Forest Services

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Armillaria root rot disease is caused by any of several species of fungi in the genus Armillaria. The genus Armillaria includes “facultatively parasitic” root and butt rot fungi that produce rhizomorphs (Figure 1). A facultative parasite in this case is an organism which survives mostly on dead, woody tissues but can, under some conditions, act as a pathogen and infect living trees. Armillaria root disease infections primarily spread underground as the fungus moves along roots of infected trees to adjacent uninfected hosts. The fungus can also spread in the form of spores produced by the mushroom it produces (Figure 2), although this is thought to be less common. There are about ten species of Armillaria that occur in North America, with seven species occurring in the Northeast.



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