Powdery Mildew – What it is and how to deal with it

Last summer (2014 as I write this) was a really bad powdery mildew summer, the worst I’ve seen in years, because it was so hot and dry.

Most people think that fuzzy white mildew is brought on by cool, wet weather, but only certain kinds are. Mildew is a fungus disease that preys on plants that are under stress. Hot, dry summers can produce lots of periods of drying-out and wilting, which will stress and weaken many types of plants. That leaves them susceptible to infection by the fungal spores floating around in the air, waiting for a victim to grab onto. Begonias, roses, Exbury azaleas, phlox, salvias, zinnias, and verbenas are just a few of the common, popular plants that fall prey to the nasty white fuzz crawling across their leaves. Begonias were particularly hard-hit in last summer’s heat because they prefer somewhat cooler summers than we have – the begonias in Scandinavia are glorious – and evenly moist soil.

So if we’re facing another hot summer, what can we do to protect our plants?

First, don’t let susceptible plants go dry in hot spells. You don’t have to water the whole yard every day. Water your baskets and planters, because they dry out fastest. For mildew-prone perennials and shrubs, leave the water trickling slowly on the roots of the problem plants in the evening so they’re comfortably moist for the coming day. Don’t worry that you’re watering in the evening and some book or neighbor said that causes mildew. This watering method doesn’t get water all over the leaves, so it is not adding to mildew danger. It’s the most efficient time and way to water because there’s little loss to evaporation. And for working people it’s when you’re home to do it. Soaker hoses are good, too, but depending on how you have it laid out they might also water plants that don’t need it so much.

If you do see mildew in spite of your watering care, spray infected plants at the first sign of white spots. Mildew will not go away without spraying, and the longer you delay, the harder it is to control. The old home remedy of 1 tsp. baking soda and 1 tsp. Ivory liquid soap in 1 quart water is a good place to start. If that doesn’t do it, you can stay organic and try a sulfur spray next. Safer brand puts out a premixed form. Spraying in the evening will prevent burning of foliage in hot weather. If organic-ness is not an issue for you, there are a number of effective fungicides you can use. Remember, no procrastination.

Many perennials are perfectly happy to be cut down to the ground in midsummer, so you can simply cut off and throw away badly infected foliage, letting the plants grow back new, clean leaves that just need a little preventative care. Of course one hates to do that if the plants are in full bloom. But once they’re done blooming, cut them off. Be sure not to put mildew-y leaves in your home compost bin. The material that goes to Marion County’s composting from the big green compost bin that most of us in Salem have will compost hot enough to kill disease spores.