Armillaria root rot
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Armillaria root rot can be caused by several species of Armillaria, and for diagnosis and management, knowing the species is not that critical. This fungal pathogen attacks several hundred woody species, as well as some herbaceous plants. It is particularly common to forested areas or areas that had been forested. Armillaria exists most of its life as a saprophyte, but it can infect trees that are weakened by stresses. Sometimes it can even infect healthy trees, which are killed outright or are weakened, leaving them vulnerable to other pathogens or environmental stresses.
Armillaria produces rhizomorphs, which grow under the bark of dead or dying trees and stumps. In at least one circumstance last year, we were able to trace the rhizomorphs from the dead trees in the plantation to the dead stump of the previous fir crop. Rhizomorphs can grow up to 60 feet advancing less than 10 feet per year. When a rhizomorph comes into contact with a root of a host species, the fungal mycelium adheres to and then penetrates the root by secreting digestive enzymes. The fungus is then able to grow through the root and subsequently up into the trunk. Armillaria can also spread from tree to tree through root grafts.
The rhizomorphs may also give rise to the sexual mushroom stage, particularly after wet periods during the fall. These mushrooms release spores that may infect the trees; however, it is unlikely that these spores play a prominent role in the disease cycle.
Infected trees exhibit symptoms similar to other root diseases. In otherwise healthy trees, crown symptoms may take several years to develop and may be accompanied by branch and shoot dieback and premature foliage loss. However, in weakened trees, foliage may rapidly turn color with tree death occurring soon after.
Because Armillaria is a natural component of the forest ecosystem, it is not possible to try to eradicate the fungus. The best way to control the disease is to prevent trees from becoming stressed. Remove dead trees and the stumps from previous plantings. Avoid planting into cut over sites, especially those with hardwood stumps. In areas where Armillaria has been a problem, do not continue to plant susceptible trees, such as balsam or Fraser fir. Keep trees as healthy and as vigorous as possible.