We had the perfect storm of weather conditions this summer to contribute to a severe infection of powdery mildew on squash, specifically our butternut and spaghetti squash. The squash leaves with mildew died back, exposing the fruit to sunscald at its formative stage. It's not uncommon for squash leaves to have powdery mildew, but as it affects yield, how can you go about treating powdery mildew in squash? Read on to learn more.

Squash with Powdery Mildew

Powdery mildew spreads rapidly and can travel a long distance. In addition to squash plants, it may afflict any number of veggies, including:

However, there is a different species of powdery mildew that attacks each different veggie. In the case of cucurbits, there are three different fungal species responsible for causing powdery mildew: Podosphaera xanthii, Golovinomyces cucurbitacearum, and Golovinomyces orontii.

Contrary to what you might think, powdery mildew on squash is not prevalent during wet growing seasons. In fact, moist conditions are not necessary at all to foster this fungus and it quite likes it hot. Hence, the aforementioned "perfect storm" here in the Pacific Northwest; we have had an unusually dry, hot summer.

So how do you identify powdery mildew in squash? This disease is fairly obvious in appearance. It appears on older leaves first, as reddish brown spots. At onset, the disease can only be identified via microscope, but quickly it will become apparent as it rapidly spreads to create white mildew covered leaves, petioles and stems. This powdery mycelium makes the leaves appear to have been dipped in talc. The leaves lose their normal dark green hue, turn pale yellow, then brown and finally shrivel, leaving the squash exposed to sunburn.

Conidia (spores) are rapidly produced in the powdery mycelium and any wind or air movement carries them to adjacent plants and leaves as well as off to plants situated even farther away. In fact, it only takes three to seven days from initial infection to appearance of symptoms. Powdery mildew thrives in dense plantings, shaded to low light exposure and high relative humidity. Infection can occur anywhere between 50-90 F. (10-32 C.), but it favors warmer temps up to 80 F. (26 C.), but not over 100 F. (37 C.). Also, powdery mildew in cucurbits is spread because the disease overwinters and is spread to successive generations of squash.

Powdery Mildew Control

Along with the perfect storm of weather conditions, we no doubt aided and abetted the disease. As mentioned above, the disease overwinters. Practicing a crop rotation will go a long way in preventing the spread of powdery mildew. Do not plant cucurbits in the same area for at least two years. We did not practice crop rotation; I blame my other half.

Additional management techniques for treating powdery mildew in squash are to destroy any diseased plant debris, space plantings since a densely planted plot is more likely to be infected, and plant resistant varieties when possible. Also, keep the garden free of weeds. Powdery mildew control may also be need to be combined with a timely application of a fungicide.

When using fungicides, it's too late for them to be of any use once symptoms become rampant. Fungicides work by preventing infection of healthy foliage, so find the infection early. There are a number of organic options available as well as traditional fungal sprays.

Sulfur and "Stylet" oil are effective products for powdery mildew control.

Fixed copper fungicides have also shown results in managing powdery mildew.

Neem oil is also an effective combatant for managing powdery mildew.

Whatever you choose, remember the key is early application, before the disease is readily apparent all over the foliage.