Just like blood oozing from a human indicates an injury, sap seeping from a tree is a sign of injury, pest infestation or disease. Specimens that are injured and develop cuts in their bark are susceptible to bacterial infections that squeeze the sap from a tree. Woodboring insects also may be responsible.
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Slime flux, also known as wetwood, is a bacterial condition that literally forces sap to ooze from a tree and down the bark. This condition afflicts trees that are still growing and have not yet reached full maturity. The bacteria typically slip into a tree via wounds caused by pruning damage or freeze injury. Over time, gasses produced by the fermenting bacteria raise the internal pressure of sap in the tree, forcing it to ooze from the specimen.
Slime Flux Features
As sap runs down the bark, it causes streaks on the wood that are light gray or white when dry. The bacteria may turn the sap frothy at the point that it slips from a crack in the tree, with the slime flux fermenting and emitting a foul odor. No means exist to prevent the disease. Although previous techniques involved boring holes in the trunk to relieve pressure, this solution is now discouraged, as it actually produces more injury to the tree.
Elm trees are extremely likely to develop slime flux and, to a lesser extent, the disease is prevalent in oak, maple, redbud, sycamore, walnut, mulberry, butternut and some birches. The likelihood of a tree contracting this condition can be minimized by being especially careful to avoid wounding the tree during pruning and other maintenance. Flower beds should not be planted around the base of a tree, as wounds to the root system may develop, providing another spot for bacteria to enter.
Borers and Canker
A wide variety of borer species inhabit trees, gaining their name from the insect's habit of "boring" into the wood of a tree, creating tunnels to feed, live and lay eggs in. Borers are the immature larvae of a number of insect species, and their activity typically causes sap to ooze from an infested tree. In addition, a multitude of fungal pathogens result in what are known as canker diseases. These illnesses affect the wood of a tree and force sap to prematurely seep from the bark.