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Why Are My Oak Trees Dying?

Submitted by Kris Randal, Master Gardener Coordinator —

CENTRAL SIERRA — The Mariposa County UC Cooperative Extension Master Gardeners’ Helpline Desk has been busy fielding inquiries about dead and dying oaks.

Many concerned landowners have asked about, or have been told that their trees have been infected with Sudden Oak Death (SOD).

Phytophthora ramorum, or SOD, is a pathogen that thrives in moist, foggy, warm coastal areas and affects coast live oak, tanoak, Shreve oak and black oak trees. Oaks of the white oak group, such as valley oak, blue oak and Oregon white oak are evidently resistant to this pathogen.

Dead Oak tree - photo courtesy Mariposa Master Gardeners

Dead Oak tree – photo courtesy Mariposa Master Gardeners

The hot, dry climate in the interior foothills of the Sierra Nevada is not conducive to supporting the SOD pathogen. The oaks that are dead or dying are a result of more than four years of drought.

Drought-stressed trees don’t typically die immediately, but instead become weakened and vulnerable to insects, fungi and diseases that hasten their death. However, many trees like our blue oaks shut down photosynthesis during the summer, causing the leaves to turn brown.

While the trees may look dead, they are simply preserving moisture through this strategy. The rule of thumb is to wait until the following spring to see whether the oaks leaf out. If not, the tree has probably died, but SOD is not the cause.

The Mariposa County Master Gardener Helpline Desk is open to the public Tuesdays from 9 a.m. – noon and Thursdays, 2 – 5 p.m. at the Ag. Building, 5009 Fairgrounds Road. You may also call (209) 966-7078 or email questions or photos to mgmariposa@ucdavis.edu

Kris Randal is a Master Gardener Coordinator

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