Magazine article Sunset

Oak Ailments: Prevention and Treatment

Magazine article Sunset

Oak Ailments: Prevention and Treatment

Article excerpt

As oak troubles go, some are more serious than others. Here we discuss the nine oak ailments you're most likely to encounter, and what you can do about them. In most cases, the best cure is prevention. For tips on keeping an oak healthy and vigorous, see opposite page.

The four worst

Any of these four can disfigure and ultimately even kill an oak.

Crown rot (Phytophthora, Pythium). Too much moisture around the base of the tree stimulates this water-mold fungus, which attacks roots. Infected trees decline slowly; foliage becomes increasingly sparse. Live oaks are most susceptible.

Don't underplant native oaks with plants that require summer water, and don't raise soil level around oak crowns; both practices encourage crown rot.

If symptoms indicate crown rot, stop all watering between trunk and drip line. Pull back soil to expose infected parts to air.

Oak root fungus Armillaria mellea). Infected trees die back slowly, and leaves can hang on and stay green for months; the tree's wood takes on a mushroom odor. If you suspect oak root fungus, peel back the bark at soil level; white or cream-colored fungus tissue underneath signals infection.

Uncover the infected root crown and leave it exposed to air, and stop watering within the drip line. In advanced cases, consult an arborist about removing the tree.

Dieback. Three kinds of fungus cause dieback: Diplodia affects branches, Cryptocline and Discula affect twigs and leaves.

Diplodia is encouraged by dry, hot summers. Cryptocline and Discula love wet weather; in wet years, they can destroy 90 percent of new growth.

Oak twig dieback occurs on coast live, valley, interior live, and sometimes blue oaks. Infection pattern among nearby trees is unpredictable; trees untouched by the fungus may grow right beside ones that are severely afflicted.

Pruning out dead twigs minimizes damage but can't eliminate it. Spray in March with the fungicide benomyl.

Pacific oak twig girdler (Agrilus angelicus). Especially troublesome in Southern California among coast live oaks, this insect attacks twigs 1/2 inch or less in diameter, causing patches of dead foliage throughout the tree's canopy.

The adult is a brownish bronze beetle, about 1/4 inch long. The female lays single eggs on twigs of a tree's most recent growth; eggs hatch two or three weeks later, and larvae-white and legless, with clear constrictions between body segments-bore directly into the twig and tunnel just beneath the bark. After two years, they pupate, then emerge (usually in late June and early July in Southern California).

Diagnose by pruning away several dead twigs and removing several inches of bark where living tissue turns to dead. If the girdler is the cause, you'll find a gallery of tunnels filled with brown frass, and possibly a larva. Prune out infested twigs.

Three borderline troubles

In general, the following afflictions are little more than nuisances. Coupled with other stresses, though, they could develop into serious problems.

Oak moth, California oak worm (Phryganidia californica). Adult moths lay eggs in live oaks twice a year, in spring and fall (three times if weather is mild). Young larvae feed on the surface of leaves (injured leaves turn brown) till about half-grown, then eat through the entire leaf. Severe infestations can almost defoliate oaks.

In spring, look for little green droppings from feeding larvae; May through June and again in September and October, look for fluttering moths. …

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