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...