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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Wednesday - August 08, 2012

From: Austin, TX
Region: Southwest
Topic: Diseases and Disorders, Trees
Title: Pecan leaves falling off the trees in Austin, TX.
Answered by: Jimmy Mills

QUESTION:

Why are the pecan tree leaves turning black, sappy and falling off the trees, in great quantities? This has happened since the recent rains, all through the Allandale area.

ANSWER:

Austin is presently experiencing an onslaught of honeydew. Honeydew is a waste-product of aphids and other sap-sucking insects such as scale. As honeydew is excreted by the sucking insects, it becomes airborne and falls and coats any surface upon which it lands in the area. Pecan trees are only one of several kinds of trees and shrubs that are being attacked by the aphids.

This article from University of Minnesota Extension, another from the University of Kentucky College of Agriculture,  and a third from the University of California Davis give a rather thorough explanation of honey dew and aphids, and their effects on plants. The blackening of the pecan leaves is because of an infestation of sooty mold that grows on the honeydew. This article from the USDA tells how to recognize and control sooty mold.
Another problem is that the sticky sap gets all over cars. This link has information about sap removal.

 

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