Oak wilt can kill even the mightiest tree
By DAVE CAMPANA
OSU Extension master gardener volunteer
The mighty oak, our nation’s national tree, can live for centuries and grow to more than 80 feet tall.
And yet this symbol of strength can be undone by the smallest of organisms. Oak wilt (Bretziella fagacearum) is a serious fungal disease that grows throughout the sap wood.
The combined actions of the fungus clogging the tree’s water conductive tissues and the tree’s own defensive response interfere with water uptake causing an often quick and deadly wilting syndrome.
Those in the red-black group (black, blackjack, pin, northern and southern red, scarlet, shingle and shumard oaks) are extremely susceptible and can die within a few weeks of infection, according to our OSU Extension Factsheet.
The white group (bur, chinquapin, post, swamp white and white oaks) are more tolerant and may survive infection for a few years.
The first signs a homeowner should look for will probably be the whole upper branches turning red-brown with red group leaves typically showing yellowing and browning at the leaf margins.
The entire leaf eventually turns brown and falls from the tree. The leaves of white oaks usually lack such indications. If the infection occurs in the late spring, wilting will likely occur in mid to late summer when trees need hydration most.
Red oaks will develop spore-bearing fungal mats under desiccating bark. These mats will crack the bark open to enable the fungus to spread.
Streaking sapwood is also an indication of the disease but, in all cases, a conclusive diagnosis can only be made in specialized labs. This is because factors and diseases like construction damage, insect attack, anthracnose and wood decays may display the same symptoms. Specific diagnosis requires quarter-inch size twigs delivered to the diagnostic lab in Columbus.
Insects spread the disease by feeding on the sap or fungal mat of an infected tree and, attracted by the smell of sap, fly sometimes miles to a newly pruned or damaged but otherwise healthy tree. It is therefore vitally important that oak trees never be pruned when sap is flowing, usually between April 15 and July 1 – but it is even better to not prune from April through October.
It is estimated that 90 percent of new infections occur from neighboring trees through root grafts. Oaks should never be planted within 50 feet of one another.
Infected trees should be cut down to a low stump and the wood removed from the area according to local and state regulations.
To learn more about this disease, go to http://go.osu.edu/oakwilt.