Be ready for Cucurbit Powdery Mildew
Posted: July 25, 2014
The first sign of the pathogen is small white powdery spots on both the upper and/or lower leaf surface of the older leaves. It is common to first see it on the underside of a leaf or within the plant canopy so when scouting it is important to through look over the entire plant. If protectant fungicides are being used, sometimes the spots on the upper leaf surface are yellow in color and it is not until you look at the underside of the leaf that you see the white powdery colony. It is also important to scout by cultivar since they can vary in their resistance to powdery mildew. The only reports so far have been on summer squash.
Keep in mind, if left unmanaged, severely infected leaves can die leading to reduced fruit size, quality and sunburn. Although powdery mildew does not infect the fruit, it can infect pumpkin handles thus reducing the overall marketability.
There is considerable concern over the development of fungicide resistance with powdery mildew. For resistance management, it is best to start applying the most effective products when you first start seeing symptoms (1 lesion on 50 leaves) and then later in the season switch to a protectant spray program rather than the reverse. In the long-run this will reduce the selection pressure for powdery mildew spores that are resistant to the fungicide because fewer spores are exposed to the active ingredient when disease severity is low. See the 2014 Commercial Vegetable Production Recommendations for a list of recommended products and rates as well as the 2013 Fungicide Resistance Management Guidelines for Cucurbit Downy and Powdery Mildew Control in the Northeast United States.
Fortunately, there are a number of organic options for helping to manage powdery mildew on cucurbits including copper, sulfur, oils like Eco E-rase (jojoba oil), JMS Stylet oil (paraffinic oil), Trilogy (neem oil) and Organocide (sesame oil), as well as potassium bicarbonate based products (Kaligreen and MilStop) to name a few.