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THREE PRIMARY CAUSES FOR DEMISE OF MATURE OAK TREES
It's sad to watch a 100-year-old oak tree die. I know, because I have several in my yard dying a slow death, and I've talked with many other folks in recent years about their mature oaks that appear to be dying. Seems like everyone's looking for a "miracle cure" that can save his or her stately trees.
There are three main culprits in the demise of mature oaks: Oak wilt disease, two-lined chestnut borer, and construction damage.
The oaks in my yard and surrounding area are bur oaks - a distinct species within the white oak family characterized by deeply notched, rounded leaves. Based on research and tissue samples I've evaluated, I think the two-lined chestnut borer is the cause of my trees' decline.
Two-lined chestnut borers are metallic beetles that attack stressed trees. The worm-like larvae of the two-lined chestnut borer feed under the bark at the treetop and at the ends of branches. Their feeding destroys the nutrient and water-conducting tissues. This girdles and kills the area above and beyond where the larvae feed. This is why the first disease symptoms are yellow leaves and bare branches at the top of trees and at the end of branches.
If you cut away the bark of a dying portion of a tree infected with two-lined chestnut borers, you should be able to see galleries, or trails, left by the white, segmented worms.
Stressed oaks are more susceptible to two-lined chestnut borer damage than strong, healthy oaks. Many mature oaks have been stressed in recent years by periods of drought, as well as deep-freeze temperatures in early April three out of the last five years that forced trees to produce leaves from secondary buds.
The best defense against two-lined chestnut borers is a strong, healthy tree. Unfortunately, while it's relatively easy to water and fertilize young oaks, watering and fertilizing giant old oaks is another story.
Other than watering stressed trees, there is no prevention or treatment for two-lined chestnut borer disease in mature oaks. Woodpiles from infected trees should be covered with plastic tarp from May to July.
Oak wilt, which infects red and black oaks much more frequently than white, bur or swamp white oaks, is a fungal disease that clogs the water conducting vessels of infected trees, causing them to wilt and die. Oak wilt is characterized by a sudden wilting of leaves at the top of the tree or at the tips of side branches - similar symptoms to two-lined chestnut borer infestations.
Oak wilt moves from infected trees to healthy trees in two ways. The first is through natural root grafts between trees. Root grafts serve as pipelines for the oak wilt fungus to travel internally from tree to tree, the same way Dutch elm disease spreads between American elm.
If you cut through the layers of bark of an oak suffering from oak wilt, you should be able to see a dark discoloration of the sapwood just beneath the bark.
The other way oak wilt spreads is from tree to tree via an insect called picnic beetle. These small black beetles are attracted to sap leaking from fresh wounds on trunks and branches, especially during spring and early summer. Through its feeding activity, the picnic beetle transmits the oak wilt fungus from tree to tree. This is why no pruning of oaks is recommended between April and August.
If ever there was an open invitation to picnic beetles to spread fungus to fresh wounds in oaks, it was the three or four windstorms in May and June. Countless oaks suffered injury during these storms and only time will tell whether the beetles used these opportunities to spread disease.
The good news about construction damage - the third enemy of mature oaks - is that it can usually be prevented. Mature oaks suffer from severed roots caused by trenching, compacted soil caused by heavy construction equipment, and changes in soil grade at the base of trees.
If you're considering building a home among mature oaks, I strongly recommend you and your building contractor fully understand these causes of construction damage and agree on a building strategy that will protect trees at your building site. If you would like a brochure on preventing construction damage to mature trees, contact me through the newspaper office.
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