An investigation of Phytophthora capsici on vegetable hosts in Michigan: Survival, spread, and response to the phenylamide fungicide mefenoxam

The incidence of root, crown, and fruit rot of pumpkins, squash, tomatoes, and peppers caused by Phytophthora capsici Leonian has increased during the last two decades in Michigan vegetable production fields. Currently recommended control strategies include the use of well drained fields, crop rotation, and the use of preventative fungicides. Growers employing all available management strategies have sustained significant losses and, at the behest of the cucurbit industry, an intensive investigation of the life history of P. capsici throughout Michigan was initiated. The primary objective of this study was to ascertain whether or not the sexual stage is active in natural populations and, if so, to determine what effect this may have on the survival and evolution of naturally occurring populations. The sensitivity to mefenoxam and compatibility type were assayed in natural populations. In 1997 and 1998, 523 isolates of P. capsici were recovered from infested fields throughout the state. In vitro crosses between sensitive and insensitive isolates from diverse locations indicated that mefenoxam insensitivity is inherited as a single incompletely dominant gene unlinked to compatibility type. All six possible mefenoxam sensitivity/compatibility type combinations were recovered from a single field and both mating types were recovered from every field sampled. Oospores were observed in diseased host tissue from four locations and all six phenotypic combinations were found in 223 oospore progeny recovered from a single naturally infected cucumber fruit. In 1999 and 2000, a single field from which only intermediately or fully insensitive P. capsici isolates were recovered in 1998 was sampled intensively in the absence of mefenoxam selection pressure. Isolates from 1998 and 1999 were analyzed using fluorescently labeled amplified fragment length polymorphism. (AFLP) markers. Clonal reproduction was significant within a single season but no clones were recovered between years. Approximately fifty percent of the AFLP markers were polymorphic: and 199 of the 263 isolates analyzed had unique multi-locus AFLP genotypes. Furthermore, the frequency of individual AFLP markers remained stable between years and mefenoxam insensitivity did not decrease over time. AFLP markers were then used to characterize an additional 383 isolates from 6 populations of P. capsici at locations ranging from 1 to >200 km distant. A similarly high level of genotypic and gene diversity was recorded in every population investigated. Cluster analysis indicated discrete clusters based on location with no blurring of the groupings based on the year of sampling. The overall picture presented by these results suggests that outcrossing occurs frequently in populations of P. capsici , that populations are large enough to withstand dramatic effects of genetic drift, and that migration between locations appears to be rare. Sexual recombination appears to have played a significant role in the integration of mefenoxam insensitivity into populations under mefenoxam selection pressure and there is no evidence that the frequency of mefenoxam insensitivity will decrease once selection pressure is removed.