Powdery mildew is caused by several different fungi, depending on the vegetable. It usually starts as small white, powdery spots on the leaves and can spread until large areas look like they’ve been dusted with flour. It may also appear on stems and leaf stalks.

Unlike many fungal diseases, powdery mildew doesn’t need standing water on the leaves to infect them. It tends to thrive with high humidity, poor air circulation, crowded plants and moderate temperatures. Dense, shady growth can make things worse.

How to Fix It

Start with cultural controls. Remove badly infected leaves, particularly early in an outbreak. Don't strip the plant bare — it still needs leaves to photosynthesize.

Give plants room to breathe. Thin overcrowded growth, control weeds and provide adequate spacing so air can move through the foliage. Give plants plenty of sun where possible.

Water at soil level and keep plants evenly watered. While wet leaves aren't required for powdery mildew, stressed plants are generally more vulnerable. Avoid pushing lots of tender leafy growth with excessive nitrogen fertilizer.

At the end of the season, clean up infected plant debris. Rotate crops where practical and choose powdery-mildew-resistant varieties when they're available.

For an organic treatment, sulfur can help prevent or suppress powdery mildew when used early. Potassium bicarbonate products can also suppress established infections, while horticultural oils such as neem oil may help when disease is still limited.

Whatever you use, coat the foliage thoroughly and follow the product label. Don't apply sulfur and horticultural oils close together — the combination can damage foliage.

And once half the squash plant looks like someone emptied a bag of flour over it, spraying isn't going to make those leaves pretty again. Remove the worst leaves and concentrate on protecting the healthy ones.

As the infection becomes severe, leaves may yellow, curl, become brittle and die. The plant loses productive leaf area, which can reduce growth and yield. On crops such as squash and cucumbers, a bad infection can cause leaves to die prematurely and expose fruit to sunscald.