You probably do not need to spray your oak with an insecticide unless you are trying to protect it from the Gold Spotted Oak Borer (GSOB). This pest has killed many native oaks including coast live oak, canyon live oak and black oak trees in the mountains east of San Diego. There is more information on GSOB at www.gsob.org .
Most insect and mite pests associated with oak do not kill healthy trees. Beneficial predators including spiders and some parasites keep most oak pests under biological control. Spraying trees with an insecticide can harm these natural enemies.
It is not unusual for an oak to have some brown leaves. Old leaves naturally turn brown when they die. Young leaves that have been damaged by powdery mildew may also turn brown. More foliage on an oak may turn brown if a tree’s root system has been damaged by disease, excess soil moisture, construction activity and other causes.
Native oaks are adapted to California's climate, which has wet winters and dry summers. Oaks are less susceptible to root diseases like oak root fungus during the summer when the soil becomes drier. Keeping soil moist during warm weather favors soil fungal diseases which can kill oaks. Established oaks should not be irrigated at all from spring through fall.
Trees can be given supplemental irrigation during late winter if there has not been enough rainfall to wet the soil to a depth of at least 3 feet. Do not apply water near the tree's trunk since this can cause crown rot. Irrigate a broad band of soil around the tree at the edge of the branches and extending out 10 feet or more. Apply water slowly until it penetrates to a depth of about 3 feet. Do not give oaks any nitrogen fertilizer since this will stimulate lush growth that is subject to damage by powdery mildew. Excessive vegetative growth will also increase the tree's water requirement during the dry season.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension,