- 1). Treat infected plants with a commercial systemic fungicide product labeled for powdery mildew and as safe for use on edible plants. Look for product containing the active ingredients triadimefon, benomyl or thiophanatemethyl. Alternatively, use a contact fungicide product containing chlorothalonil and copper. Some serious infections will require the use of both types of treatment. Always follow the use guidelines on the product label for amount and timing of applications.
- 2). Start the fungicide applications at the first sight of powdery mildew on the leaves or stems in the late spring or early summer, knocking the disease back before it spreads to all of the summer squash plants.
- 3). Repeat application of the fungicide roughly every seven to 10 days thereafter, following the product label dosing and safety directions. Spray both the top surface and underside of the squash plant leaves, as well as the lower leaves and stems where the mildew may not be as readily visible, but easily thrives.
- 4). Relocate successive and later season summer squash plantings well away from the crops presently suffering from powdery mildew in order to isolate the pathogens and preserve your next crop as disease free.
- 5). Keep spraying regularly throughout the growing season, as needed, until the plant tops die back and the ground soil is cleared.
- 6). Resume spraying late in the following spring to prevent any fungi that successfully overwintered in the soil from flaring up and infecting the new season's plants.
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