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United States Department of Agriculture

Agricultural Research Service

Title: Armillaria Root Rot

Author
item Baumgartner, Kendra

Submitted to: Pacific Northwest Plant Disease Control Handbook
Publication Type: Book / Chapter
Publication Acceptance Date: November 28, 2003
Publication Date: March 1, 2004
Citation: Baumgartner, K. 2004. Armillaria root rot. Pacific Northwest Plant Disease Control Handbook.

Interpretive Summary: Armillaria mellea, the causal agent of Armillaria root rot, infects woody roots, killing the cambium and decaying the underlying root wood. This fungal root pathogen is native to California and Oregon where it occurs on the roots of forest tree species such as Douglas-fir and oak. It also attacks planted hosts, such as grapevines, orchard trees, and street trees. The fungus spreads vegetatively, belowground, which leads to the formation of groups of dead and dying hosts known as "disease centers". Armillaria mellea can survive as a saprobe on woody host roots long after the host dies. Its vegetative fungal tissue (mycelium) decomposes the main components of wood for nutrients as it grows, thereby decaying the root wood. When forest trees with Armillaria root rot are cut down, any infected roots that remain belowground serve as a source of inoculum for planted hosts. Infection occurs when host roots grow into direct contact with partially decayed tree roots and are colonized by A. mellea mycelium. Infection can also occur when host roots contact A. mellea rhizomorphs, which are black, root-like structures that grow out from partially decayed tree roots and through the soil. The best way to control Armillaria root rot is to remove partially decayed tree roots from the soil before planting and/or fumigating the soil to the maximum depth possible. To control Armillaria root rot on a moderately symptomatic host, permanent removal of the soil from the root crown may prevent the fungus from girdling the trunk, thereby prolonging the host's life. Severely symptomatic hosts should be removed and replanted after soil is cleared of infected roots.

Technical Abstract: Armillaria mellea, the causal agent of Armillaria root rot, infects woody roots, killing the cambium and decaying the underlying xylem. This root pathogen is native to California and Oregon where it occurs on the roots of forest tree species such as Douglas-fir and oak. It also attacks planted hosts, such as grapevines, orchard trees and landscape trees. The fungus spreads vegetatively, belowground, which leads to the formation of groups of dead and dying hosts known as "disease centers". Armillaria mellea can survive as a saprobe on woody host roots long after the host dies. Its mycelium decomposes cellulose, hemicellulose, and lignin for nutrients as it grows, thereby decaying the root wood. When forest trees with Armillaria root rot are cut down, any infected roots that remain belowground serve as a source of inoculum for planted hosts. Infection occurs when host roots come in direct contact with partially decayed tree roots and are colonized by A. mellea mycelium. Infection can also occur when host roots contact A. mellea rhizomorphs. The best way to control Armillaria root rot is to remove partially decayed tree roots from the soil before planting and/or fumigating the soil to the maximum depth possible. For postinfection control on moderately symptomatic hosts, root collar excavation may prevent mycelial fans from further root collar decay, thereby prolonging the life of infected plants. Severely symptomatic hosts should be removed and replanted after soil is thoroughly cleared of infected roots.

Last Modified: 10/23/2014
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