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When oak trees are oozing what looks like sap, it’s possible that they have “wetwood” or slime flux disease, according to UC master gardeners.

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Garden Detective: Oozing oak may have ‘wetwood’

Published: Saturday, Jan. 18, 2014 - 12:00 am

Some of my mature oak trees have been really oozing and dripping a clear sap-like substance from their branches for about a month. What causes this, and can anything be done to lessen or prevent this?

–Sherry Larsen, Newcastle

One possibility for your symptoms is “wetwood” or slime flux disease, according to UC master gardeners. This condition is caused by several kinds of bacteria and yeast organisms. The condition rarely causes serious harm to the trees. The nickname “wetwood” comes from the darker hue of the affected wood; it looks wet.

Although unsightly, affected wood may be as strong as healthy wood. Prevent infestations by avoiding injuries to bark or wood.

To reduce the spread of bacteria and yeasts in an infected tree, drill a quarter-inch hole several inches long until fluid begins flowing, then install a copper tube to drain excess fluid and release the pressure of gases that form in infected wood. The tube should extend well beyond the trunk so that fluids fall on the ground, not on the tree. Do not weaken the tree’s structure by drilling drain holes at branch crotches.

Management techniques may be best handled by a certified arborist. For small infections, open wounds so they are exposed to air. Large openings should be avoided.

I have two ash trees that are side by side; both were pruned last year. This year, one had green leaves all summer; the other tree’s leaves turned brown in early summer, while a few stayed green. Could this be the result of the earlier pruning? Both trees get the same amount of water and sun. Why did the leaves turn brown on one tree?

–Lois Garcia, Roseville

According to UC master gardeners, the browned ash tree may have verticillium wilt, a fungal disease that affects the plant’s vascular system. It causes the foliage to turn faded green, yellow or brown, and sometimes wilt in scattered portions of the canopy or on scattered branches. Shoots and branches wilt and die, often beginning on one side of the plant, and occasionally entire plants die. Peeling back the bark on newly infected branches may reveal dark stains following the grain on infected wood.

Most likely, pruning the tree did not cause the brown leaves. Keep the trees vigorous by promoting new growth, with proper fertilization and irrigation. Prune out any dead wood. Regularly inspect for possible hazards; affected trees may need to be removed. If you select to remove the trees and verticillium is present in the soil, plant only resistant varieties.

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