Archive for Sunday, August 19, 2001

Wilt withers native oaks

August 19, 2001


Some of the most treasured trees in the home landscape are majestic oaks. Few problems plague these notoriously slow-growing natives, which can live for many generations.

However, a deadly disease fungal disease oak wilt has many homeowners concerned that their oak tree will soon be under attack.

Oak wilt primarily attacks red oaks, blackjack oaks and pin oaks. However, it can also cause serious damage to shingle oak, post oak, burr oak and white oak trees. Basically, almost all varieties of oak trees can fall victim to this deadly invader.

The first line of defense is to learn more about oak wilt. It's a systemic disease that attacks the tree from the inside out. Fungal mats grow just beneath the bark, interrupting the flow of water between roots and leaves.

Symptoms of the disease are easily confused with other similar problems such as a natural gas leak, Ganoderma root rot, Hypoxylon canker and drought stress.

Oak wilt symptoms usually start in mid-May to early June and continue throughout the summer. Affected trees initially show a bronzing or wilting of leaves on an individual branch or in a portion of the tree crown. Leaves exhibit a "half-leaf" symptom where the outer half scorches and turns red or brown, while the half of the leaf nearest the stem remains green.

The fungus will sometimes cause brown streaks in the sapwood, but this symptom is difficult to find in small-diameter branches and is not reliable as a diagnostic technique.

Within a few weeks, the wilted leaves begin to drop. Wilting spreads throughout the tree canopy during the summer. Most oaks are killed in one season.

Oak wilt is spread by insects and underground root graphs. Insects are drawn to the sweet fungal mats when they bloom in the spring. The insects then carry the disease to surrounding trees.

The disease spreads quickly from root to root. As a result, oak wilt is most often found in native stands of trees that share a root zone.

It's not likely to be found in urban landscapes where pin oaks or other oak species were transplanted. Without the root graphs, the chance of spread to your tree is highly unlikely.

Control of oak wilt is similar to control of Dutch elm disease. Diseased oaks cannot be saved and should be removed and destroyed.

Wood from diseased trees should not be saved for firewood. It should be shredded and then composted, burned or buried to prevent spread of the disease.

To prevent root graft transmission of the fungus, dig a 3-foot-deep trench all the way around the infected trees. To help prevent aerial spread of the disease, avoid pruning branches from April through June. The flowing sap on pruned trees can attract fungal-carrying insects.

Bruce Chladny is horticulture agent at K-State Research and Extension-Douglas County. For more information, call him at 843-7058 from 8 a.m. to 5 p.m. weekdays.

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