The Utah Pest team is just released their latest “Vegetable IPM Advisory” which included some great information about Powdery Mildew. This foliar disease is very common in our area, but can be easily avoided with some simple irrigation changes.

What to look for:

“Cucurbit powdery mildew is showing up on vine crops, especially where plantsare crowded or runningtogether. This foliar disease first appears as small circular lesions located randomly on the leaf surface or on petioles. As the infection continues, leaves turn yellow and become distorted.

Fruits areusually not directly affected, but yield and flavor can be reduced.This disease thrives in humid and shady environmentsunder moderate temperatures (up to 80° F).” -Utah Pest Team

“Severe powdery mildew infection can result in yield loss.  Both powdery mildews cause damage to the plants by reducing photosynthesis.  Once the leaves are covered with white mycelium, they absorb less sunlight and are not able to produce enough sugars to sustain plant and fruit growth.  In addition, heavily infected leaves become necrotic (turn brown and die) and fall off, which can result in sunburn of fruit. ” -Claudia Nischwitz, USU Plant Pathologist

Gardening Practices:

“Powdery mildew is favored by dew, intermittent rain, or sprinkler irrigation. Maintaining conditions that favor rapid drying of foliage will help reduce the incidence of disease. Susceptible flowers should be planted in open areas where they will not be crowded and where they are exposed to the sun. Plants in shade are more prone to mildew than those growing in the sun. Prune during the summer to thin out any dense foliage. This will increase aeration within the plant canopy. Avoid sprinkling at night during the month of August and September. Instead, soak the soil under plants as needed.” -Utah Pest Team
Chemical Options:
If you find that a fungicide is necessary to control your outbreak, here are some suggestions from the Vegetable Advisory:
  • Commercial growers: potassium bicarbonate (Kaligreen, Milstop), horticultural oil (Sporatec), sulfur (Cosavet, Kumulus, MicrothiolDisperss), Bacillus subtilis (Serenade, Cease, Rhapsody)
  • Residential growers: horticulturaloil (Monterey Garden Spray), sulfur (Bayer Natria, Bonide), myclobutanil (Spectracide Immunox).
  • Note: Do not use oil and sulfur within two weeks of each other, and do not spray when temperatures are over 90° F.