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The oomycete vegetable pathogen Phytophthora capsici has shown remarkable adaptation to fungicides and new hosts. Like other members of this destructive genus, P. capsici has an explosive epidemiology, rapidly producing massive numbers of asexual spores on infected hosts. In addition, P. capsici can remain dormant for years as sexually recombined oospores, making it difficult to produce crops at infested sites, and allowing outcrossing populations to maintain significant genetic variation. Genome sequencing, development of a high-density genetic map, and integrative genomic or genetic characterization of P. capsici field isolates and intercross progeny revealed significant mitotic loss of heterozygosity (LOH) in diverse isolates. LOH was detected in clonally propagated field isolates and sexual progeny, cumulatively affecting >30% of the genome. LOH altered genotypes for more than 11,000 single-nucleotide variant sites and showed a strong association with changes in mating type and pathogenicity. Overall, it appears that LOH may provide a rapid mechanism for fixing alleles and may be an important component of adaptability for P. capsici.

Phytophthora capsici and the closely related Phytophthora tropicalis infect different hosts that have documented overlapping geographical distributions. Phytophthora capsici attacks annual vegetable hosts whereas P. tropicalis has 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. tropicalis from rhododendron (LT232) and to P. tropicalis from 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 x LT12 were identical to LT263 whereas progeny from LT263 x 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 F2 population from the hybrids proved unsuccessful while a backcross to the P. capsici parent produced hybrid progeny. These results demonstrate that apomixis might play a significant role in species separation and that hybridization between P. capsici and P. tropicalis is possible beyond the F1 generation.

Phytophthora ramorum and Phytophthora sojae are destructive plant pathogens. P. sojae has a narrow host range, whereas P. ramorum has a wide host range. A global proteomics comparison of the vegetative (mycelium) and infective (germinating cyst) life stages of P. sojae and P. ramorum was conducted to identify candidate proteins involved in host range, early infection, and vegetative growth. Sixty-two candidates for early infection, 26 candidates for vegetative growth, and numerous proteins that may be involved in defining host specificity were identified. In addition, common life stage proteomic trends between the organisms were observed. In mycelia, proteins involved in transport and metabolism of amino acids, carbohydrates, and other small molecules were up-regulated. In the germinating cysts, up-regulated proteins associated with lipid transport and metabolism, cytoskeleton, and protein synthesis were observed. It appears that the germinating cyst catabolizes lipid reserves through the beta-oxidation pathway to drive the extensive protein synthesis necessary to produce the germ tube and initiate infection. Once inside the host, the pathogen switches to vegetative growth in which energy is derived from glycolysis and utilized for synthesis of amino acids and other molecules that assist survival in the plant tissue.

Species of Phytophthora are ubiquitous in ornamental production resulting in significant crop losses, in Tennessee, national surveys for the sudden oak death pathogen Phytophthora ramorum in 2004 and 2005 led to the isolation of Phytophthora species causing disease in nursery-grown or handled woody ornamentals or both. Isolates recovered were identified to species using direct sequencing of the internal transcribed spacer and examination of morphological characters. Six known species (P. cactorum, P. citricola, P. citrophthora, P. nicotianae, P. palmivora, P. tropicalis) and one newly described species (P.foliorum) were recovered from ericaceous hosts. The most common species recovered were P. citricola and P. citrophthora. Genetic analysis using amplified fragment length polymorphism (AFLP) markers revealed intraspecific genotypic diversity as well as isolates with identical AFLP genotypes from multiple locations across multiple years. This work provides evidence for species and genotypic diversity of Phytophthora recovered in Tennessee as well as insight into the movement of individual genotypes in woody ornamental production. [ABSTRACT FROM AUTHOR]