Is this powdery mildew?

Asked April 29, 2019, 3:46 PM EDT

I have this bush that struggles every year with white spots that also destroy the leaves. It faces west but is shaded most of the day. Last summer this spread to a house plant I had moved outside and I could not save it. I also think it spread to a tomato plant down wind of the bush. I'm not sure what the bush is but it gets yellow flowers that look like an azalea. I trimmed the bush back and have been treating it with neem oil mixed with water. I'm not sure if this will help whatever the spots are. Do I need to remove the bush or can it be treated? Pics attached. Thank you

Multnomah County Oregon

1 Response

Thank you for using Ask an Expert for help with your problem plant. The bush appears to be a deciduous azalea. These plants can be attacked by several diseases, however I can’t be sure which one is the problem you face.

First, to the question of powdery mildew. This will appear as dusty, white areas on the leaves in the later part of summer. There are many diseases called by the common name “powdery mildew,” which are caused by very different actual fungus organisms. The fungus that attacks the azalea is not the one that attacks tomatoes, though both look like and are called powdery mildew.

To help control powdery mildew on an azalea, ensure good air circulation around the plant by pruning and spacing of the plants. Do not fertilize the plants heavily; once or twice a year with a plant food for azaleas and rhododendrons is sufficient. Thin trees above the shrubs to provide a bit more light.

To control the disease with spraying, monitor the plants and begin your program as soon as leaves show spots. Recommended products are Monterey Bi-Carb Old Fashioned Fungicide or Bayer Advanced Natria Disease Control. Also Safer Garden Fungicide (Do not use this product when air temperatures are over 85°F or within a few weeks of an oil spray.)

Another disease common to deciduous azaleas is azalea leaf gall. This one attacks earlier in the season than powdery mildew. It contorts the leaves until they are thick and covered with white spores. These leaves can be removed by hand to prevent on-going infection.

If this is a yearly problem, you can get started spraying with Monterey Liqui-Cop as buds open in spring and again two to three weeks later.

The white patches on the stems of the plants in the photographs appear to be lichens. These are commonly found on many ornamental trees and shrubs in the Pacific Northwest, due to our cool, wet weather in western Oregon. These do not harm the plants, but pruning to increase air circulation may reduce their presence. The use of copper-based fungicides used when the plant is dormant can also provide some control.

When applying any fungicide, be sure to read and follow all directions and cautions.

Have a good gardening year,