All squash and cucurbits are susceptible to powdery mildew, specifically caused by the fungi Sphaerotheca fuliginea and Erysiphe cichoracearum. You might see pale yellow spots developing on the squash plant's stems and leaves, the spots expanding and white, powdery spore-producing mycelium growing to cover the plant surfaces. Squash plants can actually die from untreated powdery mildew, but you have some organic and cultural options for treating the fungal disease.
Erysiphe cichoracearum is the most common species of powdery mildew fungi to infect squash, usually requiring organic fungicide treatments. But all types of powdery mildew fungi that infect vegetables spread by wind-carried spores, thriving when temperatures are moderate -- 15.6 to 26.7 degrees Celsius -- and not excessively wet. Unlike various other fungal diseases of squash, powdery mildew fungi actually favour slightly drier conditions and can die off when water remains on the plant surfaces for long periods of time. Powdery mildew spores also die when they're exposed to direct sunlight and temperatures hotter than 32.2 degrees Celsius.
You can help to control, and even prevent, powdery mildew by using proper cultural practices while growing squash. Keep garden free of weeds, which can harbour and help to spread the powdery mildew fungi, and avoid watering your squash plants using overhead sprinklers. Although you can spray the squash plants with water to wash off the spores, watering with overhead sprinklers can encourage other disease and pest problems. Powdery mildew fungi don't need moisture or wet conditions to thrive, however, needing only warm temperatures to grow. Plant your squash in a sunny location and provide good air circulation around the plants through proper spacing. Also, avoid excessive fertilisation.
No systemic fungicides that control powdery mildew are approved for organic growing, but you can use certain organic products to prevent or treat the fungal disease in squash. Sulphur is perhaps the most widely-used and inexpensive organic remedy for controlling powdery mildew in squash, applied using a sprayer. When using any product to control powdery mildew, you'll need to coat both the upper sides and undersides of the leaves. With sulphur, the substance actually redistributes to the lower surfaces, so application is easier. You can also use "wettable sulfur" as a protectant against powdery mildew, before symptoms appear.
Other organic treatments for controlling powdery mildew on squash include horticultural, neem and jojoba oils. Don't apply these oils within two weeks of applying sulphur, or apply the oils or sulphur when temperatures are above 32.2 degrees Celsius, however, because doing so can injure your squash plants. Biological fungicides like Bacillus subtilis are beneficial microorganisms that can kill powdery mildew fungi, yet is nontoxic.
Reapply these organic fungicides every seven to 10 days if the powdery mildew persists. Read the fungicide product's label carefully to ensure that it's safe for the type of squash you're growing, and to understand all the safety precautions and application instructions.
Another organic garden-friendly control method for powdery mildew is to plant resistant cultivated varieties or "cultivars" of squash plants. Today, a wide variety of squash cultivars that are resistant to powdery mildew are available, including winter squash varieties like BonBon, Bugle, Autumn Delight, Avalon, Betternut 401, Bush Delicata, Celebration and Honey Bear. Resistant cultivars of yellow summer squash include Citlali F1, Gold Star, Sunglo, Patriot II, Prelude II, Sunray, XP 1832 III, Yellow Scallopini F1 and Success PM Straightneck. Resistant cultivars of zucchini squash include Amatista, Anton F1, Assia F1, Cuarzo, Caliph, Felix, Golden Glory, Equinox, Envy and many others.