Borers are the larvae of beetles or moths. Several kinds of borers attack oaks. Throughout the summer, females lay their eggs in bark crevices. The larvae feed on the bark, sapwood, and heartwood. This stops the flow of nutrients and water in that area by cutting the conducting vessels; branch and twig dieback results. Sap flow acts as a defense against borers if the tree is healthy; when the borer burrows into the wood, tree sap fills the hole and kills the insect. Factors that weaken the tree-such as mechanical injuries, transplanting, damage by leaf-feeding insects, and poor growing conditions-make it more attractive to egg-laying females.
Cut out and destroy all dead and dying branches, and remove severely infected young trees. Maintain plant vigor by watering during periods of drought and fertilizing regularly.
Many different species of caterpillars feed on oak leaves wherever the trees are grown. Depending on the species, the moths lay their eggs from early spring to midsummer. The larvae that hatch from these eggs feed singly or in groups on buds, on one leaf surface (these are called skeletonizers), or on the entire leaf. Certain caterpillars web the leaves together as they feed. In some years, damage is minimal because of unfavorable environmental conditions or control by predators and parasites. When conditions are favorable, however, entire trees may be defoliated by late summer. Defoliation weakens trees because no leaves are left to produce food. When heavy infestations occur several years in a row, branches or entire trees may be killed.
Apply an insecticide labeled for this pest, following label directions.
This plant disease, also called twig blight in California, is caused by a fungus (Apiognomonia quercina) that infects the leaves of white oaks and, in California, live oak. The fungus spends the winter on fallen leaves or in sunken cankers on twigs in the tree. During rainy weather, spores are blown and splashed onto young leaves. Dead spots develop where the fungus enters the tissue. The spots enlarge, and the leaves become puckered and twisted. When moist weather continues into summer, the fungus may kill the leaves, causing severe defoliation. The fungus may also enter twigs, causing canker and twig dieback.
Rake and destroy fallen leaves and twigs. Prune out diseased and dead branches. If the following spring is wet, spray young trees with a fungicide labeled for this disease, following label directions.
Tiny insects, called gall wasps or gallflies, cause hundreds of different types of growths (galls) to develop on oak trees. The galls may be round, spiny, star-shape, flattened, or elongated. Each species of insect (either a fly or a nonstinging wasp) causes its own specific gall to form. The galls are thought to be caused by a chemical that the insect injects into the plant tissue. The mature female lays her eggs on the various parts of the plant. The eggs hatch into legless grubs around which the gall forms. The insects feed and develop inside the galls, mature, and either spend the winter inside them or emerge to produce another generation. Most galls do not harm the tree. Some leaf galls inhibit food production, however, and several types of twig galls disrupt water and nutrient movement through the wood, causing twig dieback.
If galled branches are distorted and unsightly, cut them off and destroy them before adults emerge from the galls in spring.
Leaf blister is caused by a fungus (Taphrina caerulescens) that is unsightly but rarely harms the tree. It is a problem on various species of oak, particularly red, black, scarlet, and live oaks. The fungus spends the winter in bud scales on the tree. During cool, wet spring weather, it enters the leaves as they develop. Green blisters form where the fungus has entered and the infected tissue eventually turns brown and dies. In the fall, the fungus produces overwintering spores. If the following spring is cool and wet, the cycle begins again.
If leaf blister was a problem the previous year and the weather the current spring is cool and wet, apply a fungicide labeled for this disease, following label directions.
Leaf scorch is caused by excessive evaporation of moisture from the leaves in hot weather. If the roots can''t absorb and convey water fast enough to replenish this loss, the leaves turn brown and wither. This usually occurs in dry soil, but occasionally leaves also scorch when the soil is moist. Drying winds, severed roots, limited soil area, or soil with a high salt concentration contribute to scorch.
To prevent further scorch, deep-water trees during periods of hot weather to wet down the entire root space. If practical, apply a 3- to 4-inch-deep mulch over the root system. Water newly transplanted trees in drying conditions. Scorch on trees in moist soil cannot be controlled. Plant trees adapted to your climate.