powdery mildew

One Name for Many Diseases

The powdery white coating I notice on leaves of my peony and lilac plants gives them a very Mediterranean look. Not attractive, though, at least to me, because that’s a sign of a disease, appropriately called powdery mildew. If I look around the garden, the disease is probably showing itself, or soon to do so, on a wide variety of plants, including phlox, zinnia, squash, gooseberry, and many more. (This blog post is excerpted from the chapter about plant stresses in my book The Ever Curious Gardener: Using a Little Natural Science for a Much Better Garden.) Even though powdery mildew is pretty reliable in showing up each season on certain plants — this is important! — the disease doesn’t necessarily spread from one kind of susceptible plant to the next. Many different fungi are responsible for the disease we collectively call powdery mildew, and each of these fungi have specific plant hosts. Thus, the fungus that causes powdery mildew on lilac (the fungus is Microsphaera alni) cannot cause powdery mildew on rose (caused by the fungus Sphaerotheca pannosa). Read more