It is the first Saturday of the month, and I’m just back from an educational and pleasant trip to the UC Botanical Garden’s Sick Plant Clinic. This has been a wonderful year for powdery mildew on our tomato plants. I went to get the 411 on the nasty visitor.

First the good news: Mildew needs living plant tissue to live on so it won’t kill your plant. There are many species of powdery mildew and they attack specific plants—the mildew that’s on your tomato plants is different than the species on your zucchini. Usually it doesn’t spread to the fruit.

Now, the bad news: It flourishes in moderate temperatures (60° to 80°F) and shade. It spreads by spores floating on the breeze. This year has been more windy and a bit cooler than usual overall creating a perfect condition to grow and spread powdery mildew. This fungus does not need water or moist conditions to grow—that’s downy mildew, its evil sibling.

Management: Cut off all the affected leaves and stop the plant’s energy from going to the damaged leaves. Keep the plant growing vigorously to replace the leaves you cut off. Usually there are 2 leaves emerging from each leaf node so the second one will grow to replace the first you just cut off. Put the affected leaves in your green bin and wash your clippers/scissors and hands before touching healthy plant tissue.

Prevention: To prevent the spread, mix 1 part milk (some say powdered milk works better) to 20 parts water and spray the plant with it. Best to do this in the morning so the sun will dry it as the day progresses. The proteins in the milk create a barrier on the stems and leaves so the spores can’t get through to adhere. This is preventative so don’t expect the affected parts to heal.

I heard that potato leaf varieties are more resistant so I will keep an eye on our plants to see if that’s true for us. Seems like bottom line is that all we can do is manage the disease and keep in mind that the point of all this is to produce delicious tomatoes, even if the plants look like crap!!

Let us know your experiences with mildew on your plants if that’s one of your issues this year . . .