Fall garden problems: powdery mildew

As fall sets in, a light frost may kill the leaves of cucumbers, squash and, more importantly at this time of year, your pumpkins. But the stems often suffer only minor damaged and can continue to support growing fruit until a severe frost. 

The blackened leaves are discouraging and if you decide to remove them, it's best to throw them away, instead of putting them in the compost. They often have a disease called powdery mildew.

Powdery mildew on a pumpkin plant. You can see that the new leaves aren't infected yet.

Powdery mildew frequently grows on these kind of plants. It looks like white powdery spots on the leaves and is often attributed to moisture on the leaves from overhead watering or high humidity. It's caused by several different species of fungi and is spread by splashing water or wind moving the spores around.
 

When the powdery mildew is advanced on a squash or pumpkin, the leaves have white mottling all over, they're distorted and curled, and the plant starts to turn yellow, then brown.

This is an infected leaf, with discoloration and curling at the edges.

I grew gourds and small pumpkins up my trellis and only watered them with drip irrigating. I'd hoped that this treatment would prevent powdery mildew since the plants were mostly off the ground. But even in their hot location, in an area that had been covered in gravel before my soil improvements, powdery mildew made a strong appearance.

Dead and dying leaves on my trellis. The bring green leaves are grape plants and they are uninfected so far.

It can affect many species of plants including perennials, and can be controlled with various fungicides. I consider fungicides a particularly virulent type of pesticide and would recommend using it very little or not at all. If we lived in high humidity or if you are growing commercial crops, they have their place, but not in a home garden. In early days of summer, diluted milk can be applied when the first spots occur, but when the disease is well-advanced, this treatment is apparently less effective.

Prevention is a better technique. This means removing and throwing away any plant parts and plant debris from around the affected plant, especially in the fall when you clean up your garden. This won’t completely remove the fungi from your garden, but it will certainly help!