Prevent powdery mildew from getting worse

Powdery mildew is one of those problems that is immediately obvious, once you know what to look for.

Here’s the bad news: Most local gardeners know it so well because everyone deals with it this time of year.

This disease’s calling card is a white or grayish powder (hence the name “powdery”) that looks like somebody came and dusted baby powder all over your garden when you weren’t looking.

Trust me, there’s no powder fairy flitting about in the middle of the night, anointing your squash leaves. This is the work of fungus, and powdery mildews are widespread and common, especially when the relative humidity rises in places with warm, dry climates.

Unlike some other fungi, powdery mildew doesn’t need a wet surface to grow. The spores germinate when it is humid, and here you can see a proliferation of powdery mildew after even short monsoonal rains that leave the air feeling muggy for our high desert.

The other factor that seems to exacerbate the problem in my garden is the fact that by the time we get those rains, plants have grown considerably and there’s not much room for evaporation or for air to circulate between them. Moist air is concentrated right around the leaves in the garden canopy, and that shaded environment provides a great place for powdery mildew to set up shop.

While there is not much you can do once the disease has taken hold, there are some measures you can take to try to prevent matters from getting worse.

According to Colorado State University Extension, you can reduce the relative humidity by not watering from overhead (don’t sprinkle the leaves with water). Avoid late-season fertilizer with nitrogen, because it will trigger new growth on the plants that is more likely to become infected. They also recommend removing and destroying the infected plant materials (which you can do and have the plant survive if the fungus is not too advanced). In addition, you can selectively prune back plants to allow for more air circulation if it seems too crowded.

Gardeners also can use anti-fungal substances including neem oil, sulfur, triforine and potassium bicarbonate as treatments, but those work most effectively when paired with the previous methods and should be used according to the labeled instructions.

It is also important to think about next year as you battle this year’s problem.

If you look very closely at these dusty patches of fungus on your plants — whether it is your zucchini plants (the most common place I notice it) or lilac leaves or even roses or fruit tree leaves, you’ll notice there are these teensy-tiny round thingies that start out white and eventually turn black as the disease progresses. These are called “fruiting bodies” and they’re what help the fungus overwinter and come back to haunt you next year.

While there are many types of powdery mildew, and they are all host-specific (meaning the type that attacks your lilac bushes isn’t going to spread to your zucchini), they can all overwinter and come back. That’s why I throw all the infected plant material away instead of composting it, as it is unlikely that my home compost setup reaches temperatures capable of killing the spores.

Erin McIntyre is an advanced master gardener and journalist. Email her at .(JavaScript must be enabled to view this email address) with story ideas or feedback.


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