Armillaria root rotMichelle Grabowski - University of Minnesota Extension Educator
Cynthia Ash Kanner - former University of Minnesota Extension Specialist
Photo by J.O'Brien USDA FS
Photo by M.Grabowski
Photo by M.Grabowski
This fungal root rot is caused by several different species of Armillaria.
Armillaria species have a very broad host range and can infect many deciduous and evergreen trees and shrubs. In addition to living as a pathogen in live plant tissue, Armillaria can also survive as a saprophyte on dead wood.
- Infected trees have poor growth, dead branches in the upper canopy, browning needles, may produce an abundant crop of cones, and eventually die.
- Clusters of honey-colored mushrooms may grow at the base of the tree in fall.
- Flat white sheets of fungal mycelia (mycelia fans) grow between the bark and sapwood at the base of infected trees.
- Thick black, shoestring-like fungal strands sometimes grow in a net on infected trees and in the soil around the base of the tree.
- The base of the tree just below the soil surface may be encrusted in resin.
- Wood is decayed, white, soft and stringy and may extend from the base of the tree well up into the trunk. Trees frequently break or fall over in storms.
Armillaria can survive many years in colonized wood debris like an old stump or root system. New infections occur when healthy roots grow close to diseased roots, or when the black shoestring-like fungal rhizomorphs grow through the soil to encounter healthy roots. Rhizomorphs can grow 10 feet from an infected tree or stump. Once inside the roots, the fungus colonizes the roots and the base of the trunk, resulting in decay. A vigorous tree may be able to isolate the fungus and slow its growth, but stressed trees frequently succumb to the disease quite quickly. Trees die of Armillaria root rot when the infection girdles the base of the trunk, when the trees lodge due to loss of roots, or when the weakened trunks break.
- Reduce stress on trees by mulching the soil around the base of the tree, providing supplement water during drought and not wounding trees.
- Have infected trees assessed by a certified arborist to determine their structural stability.
- Remove unstable trees to prevent damage that might occur if the tree were to fall.
- Remove the stumps and as many roots as possible of infected trees.
- Do not plant spruce trees where oak trees or other hardwoods have recently been removed due to the fact that the Armillaria is likely to be present in these areas.