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Wednesday - July 25, 2007

From: Austin, TX
Region: Southwest
Topic: Diseases and Disorders
Title: Oak diseases as a result of wet, cool weather
Answered by: Damon Waitt


The mature live oak trees in our back yard have been dropping leaves just like they do in March, but this is July! Small to medium spots on the leaves turn brown before they drop. The veins of the leaves are still green. The trees are growing on a well-drained slope in shallow soil over limestone here in the hill country just west of Austin. Asiatic jasmine grows under some, but not all, of the trees. This has never happened in the past 13 years. Does it have to do with the heavy rainfall? Will they survive? Do we need an arborist consult?


It is our good fortune that James Houser, the Texas Forest Service Oak Wilt Technical Coordinator, recently addressed this question on the texasoakwilt.org website. You can click here to view a pdf version of his article with images.
"Abundant rainfall and cooler than normal temperatures this spring and summer, oak trees are infected with a variety of leaf diseases that cause significant discoloration of leaf tissues and defoliation. Symptoms include a spotting and/or yellowish mottling of the leaf tissue, blistering and deformity of the leaf margins, small twigs with brown leaves randomly distributed within the tree canopy, and small gold fruiting structures on the undersides of leaves. The trees are frequently shedding these leaves as the leaf disease matures.

Diseases that can be responsible for these problems include Actinopelte spp., Oak leaf blister, Diplodia spp., Anthracnose, and Oak rust among others. These problems are only confined to the leaf tissues, though with Diplodia spp. small twigs an inch or two long die. Branches remain alive and viable on the tree for new leaves to form. The trees are not dying. Fungicides are ineffective at this point as new infections do not occur with elevated night temperatures during the summer. Raking up and disposing of the leaves on the ground is a good strategy to remove the local inoculum. This is a natural occurrence during high rainfall years and the trees will recover on their own.

This is not oak wilt. Oak wilt leaf symptoms are completely different. Also a pattern of mortality in the stand of trees over time is not apparent, i.e. an infection center radiating out over the landscape for several years. Trees in Texas encounter many problems from our stressful weather conditions. Last year, drought was responsible for damage to the trees. This year too much moisture plays a role. Yet, trees, especially oaks, are tough. They go through these cycles of nature and keep on growing."


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