Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; firstname.lastname@example.org
In the past three weeks I have received many reports of Phytophthora blight in watermelon fields on Delmarva. This disease appears to be especially common this year on watermelons. We have had a few periods during the summer where we had high volume rain events. The threshold that usually triggers disease development is 2 inches of rain that falls over a short enough period of time to pool in the field. If soil is saturated for 5 to 6 hours, the zoospores are released and a new infection cycle will begin. Optimum temperature for spread is 28C (82°F). There are several reasons that disease might be especially severe this year. In addition to high volume rain events, soil compaction may be greater this year because growers had to work in fields during June when soil remained wet from frequent rains. Soil compaction would slow drainage and increase the length of soil saturation.
Management practices for this disease must begin prior to planting. Remove infected debris from fields, including, where possible, diseased fruit. Cultural practices for management of Phytophthora blight are to improve soil drainage through tillage, use raised beds and reduce soil compaction. Alternate hosts include beans (snap and lima), cucurbits (pumpkin, melons, cucumbers, etc.), eggplants and tomatoes.
Fumigants such as K-pam. Vapam and Telone will reduce plant death, but fumigation should not be used as a stand-alone practice. Fumigants and fungicides, used in an overall disease management program, which includes cultural practices, is the best approach.
The fungicides available for Phytophthora blight control are, at best, suppressants of disease. Forum, Gavel, Tanos, Presidio, Revus and Ranman are labeled. Bob Mulrooney wrote a good overview of treatments in a Weekly Crop Update article a few weeks ago http://agdev.anr.udel.edu/weeklycropupdate/?p=1209