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Hort in the Home Landscape

A blog devoted to sharing timely horticulture topics and answering the questions of gardeners and homeowners.
2015-10-05 15 35 09

Odd Oak Galls

Posted by Candice Hart - Trees

It seems to be the season for odd oak galls. I've had several questions come into the office and while prepping for a fall tree walk this past week, I discovered many oak galls.

Galls can be defined as irregular plant growths which are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects or mites. Most are caused by tiny cynipid wasps. The wasps secrete chemicals that cause the plant to create the gall as a home for the developing larvae, which grow tucked safe inside the gall structure. The developing insects eat plant tissue on the inside of the gall.

Galls may occur on leaves, bark, flowers, buds, or roots of many species, but galls are a pretty common occurrence on oaks in particular.

I find galls to be really fascinating. Just the idea of a plant growing this tumorous growth in response to a tiny insect is so cool. Fortunately, many galls are completely harmless to the tree, but a few can actually cause some serious issues. In Illinois, both the horned and gouty oak galls can be debilitating, even killing younger trees. There is some evidence that larger trees can also be weakened and die because of excessive gall development.

The galls that I have encountered this fall have not been horned or gouty oak galls.

1. The first gall I encountered was a fuzzy oak gall. A client brought this one into the office wondering what the fuzzy growths on his oak leaves were. No concern here as these were only covering a small portion of his leaves.

2. Oak Bullet Gall: This gall I found growing on Bur Oak in a park. Like the other galls, oak bullet gall is caused by wasps. They are lobular, hard single or clustered twig galls, 8 to 16 mm in diameter. This tree actually had quite a bit of coverage with these galls and they are likely causing stress and reducing the growth of this tree. Learn more about bullet gall here.

3. Leaf galls: On another bur oak I encountered another gall, this time red in color growing on the underside of the leaf. This tree had very few of these galls so was causing no real harm to this tree.

Learn more about oak galls in Illinois here.


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