Armillaria Root Rot

Overview

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 honey-colored mushroom that grows from the roots and base of plants. The fungus is especially prevalent in 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 oak 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 encounter. 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 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. Plant resistant plants. Plants reported to be resistant to Armillaria root rot include black cherry, bald cypress, dawn redwood, ginkgo, hackberry, holly, Leyland cypress, maple, smoke tree, sweet gum, white fir, and wisteria. These should be considered for the 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.