From: Gary W. Hickman, Horticulture Advisor
University of California Cooperative Extension
August 29, 2001
LIVE OAKS LOSING LEAVES
BACKYARD HORTICULTURE
By Gary W. Hickman, Horticulture Advisor
University of California Cooperative Extension, Mariposa County
Oaks in many parts of Mariposa County are dropping leaves, especially the live oaks. Concerned callers to our office are wondering about possible diseases, especially the Sudden Oak Death (SOD) fungus.
Fortunately, SOD has not been found in our county. It is present in the coastal counties from Monterey to Napa. Also, it occurs on coast live oak, but has not been found on our interior live oak species.
The oak leaf fall here is due to environmental conditions, notably very dry soils. Coupled with hot, drying winds, the otherwise evergreen live oaks are showing drought stress symptoms of browning and shedding of leaves. This is actually a fairly common occurrence. Many evergreen trees do not have constant leaf renewal patterns, but in fact, shed and replace leaves only periodically. If the trees are under drought stress, the leaf drop may be much larger than normal. Upcoming winter rains will determine longer-term effects on the trees. Precipitation in usual amounts will return our trees to their stately appearance.
While we generally do not recommend summer irrigation of oaks, periods of drought can be an exception. However, it is very important that any irrigation to oaks be made well away from the trunk to avoid crown rotting fungal diseases. Since most of the water absorbing roots are located starting at the dripline (outer edge of branches) and going out, this is where the water should be applied.
If you would like more information on the topic of caring for oaks, call or write our office for the leaflet “Living Among the Oaks,” U.C. Cooperative Extension, 5009 Fairgrounds Rd., Mariposa 95338, phone: 966-2417,
e-mail: cemariposa@ucdavis.edu.
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