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Brown balls on oak trees aren’t harmful

Galls are harmless growths.
Galls are harmless growths.

Q: What are the brown balls that look like fruit growing on my oak trees?

Mary B., Atascadero

A: The apple-size structures on the ground under your oak trees are probably oak galls caused by small wasps. These wasps are harmless to pets and humans, and there are a variety of species of wasp species that commonly infest oak trees. Most galls are initiated by insects, but can also be caused by bacteria, fungi or mistletoe.

Different gall wasp species form different types of oak galls on different areas of the tree during different parts of the season. One of the most common in our region is the apple gall wasp (Andricus quercuscalifornicus). These wasps cause galls that start out green on the oak branch, then turn reddish, brown or sometimes black as they increase in size to as large as 4 inches in diameter. Your oak tree may have individual galls or dense clusters.

The life cycle of California gall wasps alternates between sexual and asexual reproduction.

As with other gall wasps, no control measure is necessary or recommended. The galls do not harm the tree or weaken it any way. Gall wasps and the gall itself are important hosts for other predators including parasitic wasps, birds, spiders and lizards. As a result, chemical applications would do more harm than good by disrupting this complex food chain.

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The Master Gardeners held their 10th Annual Tomato Extravaganza and Plant Sale on Aug. 20. We would like to thank everyone who attended and announce the winners of the tomato and basil voting. Top tastes for tomato: Brandywine, followed by Black Cherry and then Yellow Pear. Top tastes for basil: Sweet Thai, with Lime Basil and Queenette Thai as a close second and third. See you next year!

Tami Reece is a UCCE Master Gardener.

Got a gardening question?

In San Luis Obispo call 781-5939, Arroyo Grande, 473-7190 and Templeton, 434-4105. Visit us at http://ucanr.org/sites/mgslo/ or email us at anrmgslo@ucanr.edu. Follow us on Instagram at slo_mgs and like us on Facebook. Informative garden workshops are held the third Saturday of every month, 10 a.m. to noon at 2156 Sierra Way, San Luis Obispo. Garden docents are available after the workshop until 1 p.m. To request a tour of the garden, call 781-5939.