeny. Crosses were made between P. capsici(LT263) from pumpkin to P. tropicalisfrom rhododendron (LT232) and to P. tropicalisfrom Theobroma cacao(LT12). The wild type isolates were analyzed for mitochondrial and nuclear DNA sequence diversity and progeny were tested for mating type (MT), AFLP marker profiles and mitochondrial DNA haplotype (mtDNA type). All oospore progeny from LT263 × LT12 were identical to LT263 whereas progeny from LT263 × LT232 were parental as well as hybrid. Hybrid progeny had either one or the other parent mtDNA type and there was no correlation between MT and mtDNA type. Attempts to generate an F2population from the hybrids proved unsuccessful while a backcross to the P. capsiciparent produced hybrid progeny. These results demonstrate that apomixis might play a significant role in species separation and that hybridization between P. capsiciand P. tropicalisis possible beyond the F1generation.