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Garden Q&A: insects suspects in azalea damage

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Q: I have several azaleas in front of my home that have been there for approximately 15 years. Last fall I noticed one of them had black on the branches and on the rock underneath. This spring it seems to be spreading. What is this? Is there anything I can do to stop it?

A: I suspect that there is an insect to blame for your azalea issues; the only trouble is, I'm not positive exactly which insect it is. I'll describe two different problems, and you'll have to do the detective work to determine exactly who the culprit is.

When a black, splotchy substance is found on areas around and underneath a tree or shrub, as well as on the plant itself, I feel confident that it is a fungus known as sooty mold. Sooty mold itself is not a big problem, other than the fact that its presence on foliage can cut down on photosynthesis. The real problem is that the sooty mold is there because it is feeding on “honeydew” — the sweet, sticky excrement of several different soft-bodied insects. When you see sooty mold, it is nearly certain that there is an insect issue to deal with.

Look along the stems of your azaleas and on the leaf undersides for signs of pests such as mealybugs, scales and aphids, all of which excrete honeydew. You can search for images of them on the Internet to help you properly identify them or take a sample to a nursery in a sealed, plastic baggie. The aphids and mealybugs are fairly easy to manage with horticultural oil or insecticidal soap. Scale is a bit tougher because of its hard shell, but it, too, can be managed with a neem-based organic insecticide.

The other insect that may be plaguing your azaleas is lace bugs. Lace bugs are very tiny ( 18 inch), dark insects with lace-like wings. They can be found on the leaf undersides along with small black spots of their excrement (known as tar spots). These tiny black specks of excrement do not wipe off and, in severe infestations, can be found on items beneath affected plants. Damage to the leaf surface appears as whitish dappling, and even when the insects are eliminated, the discoloration will remain. Lace bugs also can be controlled with horticultural oil, insecticidal soap, or neem-based insecticides applied to all leaf surfaces.

All four of these insects are quite common on azaleas and rhododendrons planted in locations that get too much sun. Azaleas and rhododendrons are shade-loving, understory plants, and when they are sited in full sun, they are weakened and prone to insect attacks (in wooded areas, where these plants are well-adapted, we almost never see issues with insect pests like these). If your azaleas are in full sun and you want to prevent this problem from repeating itself, you may want to consider replacing them with a sun-loving shrub variety or relocating them to a shadier spot.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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