Armillaria root rot
Fig. 1 Black, coordline rhizomorphs
Photo: U of MN Department of Plant Pathology
Armillaria root rot is a fungal disease affecting hundreds of species including woody and herbaceous plants. The disease occurs worldwide in both temperate and tropical regions.
The name "Armillaria root rot" is a generic term for this disease, which is caused by at least 11 different species of fungi. This disease is commonly called "shoestring root rot," which describes the black fungal structures of the fungus.
Armillaria is often associated with forest and landscape trees. Symptoms of the disease are not clear-cut and may be similar to those caused by other diseases or environmental conditions. Commonly, an infected tree will show a gradual decline, evidenced by small, yellowish leaves, reduced growth, and dieback of branches. Trees which are chronically infected may wilt suddenly during periods of stress, such as drought.
Armillaria is easily identified by white fungal mats found under the bark, black shoestring-like structures (called rhizomorphs) found under the bark, around roots, or in the soil, and honey-colored mushrooms found at the base of the plant.
The white mycelial mats are often shaped like fans with veined margins. These mats may extend upward several feet from the base of the tree for and may be associated with decay of the plant. On dead and decaying wood these mycelial mats may glow in the dark, indicating the presence of the fungus.
Fig. 2 Mushrooms of armillaria
Photo: Chad Behrendt
The cordlike rhizomorphs ("shoestrings") found under the bark appear flattened (Fig. 1), while rhizomorphs , growing freely in the soil or decayed wood, appear rounded. Rhizomorphs have a white core and black outer layer. As rhizomorphs grow through the soil, they branch and penetrate roots, causing new infections. Rhizomorphs, in addition to direct root contact, are the principal means of tree-to-tree spread.
Mushrooms of armillaria may develop annually in the fall, during wet periods. They usually appear in groups, with up to several hundred on or near decaying wood, stumps, or roots (Fig. 2). Although spores from these mushrooms can infect recently wounded plants, they apparently do not play an important role in the spread of the disease. It is important to note that many other fungi also produce mushrooms on or near decaying wood.
Infection often develops after trees have been weakened by freezing, drought, or other environmental stresses. Healthy trees have a number of defenses against armillaria, with mature trees sometimes walling off and enclosing infections by callusing over them. Weak virulent strains of the fungus can penetrate roots only after stress-related events have weakened the defense system of the roots.
The best strategy for preventing armillaria root rot on ornamental trees and shrubs is to promote the vigor of the plants. Watering thoroughly once a week during dry periods is very important. Avoid injuring trees and avoid damaging or stressing the roots.
Red pine saplings and other conifers are particularly susceptible when planted in light soil with insufficient rainfall. Conifers should not be planted in areas where broad-leaved trees, especially oaks, have been recently removed. The stumps and roots of these trees provide a good food source for the fungus, allowing it to grow through the soil and infect young conifers. After harvesting, remove as much of the debris, stumps, and roots as possible.
Agrios, George N. 1978. Armillaria Root Rot of Fruit and Forest Trees, pp. 419-421 In: Plant Pathology, Second Edition. Academic Press, Inc., Orlando, Fla. 703 pp.
French, D.W. and Cowling, Ellis B. 1975. Armillaria Mellea Root Rot, pp. 42-44 In: Forest and Shade Tree Pathology. University of Minnesota, Plant Pathology. 258 pp.
Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 1987. Armillaria Root Rot, pp. 308-312 In: Diseases of Trees and Shrubs. Comstock Publ. Assoc., Cornell U. Press, Ithaca, N.Y. 574 pp.
Revised by Chad Behrendt and Crystal Floyd 1999