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Monday - August 09, 2010

From: Austin, TX
Region: Southwest
Topic: Diseases and Disorders
Title: Ailing Lacey oak in Austin
Answered by: Barbara Medford

QUESTION:

I have a five-year-old lacey oak that is about 5 feet tall. Last last spring it became infected with oak phylloxera, which was initially misdiagnosed. I treated it with Neem oil several time but this did not help. By fall all the leaves had dried up and fallen off. I treated the soil with beneficial nematodes. This spring the tree leafed out beautifully and looked great until it got hot last week. Now the infestation has returned. I treated it with insecticidal soap but this has not helped. Can you suggest a treatment? If I cannot eliminate or control the infestation I plan to remove the tree. I am willing to pay for professional treatment but need to know if anything will actually work.

ANSWER:

Mr. Smarty Plants is always learning. We had never even heard of oak phylloxera, and when we searched on it, at first all we found was that it infested grapevines. Scratching a little harder, we found this article from Texas A&M AgriLIFE Extension on Gall-Making Insects and Mites. Apparently, these very tiny insects cause swellings, or galls, on the leaves of oaks that are susceptible to such an infestation. All we were really able to pull out of this material was that, yes, it got galls on its leaves, but no, it wasn't particularly of concern.

We are not entomologists, and certainly no good at diagnosing plant diseases at a distance. From our viewpoint, about all we can suggest is that you may be overtreating the tree. In the first place, don't fertilize it, that will encourage more leaves, which will in turn encourage more gall-making insects to move in. All these treatments in the heat of summer must be stressful for the tree, as well. Quercus laceyi (Lacey oak) is considered a good choice for the Central Texas area because it is resistant to Oak Wilt, and can therefore be used as a replacement for live oaks. 

Please read the entire article above from Texas A&M. At the very end is information on phylloxera in trees, with particular emphasis on pecans, which apparently can actually be damaged by it. We thought a couple of extracts from that portion were interesting:

"Because most galls and gall-forming insects are not a threat to plant health, attempting to control them is not usually warranted. Learning to recognize different galls and the insects or mites that cause them may provide peace of mind and can be a good way to enjoy nature at our doorstep! If galls are considered unsightly, they can be removed by hand or infested plant parts can be pruned and discarded. However, this may not prevent future infestations. Removing the host plant and replacing it with a non-susceptible species or a more resistant specimen is the only sure method of control." 

"Biological control. Several species of wasps parasitize gall-forming insects and reduce the number of galls formed. These wasps are natural enemies of gall-making insects and function as their biological control agents. To protect these beneficial wasps, avoid using broad-spectrum insecticides during the time they are searching for hosts (from late spring through early summer)." 

"Chemical control. Although there are some insecticides and miticides registered for use against gall-making insects (including insecticidal soaps, horticultural oils and several formulations of acephate and carbaryl), their use is generally unwarranted. It is very important to know the basic biology of the specific gall-making insect or mite involved before applying a pesticide. Unless pesticides are applied when adults are laying eggs or during the crawler stage (pecan phylloxera), they may not control the pest. Once galls begin to form, the insects and mites are protected inside them and can not be killed with either a surface-applied pesticide or a systemic pesticide. Furthermore, pesticides may kill the beneficial insects that help control gall-making insects."

In summary, the experts seem to be saying, "hey, it's no big deal-either let them alone to deal with it, or replace the tree." We would suggest you contact the Texas AgriLIFE Extension Office of Travis County; they DO have entomologists they can confer with. It would appear that the printed materials they have deal with the pecan galls, but perhaps they have added other information for the oaks.

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