Q. What are the silky spots on the back side of my avocado leaves?
F.B., Arroyo Grande
A. Persea mites. Typically, they leave distinct circular, yellow or brown spots along veins on the undersides of leaves. The spots become visible on the upper leaves as the number of mites grows.
Avocado crown mites and Sixspotted spider mites can also feed on avocado leaves. But they don’t leave spots.
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Avocado brown mites feed on the upper leaf surface of avocado leaves, causing the upper leaf to appear bronzed or scorched. Sixspotted spider mites feed primarily on the underside of leaves, producing brown to purplish irregularly shaped blotches along the veins.
Persea mites are yellow to green with two or more dark spots. Adults have eight legs. They start to appear around March and gradually increase through spring as they feed on new leaf growth. Populations peak in July and August, but are suppressed when humidity is low and the daily high temperature is 100 degrees Fahrenheit or warmer for several consecutive days.
Persea mites are most damaging to Hass, Gwen and a few other avocado varieties. Bacon, Fuerte, Lamb Hass, and Zutano varieties are far less affected.
If there are many persea mites, they can severely stress trees and cause leaves to drop prematurely, resulting in sunburned bark, dropped fruit and smaller yields.
You can ward off persea mites by maintaining healthy trees. Avoid over-watering and over-fertilizing. Avocado trees are susceptible to root rot diseases, which is exacerbated if you fail to water them properly. Excess nitrogen fertilizer will promote leaf growth and attract more mites.
If you detect evidence of too many mites, hose down your tree’s leaves, especially the undersides, to remove mites and destroy nests. If the water doesn’t work, apply insecticidal oil, although this may not be practical on a large tree.
For more information, go to http://ipm.ucanr.edu/PMG/r8400211.html.
DON’T MISS: The 11th annual Tomato Extravaganza and Plant Sale, 10 a.m. to 2 p.m. on Saturday, Aug. 19, 2156 Sierra Way, San Luis Obispo.
Polly Nelson 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 firstname.lastname@example.org. 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.