Persistent powdery mildew - Ask Extension
Hi, I have a garden bed that has been plagued by powdery mildew for several years. Aside from removing the affected plants, what can be done to remedi...
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Persistent powdery mildew #900411
Asked May 07, 2025, 10:01 AM EDT
Hi, I have a garden bed that has been plagued by powdery mildew for several years. Aside from removing the affected plants, what can be done to remediate the area? It’s clearly in the soil and not going away. It affected brown eyed Susan’s when it was full sun, now it’s more shaded with a river birch. The birch seems to have gall and a lot of aphids. Should I lay down mulch and try again next year or is there any chance of planting this year? Thank you
Montgomery County Maryland
Expert Response
Powdery mildew doesn't reside in the soil (spores are windborn and blow around on the breeze, or splash around in heavy rains), so that shouldn't be a problem for remediation. Plant disease severity and spread greatly depends on weather, and outbreaks won't necessarily recur every year, especially when the weather skews dry, as it has been for much of the past year. Therefore, it's hard to predict whether plants will have recurring issues with a certain disease, though if you use a sprinkler or hose to irrigate plants when they need water, avoid wetting the leaves too late in the day (they should ideally dry by nightfall), because wet leaf surfaces are more vulnerable to infection. Powdery mildew spores actually can be hampered by wet leaves (they need high humidity, not actual wetness), but as that moisture evaporates in conditions with stagnant air (insufficient air circulation), that can still contribute to outbreaks.
We don't see any indications of powdery mildew in the photos. The birch, as you noted, has a population of aphids (the kind that use witchhazel as their alternate host plant for part of the year), which is quite common. The leaf wrinkling as a result can be an eyesore, but it doesn't cause the tree serious harm, and predators like the ladybug larva you pictured will consume them and reduce their numbers. Aphids release honeydew (sugar-water, basically) as they feed, and this falls onto plants and surfaces below them. Then, windborne pollen (which has been high recently) can stick to those surfaces and accumulate, looking like mildew or other leaf diseases. We can see numerous white shed aphid skins sticking to the plant in that last photo, which are falling off the birch above.
If the existing mulch is thinning, where it's an inch or less thick, then you can top it off with more mulch to a thickness of about 2-3 inches as a weed-discouraging layer. Otherwise, you don't need to amend it. Many species of perennials and shrubs can contract powdery mildew when the weather conditions are right, but even though the symptoms all look alike, they are not the same species of mildew, so one infection won't cross over to another unrelated plant. You can plant now, and if mildew arises, treatment options would include a general-purpose fungicide (if needed).
Miri
We don't see any indications of powdery mildew in the photos. The birch, as you noted, has a population of aphids (the kind that use witchhazel as their alternate host plant for part of the year), which is quite common. The leaf wrinkling as a result can be an eyesore, but it doesn't cause the tree serious harm, and predators like the ladybug larva you pictured will consume them and reduce their numbers. Aphids release honeydew (sugar-water, basically) as they feed, and this falls onto plants and surfaces below them. Then, windborne pollen (which has been high recently) can stick to those surfaces and accumulate, looking like mildew or other leaf diseases. We can see numerous white shed aphid skins sticking to the plant in that last photo, which are falling off the birch above.
If the existing mulch is thinning, where it's an inch or less thick, then you can top it off with more mulch to a thickness of about 2-3 inches as a weed-discouraging layer. Otherwise, you don't need to amend it. Many species of perennials and shrubs can contract powdery mildew when the weather conditions are right, but even though the symptoms all look alike, they are not the same species of mildew, so one infection won't cross over to another unrelated plant. You can plant now, and if mildew arises, treatment options would include a general-purpose fungicide (if needed).
Miri
