Issue: June 30, 1997

Leaves turning yellow and falling from locust tree

A lot of leaves on my locust trees are turning yellow and falling. It looks like autumn now. I=m afraid the rest of the leaves on the tree are going to drop before mid-summer. Is there a disease going around? What can I do to protect my trees?

The loss of numerous leaves from trees is not uncommon in New Mexico in June. The leaves that formed early on the trees developed under vastly different environmental conditions. It was cooler and water loss by the leaves was much less. As the weather has warmed and the moisture decreased, increased water loss by the trees has caused them to dispose of excess leaves. This is a natural strategy to conserve water. This year the cooler and wetter-than-normal spring has enhanced this effect. I have noticed this is especially true in landscapes in which the owner has decided to reduce landscape water use, converted to xeriscape, or just not noticed the drastic changes in environment affecting the landscape. Some people have not watered as much as usual because we did have a relatively moist spring in much of New Mexico. The soil is now quite dry and unless the trees are irrigated, severe leaf drop must occur. Even with irrigation, the drier air and hot temperatures will cause considerable leaf drop. The late cold spells may also have played a role, injuring some leaves just as they were developing in the buds or beginning to enlarge. These are the first leaves the trees will drop to conserve water. If you are providing adequate water for your trees, leaf drop with no other symptoms of insects or disease is not a cause for concern.

Locust trees prefer a more moist climate but are able to adapt to our hot, dry environment by disposing of excess leaves. Locust trees are also very susceptible to aphids, insects common in locust trees this year. Check your trees to see if aphids are causing some of the problems. You will find them on the undersides of leaves and on young twigs. You may also notice a sticky mist falling from the trees and collecting on surfaces under the tree. This is honeydew, a sugary substance excreted by the aphids, and often an indicator of some insect infestations. If the trees are well irrigated and otherwise healthy, the aphids should not cause concern. Natural predators and parasites which kill aphids will usually limit the damage due to aphids. If the population of aphids or their damage is too severe, an insecticide may be needed. If you feel you need to use an insecticide, be sure to select one which says on its label that it is for the control of aphids on ornamental trees; then follow the label directions to the letter. Before using insecticides, however, try blasting the aphids from the trees with forceful jets of water. This is often successful on smaller trees and the lower branches of larger trees.

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.

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