Ammillaria root disease is not just a disease that attacks oak trees; it is a disease that affects hundreds of species of trees and shrubs. Every state in the continental U.S. has reported the presence of armillaria root disease. It is a fungal disease caused by the fungus Armillaria mellea and several related fungal species.
Natural Component of the Forest
The fungus Armillaria mellea is a natural component of the forest, and it is found on the roots as well as the lower stems of many broad-leaved trees. Conifers are also a host tree to this fungus. This fungus is a parasite that lives on the tissue of the living trees. The fungus also lives off of dead wood, in which case it is called a saprophyte as opposed to a parasite.
Symptoms of the Disease
The symptoms of armillaria rot appear above and below ground level. Above ground level the leaves will appear yellow and wilted; eventually they will die. Below ground level as well as at ground level there will be fungal growth that exists between the bark and the wood of the oak tree. Also, rhizomorphs or black strands, which appear to be fine roots will be seen throughout the surrounding soil. Mushrooms might also appear at the base of the diseased tree.
Chemical fumigants such as chloropicrin, methyl bromide and carbon disulfide can be applied to reduce the infection of the disease. An arborist will apply the fumigant in and around the base of the infected tree. Removal of a 3-foot radius of soil around the trunk reduces the favorable conditions of the disease.
One way to reduce the potential for armillaria rot on oak trees is to water deeply only when watering is required. Also, do not surface water, and avoid getting the crown and trunk root area wet.
Maintaining a Healthy Tree
Healthy trees are able to resist disease, so gardeners should keep their oak trees healthy through deep watering, following correct pruning procedures and planting in well-drained soil.
Tree Removal / Replacement
Removal of trees infected with armillaria rot requires that the root system be thoroughly dug up. (Even after the tree is removed, the fungus remains active on any remaining roots.) Replace the removed tree with a species that is resistant to this disease.