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.

The potential for outcrossing, occurrence of oospores, and inheritance of mefenoxam sensitivity was assessed in naturally occurring populations of Phytophthora capsici. Between 1997 and 1998, 14 farms were sampled, with 473 isolates recovered from cucurbit hosts and 30 from bell pepper. The A1 and A2 compatibility types were recovered in a roughly 1:1 ratio in 8 of 14 farms with sample sizes larger than 15. In 1997, one isolate was designated as insensitive and four as sensitive to mefenoxam. In 1998, 55% of the 498 isolates sampled were sensitive, 32% were intermediate, and 13% were fully insensitive to mefenoxam. In vitro characterization of mefenoxam sensitivity was conducted by crossing field isolates. Chi-square analysis of crosses between sensitive, intermediately sensitive, and insensitive isolates indicate that mefenoxam insensitivity segregated as an incompletely dominant trait unlinked to compatibility type (P = 0.05). Oospores were observed in diseased cucurbit fruit from four farms in 1998, and 223 oospore progeny were recovered from a single diseased cucumber. All six mefenoxam sensitivity-compatibility type combinations were present in these oospore progeny and within single fields. Based on these findings, we conclude that oospores likely play a role in the survival of P. capsici and that sexual recombination may significantly influence population structure.

Since L. H. Leonian's first description of Phytophthora capsici as a pathogen of chile pepper in 1922, we have made many advances in our understanding of this pathogen's biology, host range, dissemination, and management. P. capsici causes foliar blighting, damping-off, wilting, and root, stem, and fruit rot of susceptible hosts, and economic losses are experienced annually in vegetable crops including cucurbits and peppers. Symptoms of P. capsici infection may manifest as stunting, girdling, or cankers for some cultivars or crops that are less susceptible. P. capsici continues to be a constraint on production, and implementation of an aggressive integrated management scheme can still result in insufficient control when weather is favorable for disease. Management of diseases caused by P. capsici is currently limited by the long-term survival of the pathogen as oospores in the soil, a wide host range, long-distance movement of the pathogen in surface water used for irrigation, the presence of fungicide-resistant pathogen populations, and a lack of commercially acceptable resistant host varieties. P. capsici can infect a wide range of hosts under laboratory and greenhouse conditions including cultivated crops, ornamentals, and native plants belonging to diverse plant families. As our understanding of P. capsici continues to grow, future research should focus on developing novel and effective solutions to manage this pathogen and prevent economic losses due to the diseases it causes.

Phytophthora capsici isolates were recovered from pepper and cucurbit hosts at seven locations in Michigan from 1998 to 2000. Isolates were characterized for compatibility type (CT), mefenoxam sensitivity (MS), and amplified fragment length polymorphism (AFLP) marker profiles. In total, 94 AFLP bands were resolved. Individual populations were highly variable. Within populations, 39 to 49% of the AFLP bands were polymorphic and estimated heterozygosities ranged from 0.16 to 0.19. Of the 646 isolates fingerprinted, 70% (454) had unique AFLP profiles. No clones were recovered between years or locations. Pairwise F statistics (Φ_ST) between populations from different locations ranged from 0.18 to 0.40. A tree based on unweighted pair-group method with arithmetic average cluster analysis indicates discrete clusters based on location. Isolates from the same location showed no clustering based on the year of sampling. Analysis of molecular variance partitioned variability among (40%) and within populations (60%). The overall estimated Φ_ST was 0.34 (SD = 0.03). A1/A2 CT ratios were ≈1:1, and MS frequencies were similar between years for the two locations sampled over time. These data suggest that P. capsici persists in discrete outcrossing populations and that gene flow among locations in Michigan is infrequent.