Two years ago, in the middle of the COVID-19 pandemic, Jim and I didn’t have much to do, so we ordered a large number of perennial plug packs from a favorite supplier. In fact, we ordered so many plug packs that I didn’t have room to plant a batch of Monarda didyma, commonly called bee balm, bergamot, or Oswego tea. Bees and butterflies flock to Monarda, and I didn’t want it to go to waste, so I planted the plugs in a half whiskey barrel with a plan to move half of them the next year. I didn’t of course, and by this spring, the barrel was packed full of the tall plants, too closely, it turns out, because some of the plants now have powdery mildew on the leaves.

Powdery mildew is a fungal disease, caused by one of several organisms, and can affect a wide variety of plants besides Monarda, including oak, dogwood, azalea, rhododendron, zinnia, dahlia, and even squash and cucumber, among others.

The most common and easily recognizable symptom of powdery mildew is a layer of pale gra...