The Norman Transcript

March 22, 2013

Oak disease is a growing problem


The Norman Transcript

NORMAN — Losing a tree can be stressful for homeowners, financially and emotionally.

For all the years it takes a tree to mature, it is devastating for one to die rapidly because of disease. This is true with hypoxylon (Biscogniauxia) canker in oaks. Hypoxylon canker has been in Oklahoma a long time. However, significant reports didn’t occur until 1979 when outbreaks were documented in 14 counties.

Hypoxylon canker is a fungal infection occurring on various oak species, including blackjack, live, post, southern red and white oaks. The fungus is found in wooded areas, pastures, new construction areas and established residential areas. I’ve seen the most devastating outbreaks in the cross-timber areas of eastern Cleveland County, where acres of trees have been lost.

Hypoxylon canker is exacerbated by environmental stressors such as drought, heat, physical injury to the tree or roots and chemical drift. Thus, healthy trees are more resistant to the disease.

Like most fungi, hypoxylon canker is spread from one tree to the next by wind, rain, tools and insects. Research shows the fungus enters the tree through wounds, growing through the wounds and sapwood and causing decay. Hypoxylon canker may be dormant in seedlings and young trees that show no outward symptoms.

However, due to environmental stresses, the fungus can overcome the tree’s resistance and begin causing damage.

Symptoms of hypoxylon canker include yellow, wilted leaves followed by death of entire branches. As the disease progresses and branches die, bark is lost exposing the fungal growth or stroma. The stroma appears as sunken areas that are brown, silver, black, dark gray or white, depending on the life stage of the fungus. As the fungus matures, the stroma hardens.

The exposed lesions or cankers may be seen on upper branches before progressing to the trunk. In one year, the canker may not spread at all, up to 3 feet on a single branch, or the entire length of the tree. Death of the tree may take one or two years but may seem like weeks if early symptoms go unnoticed. There are no effective means of control for hypoxylon canker.

Infected trees should be removed to prevent secondary infections on other trees. The homeowner should completely remove trees and grind the stumps when greater than 15 percent of the tree canopy is dead. Remaining branches and stumps can harbor the fungi, causing a source of reinfection. Trees with less damage should be kept as healthy as possible; deep watering at the drip line during drought and applying fertilizer in early spring.

Also, prevent injury to trunks and roots from weed eaters, lawn mowers and tractors. If you plan to use infected trees as firewood, be aware that hypoxylon canker fungus is still active on dead wood and should be burned as soon as possible.

In other southern states, additional species of hypoxylon canker fungi affect pecan, hickory, America sycamore and London plane trees. Although there are reports of hypoxylon canker on pecan trees in Oklahoma, hypoxylon canker of oak is the most devastating.

If you have additional questions about diseases in the lawn or garden, contact a Cleveland County Master Gardener at 321-4774 or ccmastergardener@yahoo.

com.

Tracey Payton Miller is Cleveland County’s horticulture extension educator for the Oklahoma Cooperative Extension.

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