Armillaria

    Armillaria Root Rot

    Armillaria mellea

    armillaria

    Mycelial fans of Armillaria (William Jacobi, Colorado State University, Bugwood.org)

    Armillaria

    Armillaria rhizomorphs (Minnesota Department of Natural Resources, Bugwood.org)

    Armillaria

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

    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.