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October 25, 2014

Blisters on your Oak Leaves?

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Posted: Wednesday, July 9, 2014 2:47 pm

Question: I have a large stand of oaks that provides a beautiful natural landscape for my home. Recently, the trees have been dropping leaves as they would in the fall. I have noticed that some of the leaves have raised spots on them and others have small brown spots. There are so many leaves falling that I am afraid I am going to lose my trees. What can I do to save them?

Answer: This sounds like the classic symptoms of oak leaf blister, a common ailment of oak trees this time of year. This disease has been very common this year due to the cool, wet spring weather. While the fungus may cause leaves to look strange or deformed, it will not cause serious damage to the tree.

Oak leaf blister (OLB for short) is a very common foliar disorder caused by the pathogen Taphrina caerulescen. OLB affects more than 50 species of oak including water oak, Southern red oak, Northern red oak, Nuttall oak and pin oak.

During years that are cool and moist during bud break, there will be a high occurrence of affected trees. Symptoms of the disease can include raised bulges or blisters and distortion of the new leaves, brown or dead tissue on older leaves and partial defoliation later in the season. OLB is especially common during years that we have cold and rainy weather as the leaves are emerging from the bud early in the spring. Leaves that emerge later in the spring often show no signs or symptoms of infection.

While oak leaf blister will not cause any lasting damage to the health of the tree, the foliar spots and falling leaves are not esthetically pleasing. If enough leaves drop, the tree will respond by initiating new growth. Generally, mature and healthy trees have many more leaves than they require for food production, so a few lost is not a big deal.

While a quick internet search may show you that there are some chemical solutions to help with OLB, it is not recommended for two main reasons. The first is that by the time you see the damage, it is much too late to treat the tree. In the lab setting, leaves that were inoculated with the fungus did not show damage until up to four weeks later.

Timing of the application would be critical -- just as leaves are emerging from the bud -- and adequate coverage would be necessary, which leads to the second and biggest problem. Mature oak trees can reach to heights of 100 feet or more with a span that can equal that. Most homeowners would not be able to effectively treat trees that are this large, and the chemicals needed to treat such a large tree could be prohibitively expensive.

The cheapest and most effective control is to rake and remove the leaves from the area. Unfortunately in natural areas with many trees, it could be impossible to achieve this. Remember, although OLB is not particularly attractive, it will not kill the tree and if we have a dry spring next year, we may not see this disease.

For more information on this or other horticulture related topics, contact Hunter McBrayer, Plant Diagnostic Lab Technician, of the Alabama Cooperative Extension System, C. Beaty Hanna Horticulture & Environmental Center, which is based at the Birmingham Botanical Gardens.

This column includes research-based information from land-grant universities around the country, including Alabama A&M University and Auburn University. Email questions to Hunter McBrayer at rhm0015@auburn.edu or call 205-879-6964, Ext. 19.

Learn more about what is going on at the St. Clair County Extension office by visiting the ACES website, aces.edu/StClair.

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