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Armillaria root rot

Michelle Grabowski and Cynthia Ash Kanner

Back to Diseases of spruce trees in Minnesota

trunk with sections rotted out at base

Photo by J.O'Brien USDA FS

tree trunk with lines of black rot

Photo by M.Grabowski

mushrooms at tree base

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.



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.


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