Issue: June 17, 2006

Curled leaves on trees

The leaves on some of my fruit trees and shade trees are twisted into knots. It doesn't look good, and I'm worried that the trees are dying. My ash tree is especially bad. Should I be worried?

I have observed this problem in several New Mexico communities this year. There are small aphids that attack a variety of plants and cause leaf curling. The aphids that attack one type of plant will often not attack other plants, so you may have several species of aphids in your trees.

The aphids inject a substance with their saliva before they begin to suck the juices from the leaves. There are chemicals in the saliva that cause the leaves to curl and twist. The curled leaves protect the aphids from the wind, rain, and pesticide applications.

You can apply an insecticide if you wish, but the effect may be less than desired because of the protection for the aphids afforded by the curled leaves. A systemic insecticide (used on plants as permitted by the label) may be more effective. However, you shouldn't use a systemic insecticide on fruit trees unless specifically allowed by the instructions on the label. This includes use on so-called "flowering plums" because they may produce very tasty fruits some years and should be considered a "fruiting tree" even though they don't produce fruit every year. Your ash tree can be considered an ornamental tree that does not produce edible fruit.

If you prefer to not use insecticides, there is some good news. This aphid tends to infest the trees and cause damage early in the growing season and then usually disappears as the season progresses. Chemical treatment my not be needed. One reason the aphids disappear as the season progresses is that the population of natural predators increases with time. These natural predators will minimize the aphid problem. You can purchase some of these natural predators from insectaries, or you can just wait for them to appear. If you apply insecticides, you may kill the predators and actually allow the aphid population to increase.

Should you worry? No, unless your fruit trees are a commercial orchard. The aphids can lower fruit quality and yield if the infestation is severe enough. In home orchards, a strong spray of water will often dislodge many aphids even from inside the curled leaves. This should reduce the aphid population enough to minimize the problem without harming the natural predators. You must evaluate your specific situation to determine if insecticide application is warranted. If you decide that an insecticide is necessary, contact your local Cooperative Extension Service office for recommended chemicals that are effective and have the least damaging effect on the environment. Also, ask for a product that is safest to use in the environment occupied by you and your family.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!