Issue: June 15, 2002

Locust tree leaf rust


My purple robe locust has rust on many leaves. Is this a result of over-watering?


The locust trees are subject to leaf diseases such as rust and powdery mildew. These tend to be problems in wet years. For leaf diseases to be developing this year, frequent watering may be the cause. Over-watering would be a different problem. Leaf diseases on the trees require moist air around the tree during the cool evening hours. The diseases are dependent on having both proper temperature and humidity for their growth. Night provides the temperatures, and we often provide the humidity by irrigating often and at the wrong time of day.

Deep watering to moisten the soil to a depth of two-to-three feet for trees and approximately one-foot for turf areas should allow watering on a more infrequent basis. The surface soil dries rapidly by evaporation, but moisture deeper in the soil persists longer and is more likely to be used by the plants. Allowing the upper levels of the soil to dry between irrigations encourages development of deep roots in the plants and reduces humidity around the plants at night. This reduces leaf diseases, at least in dry years. During monsoon season, we can't control night humidity but, while it is dry, we do have greater control.

In order for deep watering to be effective, the soil must be able to absorb water to the depths mentioned and plant roots must explore greater soil depths. Caliche layers often interfere, but another common problem is compacted soil created following construction (when the home was built or remodeled). Compacted soils are difficult to moisten and discourage development of deep roots. Deep aeration and careful irrigation can help overcome the problem, but the best solution is to prepare the soil over the whole landscape before any landscape plants are installed. This would involve deeply rototilling the entire area of the future landscape. This cannot be done in established landscapes without injuring the roots of existing trees and shrubs.

Deep watering also conserves water. With each irrigation, evaporation claims a portion of the water applied during application and some of the water at and near the surface. Deeper water is not lost to the air until it is used by the plants. The less frequent the irrigation, the less the evaporative loss and the greater the irrigation efficiency. This is also how we create conditions less conducive to leaf diseases.

Irrigation timing is also important in reducing leaf diseases and in conserving water. Irrigation in the evening will result in more moisture around plant leaves in the evening, but early morning irrigation allows time for the evaporate moisture to dissipate and lowers humidity around leaves at night. Evening temperatures are higher than morning temperatures. This increases evaporation rates in the evening. Morning water evaporates less and is a more efficient time to irrigate. (Wind is often less in the morning also increasing watering efficiency and reducing evaporation). Be certain to check city irrigation ordinances regarding times to irrigate.

Finally, do you really have leaf rust disease? Do the rust-colored fungal spores rub off on your finger when you gently rub the leaf, or do you just have dry, brownish dead spots developing as a result of dry air and wind? We often see a lot of leaf drop in June as a result of the increasing heat and dryness. Leaves (leaflets on the locust tree) that were produced under cooler conditions are often dropped by the tree as wind and heat increase water loss by the tree. This is just the tree's water conservation technique. If a lot of leaves are falling, the problem may be the result of improper irrigation or due to the tree conserving water.

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Send gardening questions to Southwest Yard and Garden, Attn: Dr. Marisa Thompson at or at Please include your county Extension Agent ( and your county of residence with your question!

For more gardening information, visit the NMSU Extension Horticulture page:

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, office: 505-865-7340, ext. 113.