Oak wilt is a fungus that inhabits the water conducting vessels of all species of oaks.  The fungus disrupts water movement to the crown and the leaves eventually wilt and fall off.   Spread of the disease is typically through root grafts of infected trees to uninfected trees.  However, beetles carrying the fungus spores can cause an infection as well.

SYMPTOMS

The leaves of Red Oaks will transition from dull green to bronze and finally tan. This process begins on the outer portion of the leaf and works towards the mid-vein of the leaf. Wilting in Red Oaks, the most obvious symptom, generally starts near the top of the tree and then rapidly involves the entire crown. The tree wilts completely within a few weeks after the first symptoms appear. Brown to black discoloration is present in the outer sap wood where the fungus has induced the tree to produce tylosis and gums which account for this discoloration.

The leaves of White Oaks and Bur Oaks turn brown starting at the leaf tip and moving to the base of the leaf. Leaf decline from oak wilt in the white oak group of trees can resemble normal fall coloration. Oak wilt infection generally occurs in random branches throughout the canopy of the tree. These trees may die in one year but more often die slowly over a period of several years. Bur Oaks are intermediate in susceptibility and may be killed as quickly as Red Oaks or as slowly as White Oaks.

Field identification of oak wilt is difficult. An arborist can conclude a tree is likely to be infected with oak wilt based on the symptoms presented by the tree, but positive identification of oak wilt often requires lab testing.

CONTROL

Pruning and trimming of Oak trees should only be done in the dormant month’s to prevent the possibility of infection through beetles carrying the fungus spores. If oaks are damaged in the spring by storm or construction, the wounds should be covered immediately with a tree wound dressing. Based on a large number of trees wounded throughout the growing season, infection from insect-carried spores occurred only in trees wounded in May and June.

Once oak trees are infected, there is no known way of saving them. Control must first be aimed at halting the spread of the fungus through the root grafts to other trees. A trench between diseased and healthy trees immediately disrupts root grafts between these adjacent trees. 

Since the fungus spores can live on a dead tree for quite some time it is best to remove the dying and dead diseased Oak trees to help prevent the possibility of the spread of the disease to healthy trees via bark beetles.

CHEMICAL CONTROL

A fungicide which is macro infused into the vascular tissue of the healthy tree is very effective for Oak wilt control by protecting the water conducting tissue where the fungus grows. The fungicide is injected into the tree near the base of the root flare. The tree then distributes the chemical throughout the whole tree evenly. This application is done every two years.

Photos courtesy of www.forestryimages.org