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Saturday - May 08, 2010

From: Austin, TX
Region: Southwest
Topic: Trees
Title: Sap flowing from live oak trees in Austin
Answered by: Barbara Medford

QUESTION:

I have live oak trees in my backyard,17 to be exact. I live in Northwest Austin, in Williamson county. There are two live oaks growing about 4-5 feet apart in the center of my yard. One appears to have a wound where sap is seeping out. Butterflies or moths have been flocking to it to drink the sap. This happened a few years ago to the other tree. Are my trees sick? I have mostly natives planted in my yard with a flower bed around these two trees. What can I do?

ANSWER:

The first thing we want to ask is, what do you have growing in your flower beds under the oaks? This probably has nothing to do with the wound on the oaks, but ordinarily, it is difficult to impossible to grow much of anything under oaks because of allelopathy, or the ability to emit substances that will retard the growth of competing plants beneath the oak.

The second question: Is the first tree that showed the symptoms still healthy? Or alive? Read this article from Texas A&M Horticulture Things You Should Know About Live Oak Decline. We don't know what caused the wound, disease or a blow from some mechanical source, but we know the sap being available between February 15 and June 15, when the nitulidid beetle is active, is bad news. This beetle also feeds on the sap, but if the little beastie has been visiting an oak tree with Oak Wilt, he will carry the fungus for Oak Wilt on his body, and infect the wounded oak.

So let's get back to the wounds and the insects visiting the wounds. From Eric Becker of the Texas Forest Service:

"After being stressed by the drought many of our oaks were further damaged by Hypoxylon and borers. As soon as you hear someone say the bark has fallen off, you can bet that their drought stressed tree has hypoxylon cankers (first brown and powdery, and then a silvery crust) and the prognosis is not good."

Go to the TAMU AgriLife Extension Plant Pathology page to find three fact sheets about hypoxylon: Hypoxylon Fact Sheet #1, Hypoxylon Fact Sheet #2, and Hypoxylon Fact Sheet #3.

If you live in the areas north of a Wimberley to Kyle line then there is the chance that oak wilt could be at play as well. Trees damaged by oak wilt can also display hypoxylon cankers, so then we look for the pattern of mortality, groups of oaks dying side by side, and the brown vein leaves. Drought mortality accompanied by hypoxylon cankers usually occurs on one or two oaks over here and another two or three over there—the sick trees are scattered about with healthy ones in between."

However, you should visit the Texas Oak Wilt site to familiarize yourself with the symptoms, prevention and treatment of the disease."