Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Be on the lookout for Phytophthora fruit rot on cucurbits, especially watermelon. After last seasons’ losses from Phytophthora fruit rot due to the excessive rainfall in 2009, growers should be thinking about the possibility of fruit rot this season. We are at risk for Phytophthora blight if the scattered thunder storms, along with the frog-strangling rains that we can get, occur like they did Tuesday evening. Phytophthora blight is a tough disease to control, but if you have cucurbits in fields that had fruit rot last season you are at very high risk if the soil stays saturated even for a few hours. This is a fungus that moves in water and the spores will move where water goes. (Spores will not move more than a few feet in the air.) Some additional cultural controls would be rotation (5 years or more) for watermelons, sub-soiling between the rows before they close to help water drain faster and to keep the fruit out of standing water. Fungicides will only suppress the disease and those that have the best activity are the following: Presidio, Revus, Ranman plus a surfactant (see label), Forum, Gavel and Tanos. Depending on the test, the season, and the location, the efficacy of these fungicides varies. However, proper application of these products will result in better yields than in untreated fields. Remember that Revus and Forum are Group 40 fungicides and have the same mode of action, so they should not be applied in succession. All of these fungicides, except Ranman, should be tank mixed with fixed copper if the label allows. Fixed copper is not compatible with Ranman plus the surfactant. Good coverage of fruit is very important.
On pickling cucumbers, fruit rot fungicides should be applied soon after flowering when the fruit are one inch long and repeated once they are three inches long for the best results. Data from Michigan State indicate that Presidio, Revus, Gavel , Forum (Acrobat), and Ranman provide suppression of fruit rot. Remember that Revus and Forum are Group 40 fungicides and have the same mode of action, so they should not be applied in succession.