Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
June was very favorable for the development of Phytophthora fruit rot in several Delmarva watermelon fields. Conditions that favor Phytophthora fruit rot are rainfall amounts that lead to saturated fields for several hours. For example, we have had many heavy rains that produced up to 2 inches or more of rain and resulted in pooling in fields. When soil is saturated for 5 to 6 hours, the disease progress is greatly increased. Optimum temperature for the disease is 82°F, and it will spread well even at higher temperatures.
The best way to manage Phytophthora fruit rot is to implement good cultural disease management practices such as removing infected debris and diseased fruit from fields, using raised beds, and improving soil drainage through tillage. Also avoid susceptible host plants in the field rotation (snap and lima bean, cucurbits, eggplants and tomatoes).
Phytophthora fruit rot on watermelon
“Felt-like” sporulation of Phytophthora on cantaloupe fruit.
Several trials have been conducted in the last 10 years by Dr. S. Kousik in South Carolina to evaluate efficacy of fungicides on managing Phytophthora fruit rot in watermelon. In these trials, Revus, Presidio and Zampro were the products that were most often included in the best treatment programs. In addition, Prophyt also improved management. One example of a good program for Phytophthora fruit rot is Actigard plus Prophye plus Kocide applied and alternated with an application that includes Zampro, Revus or Presidio. While it is expensive, Zampro applied four days before harvest performed well on post-harvest efficacy, which should relate to reduced disease spread during shipment. Revus and Presidio also performed well when applied four days before harvest in reducing post-harvest rot.
Watermelon fruit are susceptible to Phytophthora fruit rot at all growth stages. Therefore sprays targeted for Phytophthora fruit rot should begin when fruit are approximately grapefruit size.
Another question to consider before deciding whether to aggressively manage a Phytophthora fruit rot epidemic is how effective the best management program will be. In the same trials mentioned above, the best treatments, sprayed weekly for four or five weeks, reduced fruit rot between 60 to 75% over the nontreated plots. Therefore even in treated fields many fruit won’t be harvestable.