Mefenoxam insensitivity and the sexual stage of Phytophthora capsici in Michigan cucurbit fields

The potential for outcrossing, occurrence of oospores, and inheritance of m efenoxam sensitivity was assessed in naturally occurring populations of Phy tophthora capsici. Between 1997 and 1998, 14 farms were sampled, with 473 i solates recovered from cucurbit hosts and 30 from bell pepper. The A1 and A 2 compatibility types were recovered in a roughly 1:1 ratio in 8 of 14 farm s 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 iso lates sampled were sensitive, 32% were intermediate, and 13% were fully ins ensitive to mefenoxam. In vitro characterization of mefenoxam sensitivity w as conducted by crossing field isolates. Chi-square analysis of crosses bet ween 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 diseas ed cucurbit fruit from four farms in 1998, and 223 oospore progeny were rec overed from a single diseased cucumber. AII six mefenoxam sensitivity-compa tibility type combinations were present in these oospore progeny and within single fields. Based on these findings, we conclude that oospores likely p lay a role in the survival of P, capsici and that sexual recombination may significantly influence population structure.