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Oak ailments: prevention and treatment.

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.

Bacillus thuringiensis is an effective control if applied to coincide with the appearance of young larvae.

Oak or crown whitefly (Aleuroplatus coronatus). Dense colonies cluster on leaves of coffeeberry, manzanita, and toyon as well as on many kinds of oak throughout the West. Adults are snowy white, but larvae and nymphs do the damage. They suck the tree's sap and excrete honeydew, which serves as a growth medium for sooty black mold. With a magnifying glass, you can see their most distinctive feature: a dark pupa case surrounded by flat, waxy white bands in a shape resembling a crown.

Often, no countermeasure is required. In severe cases or for high-value trees, have the tree sprayed with horticultural oil.

Pit scale (Asterolecanium). Often associated with Cryptocline twig blight, pit scale attacks oak twigs. Pinhead-size insects are green, golden, or brown; they live, immobile, in a depression in the bark. By sucking the sap, they reduce an oak tree's vigor.

Microscopic young scale crawlers arrive in spring and early summer; they don't move far, and they colonize current growth and one-year-old wood.

Poor growth and dieback of twigs are common results of infestation. Dieback is most noticeable during the summer and early fall. Affected twigs of deciduous oaks retain their dead leaves throughout winter. Severe infestation delays leafing out of deciduous oaks in spring. Control mature pit scale with sprays of horticultural oil in January.

Any healthy oak can handle these

Gall wasps (Cynipid wasps). Galls on oaks are common on twigs, leaves, flowers, and roots, or in acorns; they assume different sizes, shapes, and colors-from rosy, pea-size blobs to apple-size goiters, depending on the insect that makes them. Rarely a serious menace, the insects puncture leaves to deposit eggs, causing leaves to turn brown.

Acorn woodpecker (Melanerpes formicivorus). The bird drills acorn-size holes in oak bark to store its winter acorn supply. Each woodpecker drills only a few holes per year; a large valley oak with thousands of holes in its trunk reflects several generations of birds.

Holes don't harm the tree; the bark is thick, and the holes don't penetrate the cambium layer underneath.

Keeping native oaks healthy

The best way for you to protect your oaks is to keep them in good health.

Mulch from the trunk to beyond the edge of the canopy; fallen leaves, left to decompose naturally, are best. For a neater look, try a layer of ground bark or rock.

Avoid unnecessary watering and don't water within the tree's crown area in summer. During drought years, one deep watering in January or February should be enough.

Feed only as needed. Yellowish leaves and lack of new growth in spring may indicate a need for nitrogen. In early winter, spread a 10-10-10 fertilizer (about 20 pounds per 1,000 square feet of soil surface) to slightly beyond the drip line.

Prune only to remove weak or dead branches or to limit pest damage (or also, occasionally, to thin the canopy of a mature tree to reduce wind resistance). For oaks, as for other large trees, have an experienced arborist handle major pruning (and spraying).

Avoid overthinning or stubbing branches. Stubbed branches respond with vigorous watersprout growth that is susceptible to mildew. Large pruning wounds leave the tree more subject to decay.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Publication:Sunset
Date:Oct 1, 1990
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