Armillaria root rot, also known as shoestring root rot, is a soilborne disease that can affect several fruit crops. However, it is most common in the eastern United States on peach and apple trees. Its host range also includes numerous species of deciduous and evergreen trees, shrubs, and woody vines. The greatest prevalence of the disease is in orchards planted on newly cleared land in which the soils are sandy and well-drained. In these locations, pieces of wood invaded by the fungus serve as inoculum for infecting roots of fruit crops.
|Figure 1. Mushrooms of Armillaria mellea, cause of Armillaria root rot. Photo: Alan R. Biggs, West Virginia University.|
Affected trees exhibit reduced terminal shoot growth and may exhibit reddish to purplish foliage much earlier in the season than healthy trees. Diseased trees are prone to collapse suddenly in midsummer, and the dead leaves may remain on these trees into early winter. Trees usually die in a circular pattern in the orchard, often expanding over time from an initial infection of one or two trees.
Several Armillaria species have been associated with root rot in the eastern United States. One of the more common ones, A. mellea (fig. 1), or the honey mushroom, is a common inhabitant of eastern forests (as are the other species). Under favorable conditions, these organisms can survive for several decades as mycelia and rhizomorphs in stumps and dead root tissue. Disease spread can occur via windblown basidiospores produced by the mushroom phase. However, it is believed that Armillaria more commonly spreads via mycelia when a healthy root comes into contact with an infected one and via rhizomorph growth in soil.
Diagnosis of the causal organism is achieved by removing soil from around the bases of declining trees or pulling the trees out of the soil. The Armillaria root rot pathogens cause a white rot of the wood, giving infected tissues a bleached appearance that can also be spongy and stringy when the outer bark of the roots and crown are removed. Typically, the fungi produce fanlike mats of white mycelial growth under the loosened bark near the crown of infected trees (mycelial fans). Reddish-brown to black shoestring-like rhizomorphs also form under the bark, and these are diagnostic for the disease (fig. 2). The presence of clusters of the characteristic light brown or honey-colored mushrooms at the base of infected trees is also diagnostic; however, their production tends to be erratic. Mushrooms may form in late summer or fall. The Armillaria mycelium is bioluminescent in actively decaying tissues.
|Figure 2. Rhizomorphs, or “shoestrings,” of Armillaria root rot on an apple rootstock. Photo: Alan L. Jones, Michigan State University.|
Control of Armillaria root rot of tree fruits depends primarily on avoiding the pathogen by not planting trees in soil that harbors the pathogen. When clearing a woodlot for an orchard, attempt to remove all tree roots and allow a year or two for any roots left behind to completely decompose before planting new fruit trees. Promote tree health by following good orchard renovation and cultural practices, including soil pH adjustment and cover cropping. Trees stressed by other biotic and/or abiotic factors may be more vulnerable to attack. Generally, stone fruit rootstocks are the most susceptible, apple rootstocks are moderately susceptible, and pear rootstocks are considered tolerant to Armillaria. Because of the variability within and among Armillaria species causing root rots, rootstocks that show resistance in one test may show susceptibility in another test.
Alan R. Biggs, West Virginia University