Annosum Root Rot

Image of Annosum Root Rot


This fungus may be called many names - including annosum root rot, annosus root rot, or Heterobasidion root rot – and is caused by Heterobasidion irregulare (formerly named Heterobasidion annosum and Fomes annosus). This fungus is present throughout North America, has a very wide host range, and is commonly found in southeastern U.S. forests. The fungus causes root decay, although infected trees may survive for many years after infection. Weakened roots are at an increased risk of windthrow. Fungal spores are also spread by wind, and often infect stumps from recently harvested forest stands. Annosum root rot is most common on deep, sandy soils or former agricultural land. 


Symptoms first develop 1-3 years after thinning. Crowns may appear thin; needles may be chlorotic, stunted, and tufted at shoot tips. Trees that have been killed may stay green through the winter, but needles will turn brown the following spring/summer. Wind-throw is commonly observed in stands where Annosus root rot is present. Rotted roots are unable to provide the necessary structural support. Windthrow may also be the first indication of infection; green and otherwise healthy looking trees have been known to fall over prior to the development of needle symptoms. Roots may be resin-soaked; white and stringy rot may be present in wind-thrown trees. In late winter or early spring, fungal fruiting bodies (conks) may be produced at the base of the tree (possibly under the litter layer). The conks are brown on top, white and porous underneath, and have a creamy-white edge. 


Thin stands during summer months when temperature is above 85oF; spores can only germinate on stumps in cooler weather. If thinning during cool weather, stumps should be treated immediately after cutting with granular borax (or alternative product) to prevent infection. Treatment with borax is ineffective once infection has occurred. Clearcut stands with greater than 50 percent infection; removal of diseased trees will result in an under-stocked stand. Wide spacing is recommended when planting to increase time to first- thinning, and to reduce the number of thinnings necessary in the stand. Wounding of roots and lower stems should be avoided during logging, fire break installation, and road building because these wounds are also suitable infection courts. 

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