Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Last Thursday, August 6, my pumpkin plants looked great with large green leaves and just a little powdery mildew (Fig. 1). Just a few days later and they looked horrid (Fig. 2). Some leaves were just splotched with little yellow spots, while other leaves had necrotic edges (Fig. 3). By the calls I have been getting so far this week this same sort of thing is happening in many other pumpkin fields around Maryland. Exactly what could cause such a rapid decline of the foliage is still not clear. The weather over the weekend and for the last few days was some of the hottest and most humid we have had so far this summer. Pumpkins are sizing on the vine and the plants are under stress. In some of the fields the soil moisture seemed adequate, but greater levels are needed when it is this hot and plants and fruit are large. In most of the fields downy mildew was either established or just getting started. Whether it was the weather that weakened the plants and the downy mildew could take-off or the low level of infection of the downy mildew further stressed the plants when it was so hot and humid is unclear and does not really matter as the downy mildew needs to be controlled before it gets any worse. Best management practices for downy have been reported by Bob and Kate over the last few weeks. (Editor’s Note: See articles in WCU 17:17, WCU 17:18, and WCU 17:19 for fungicide recommendations from Bob Mulrooney and Kate Everts.)
Another factor may have been soil pH that was a little low but certainly tolerable, i.e., 5.7-6.0. These pH levels usually are not a problem but when the plant is under stress any additional stress will just add to the misery of that plant. At this point in time you cannot do anything about your pH, so be sure to reduce as many other stresses as you can—disease, insects (squash bug and cucumber beetles), and lack of water.
Figure 1. Pumpkin field on Thursday, August 6, looking good
Figure 2. Same pumpkin field on Wednesday, August 12, downy mildew is at low levels in the field
Figure 3. Pumpkin leaf with splotchy yellowing — downy mildew was found to be just starting on this leaf