There are low levels of powdery mildew showing up in the pumpkin and squash plots on campus this week. Powdery mildew is the most important disease of pumpkins and squash. Melons and cucumbers are less susceptible, although infections also occur in these crops.
Powdery mildew does not overwinter in Ontario. It typically appears in late-July each year. While some varieties have a good level of resistance to this disease, many do not. Severe infections result in decreases in yield, sugar content and harvest quality. Control is especially important in Halloween pumpkins where infections cause the stems to decay resulting in poor (or absent) handles.
Many of the products commonly used for downy mildew control in cucumbers do not control powdery mildew. Despite the similar names, the two diseases are quite different. Each disease has its own set of registered fungicides. An effective fungicide program for pumpkins and squash is very different from one used in cucumber crops.
Now that the disease is present in Ontario, inspect pumpkin and squash fields twice weekly. Look for white, powdery lesions on both the upper and lower leaf surfaces. Apply powdery mildew fungicides at the first sign of disease. Applications made late in the disease progress do little to control the disease. Depending on the weather conditions, follow a 7-14 day spray interval. Always rotate between different fungicide groups.
For photographs of the disease and more information on the biology of powdery mildew in cucurbit crops, visit the Ontario CropIPM website.