Armillaria Root Rot

    Armillaria mellea


    Mycelial fans of Armillaria (William Jacobi, Colorado State University,


    Armillaria rhizomorphs (Minnesota Department of Natural Resources,


    Mushrooms at the base of a tree, a sign of Armillaria root rot (USDA Forest Service,

    Hosts, Symptoms & Signs

    • trees
    • aboveground symptoms: over time the foliage turns yellow, thins and branches die back; eventually the tree dies
    • below ground, the roots are rotting leading to the above ground symptoms; infected conifers may exude resin at the base of the tree

    Three signs to look for to diagnose Armillaria:

    • the mycelial fan under the bark that can be seen when the bark of a dead tree is peeled back
    • rhizomorphs on the roots and under the bark (rhizomorphs are thick strands of hyphae that are dark brown; they can look like shoelaces)
    • mushroom clusters at the base of the tree in late summer and fall

    Disease Cycle

    • Armillaria fungus is soilborne
    • colonizes the roots and causes root rot
    • rhizomorphs can grow through the soil from one tree root to neighboring tree roots, infecting neighboring trees
    • once the tree is dead, Armillaria can survive on dead roots and other wood for decades in the soil until a new host plant is found
    • Armillaria can be a problem in areas where woodlands used to be

    IPM Recommendations

    • Keep trees vigorous and avoid excessive moisture.
    • Remove affected trees and remove as much of the roots as possible (roots can extend several feet away from the trunk).
    • If Armillaria has been a problem in the past, plant resistant trees.