The Spatiotemporal Genetic Structure of Phytophthora capsici in Michigan and Implications for Disease Management

Abstract

Root, crown, and fruit rot caused by Phytophthora capsici Leonian is a limiting factor for the production of peppers, tomatoes, and cucurbit crops in Michigan and the United States. Like many species in the genus Phytophthora, P. capsici has the potential for rapid polycyclic disease development from a limited amount of initial inoculum (6). P. capsici produces caducous sporangia that can be spread by wind-blown rain or release 20 to 40 motile zoospores in the presence of free water. The polycyclic phase of disease development is thought to be driven primarily by asexual spore dispersal at a local scale (within and down rows). Sexual reproduction requires both the A1 and A2 compatibility types (CTs) and results in the production of thick-walled oospores. Oospores are thought to serve as the primary survival structure outside of host tissue. Recommended disease management strategies stress the importance of avoiding excess water in the plant rhizosphere by using well-drained fields, conservative irrigation, and planting on raised beds. Additional recommendations include rotation to nonsusceptible hosts for at least 2 years and the use of fungicides. The phenylamide fungicide (PAF) mefenoxam is a systemic compound with high activity against P. capsici and has been used by growers throughout the United States to control P. capsici. Insensitivity to PAF has been reported for a number of other oomycetous organisms (Bremia lactucae, P. infestans, and P. sojae, etc.) and appears to be conferred by a single incompletely dominant gene of major effect (1). Growers in Michigan practicing 2+-year rotation in well-drained fields using an array of fungicidal management tools have experienced significant losses to P. capsici. Michigan is the number one producer of cucumbers for pickling in the United States and it was at the request of grower groups associated with this industry that research into the epidemiology and reproductive biology of P. capsici on cucurbit hosts was initiated. Although many researchers cite oospores as the most likely propagule for survival outside of host tissue, there have been very few investigations specifically aimed at determining the impact of sexual reproduction in natural populations of P. capsici. Our hypothesis was that the sexual stage may play an important role not only in survival but also in the adaptation of P. capsici populations to environmental stresses (e.g., fungicides). Our goal was to perform a comprehensive investigation of the phenotypic and genetic diversity present in P. capsici populations from the major vegetable production regions of Michigan, with the implicit intention of addressing questions concerning epidemiology, reproductive biology, and the durability of currently recommended management strategies.