Continuous rain = Phytophthora Fruit rot
Posted: September 8, 2011
Phytophthora capsici causes a wide array of diseases ranging from seedling damping-off, leaf spots, foliar blight, root and crown rot, stem lesions to fruit rot on numerous vegetables including pepper, tomato, eggplant and most cucurbits. This time of the year, we are most concerned about fruit rots. With the recent heavy rains, I am hearing of more reports of Phytophthora fruit rots in the field.
Initial symptoms of Phytophthora fruit rot are water soaked or depressed spots typically on the underside of the fruit where it is in contact with the soil. Symptoms can develop on the upper side of the fruit following rain or an irrigation event that splashes infested soil and spores up onto the fruit. Eventually the fruit will become covered with white sporangia and will rapidly collapse either in the field or shortly after harvest. The sporangia form when the soil is at field capacity and the infective zoospores that cause new infections are released when the soil is saturated which is why this disease is most prevalent poorly drained soil and/or after significant rainfall.
Managing soil moisture by avoiding planting in poorly drained soils or low lying areas, sub-soiling to break-up hard pans, raised beds and avoiding excessive irrigation. Do not irrigate from ponds or surface water sources that may contain water that drained from infested fields. Minimize splash dispersal of spores and surface water movement between rows or fields.
In fields where Phytophthora is a problem minimize movement of people and equipment from infested to uninfested fields. Remove diseased plants and fruit from the field. Avoid culling infected fruit into production fields.
Currently, fungicides will only suppress Phytophthora at best. Several including mefenoxam are registered for application through the drip for suppression of the root and crown rot phase early in the season. Foliar applications of oomycete specific fungicides containing mandipropamid, flucopicolide and cyazofamid initiated prior to the development of symptoms will also offer some suppression. See the 2011 Pennsylvania Commercial Vegetables Production Recommendations for a listing of specific products and rates.