Armillaria Root Rot
Click for larger image Developing fan-shaped mat of fungal strands caused by armillaria root rot at base of sycamore trunk (Platanus)

The fungus, Armillaria mellea, occurs sporadically in this region and has been reported to infect over 25 species of ornamental trees and shrubs. The most distinctive sign of Armillaria infection is the honeycolored mushroom that grows from the roots and base of plants. The fungus is especially prevalent on oak but also affects many different kinds of fruit and nut trees, ornamentals, and herbaceous plantings. It is often referred to as oak root rot fungus because it is commonly found on oaks or in areas where oaks trees once were grown, such as cleared forest land.

Symptoms and Diagnosis

The symptoms of Armillaria are similar to those of other root disorders; height growth is reduced, foliage is sparse, and leaves that remain on plants are stunted and yellow. An abnormal flow of sap may be visible on the root collar. When soil is removed from the base of the trunk, black, root-like strands are visible and attached to larger roots. White to dark, fan-shaped mats of fungal strands develop between the bark and wood in infected root and trunk tissues. The most positive sign of infection is the production of clusters of honey-colored mushrooms at the base of the tree near the soil line. The mushrooms may have stalks 4 to 6 inches high with caps 1 inch high and 2 to 4 inches wide.

Life Cycle

Armillaria is found in the soil sporadically throughout the Midwest. The fungus spreads primarily by root-to-root contact or by root-like fungal strands. Root-like fungal strands grow through the soil and adhere to the host roots or root collar that it encounters. Successfully attacked trees do not die until infections girdle the base of the tree. On healthy, vigorous trees, Armillaria is not lethal but if present, it may begin to grow when the tree dies of other causes. Young trees are more likely to be killed by Armillaria. Trees that are 15–20 years old are more tolerant to attack.

Integrated Pest Management Strategies

1. Remove and destroy infected material. Remove as much of the stump as possible. Sterilize tools with a solution of 1–part bleach with 9–parts water after use.

2. Provide adequate moisture in a well-drained soil to maintain vigor and resistance to infection. Plants suffering from drought are extremely susceptible to infection. Fertilize trees appropriately in late winter or early spring.

3. No effective chemical controls are known. Plants reported by Auburn University to be resistant to Armillaria root rot include ginkgo, tulip tree, ash, bald cypress, cherry, Chinese elm, Chinese pistache, crabapple, cryptomeria, dawn redwood, eucalyptus, hackberry, holly, incense cedar, Leyland cypress, maidenhair tree, maple, privet, smoke tree, sweetgum, tree-ofheaven, white fir, and wisteria. These should be considered for replacement of diseased trees. New soil should be used to amend the planting site. All diseased material and associated roots, as much as possible, should be excavated before replanting.

Organic Strategies

Strategies 1 and 3 are strictly organic approaches. Organic tree fertilizers are available and could be a viable organic approach to Strategy 2.

More images:

Click for larger image
Sideview of a cluster of the honey-colored mushrooms associated with ringless armillaria root rot growing on the roots of a dead shingle oak (Quercus)
Click for larger image
Honey-colored mushrooms of ringless armillaria root rot growing on the roots of a dead shingle oak (Quercus)
Click for larger image
Closer view of the honey-colored mushrooms of ringless armillaria root rot growing from the roots of a dead shingle oak (Quercus)
Click for larger image
Honey-colored armillaria mushrooms and the fruiting body of ganoderma root rot growing from the base of winterberry holly (Ilex verticillata 'Nana' RED SPRITE)
Click for larger image
Honey-colored armillaria mushrooms and ganoderma root rot growing from the base of winterberry holly (Ilex verticillata 'Nana' RED SPRITE)
Click for larger image
Honey-colored armillaria mushrooms growing from the base of winterberry holly (Ilex verticillata 'Nana' RED SPRITE)
Click for larger image
Honey-colored armillaria mushrooms growing from the base of winterberry holly (Ilex verticillata 'Nana' RED SPRITE)
Click for larger image
Clusters of honey-colored mushrooms scattered throughout this lawn indicate that the silver maple (Acer saccharinum) in the center has armillaria root rot
Click for larger image
These clusters of mushrooms are the fruiting bodies of armillaria root rot possibly infecting the nearby silver maple (Acer saccharinum)
Click for larger image
Close-up of a cluster of mushrooms from the armillaria root rot possibly infesting the nearby silver maple (Acer saccharinum)
Click for larger image
Armillaria root rot on dogwood (Cornus). FL DIP Archive, FDACS, Bugwood.org
Click for larger image
Symptoms of armillaria root rot on yew plum pine (Podocarpus macrophyllus). FL DPI Archive, FDACS, Bugwood.org
Pests and Problems

Click a link in the site map below to see other "Pests and Problems" pages