Anne of Green Gardens

What is Powdery Mildew?

Powdery mildew is a white, powdery-like fungus that sometimes resembles bird droppings. The fungus is composed of tiny white spores that you can see with an eye loupe or a microscope. This unsightly fungus seldom kills plants, but can weaken them and leave them open to new infection by other diseases or by insects.

Powdery mildew does best when temperatures are between 60-80F. Plants growing in shade that need full sun often get powdery mildew.  Spores not killed will overwinter and reinfect plants the following year.

Killing and Preventing Powdery Mildew

Interestingly, overhead sprinkler helps reduce the spread of  powdery mildew. If you use sprinklers, turn water on in the morning. This will wash off the spores which are killed on contact with water.

If your favorite plant gets infected each year, you can prevent it with a fungicide. Horticultural oils can act as fungicides to both kill and prevent the fungus. However, completely eradicating the fungus once it becomes established can be difficult. You’ll need to treat the following year in spring when weather reaches 60F, or as soon as you see signs of powdery mildew. Apply the oil every 7-10 days until weather warms above 80F.

Horticultural oils may be composed of petroleum products, or plant-based products like neem or jojoba oil. (they are also excellent for controlling aphids, scale insects, mealy bugs and white fly). Most oils are found at nurseries or garden centers, read the label for any precautions. For example, spraying when temperatures are above 90F or spraying water-stressed plants can cause plant injury. Your plant will react by looking “fried” or “crispy.” Of course I don’t have any personal experience with this…LOL.

Some varieties of roses, crape myrtle, and London Plane Tree (a type of sycamore) get powdery mildew every year. Some types of fruit trees, berries and vegetables also get mildew. If constantly monitoring and spraying your plants is too tiresome, you could replace infected plants with resistant cultivars. Ask a local master gardener or a  local nursery for help.

Originally Published: April 18, 2012
Updated: August 21, 2016

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