Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Phytophthora on leaves and fruit of watermelon and cantaloupe is occurring throughout Maryland and Delaware. The disease has been favored by the excessively wet weather. Phytophthora is a soilborne pathogen, and will only occur in fields where the pathogen has been introduced. The symptoms on the fruit begin as small dark brown circular areas (about one inch in diameter) on the fruit. The lesions expand rapidly and under favorable conditions support white fluffy growth of the pathogen.
Over the years I have observed less Phytophthora blight on watermelon and cantaloupe than on other susceptible crops such as tomato, pepper, squash, pumpkin, etc. However watermelon and cantaloupe are still very good hosts of the disease and our saturated fields are prone to high disease if the pathogen is present.
Several fungicides are registered for suppression of Phytophthora blight. These include Revus, Ranman, Presidio, Forum, Gavel and Tanos. Some of these materials can be used through the drip, and have had good success in reducing crown rot of cucurbits when they were applied through the drip. Drip application is relatively new for these chemistries, and little data exists on whether it will manage fruit rot as well as, or better than foliar application.
Initial lesions (top) and sporulating lesions (bottom) on watermelon fruit. The lesions sometimes have a concentric ring appearance as seen on the fruit on the bottom.
Top image courtesy of Jason Brock, University of Georgia, Bugwood.org