Introduction

Armillaria root disease is worldwide in distribution and found throughout temperate and tropical regions. In the continental United States, the disease has been reported in nearly every state. It is one of the most prominent killers and decayers of deciduous and coniferous trees and shrubs in natural forest stands, plantations, orchards, and gardens throughout the world.
 
 

The fungus most often identified as causing the disease is Armillaria mellea (Vahl: Fr.) Kummer. Recent research, however, indicates that several different but closely related species are involved. Therefore, the generic term Armillaria is used to refer to this group. A. mellea is not a soil inhabitant and must survive as mycelia or rhizomorphs on large root and stem pieces. The fungus may remain alive for years on these roots and stems, as parasites on living host tissue or as saprophytes on dead woody material.

Armillaria mellea, and probably other closely related species, is one of the most common fungi in forest soil. They live on the coarse roots and lower stems of conifers and broad-leaved trees. As parasites, the fungi cause mortality, wood decay, and growth reduction. They infect and kill trees that have been already weakened. The fungi also infect healthy trees, either killing them outright or predisposing them to attacks by other fungi or insects. Such behavior typically occurs in the relatively dry, inland coniferous forests of the Western United States. The most spectacular losses occur in orchards or vineyards planted in recently cleared forestlands or in forest tree plantations. In coastal forests, the fungus occurs commonly as a butt rotter in old trees and a decayer of dead and down trees. The losses caused by Armillaria root rot are steady but conspicuous.

Common Names

Armillaria root rot has several common names. Shoestring root rot refers to the root-like fungal structures; called rhizomorphs that spread the fungi. The names honey mushroom, honey agaric, mushroom root rot, or toadstool disease refer to the mushrooms produced. Conifers often respond to infection by producing a copious flow of resin, hence, the names resin glut or resin flow. When oaks are the common host, Armillaria is often called the oak fungus.
 
 

Host range and distribution

Armillaria mellea occur on a wide range of angiosperms and gymnosperms. It is an important pathogen of various hardwood species and to lesser extent conifers in Europe, Asia and North America. Hosts include fruit trees, vines, shrubs, and shade and forest trees. Apart from woody hosts, infection of some herbaceous species has been reported from strawberry, potato and carrots. Some other unusual hosts include herbs belonging to Hyssopus, Lavendula, Origanum, Salvia, and Sideritis. Fungus is more common in areas of relatively mild winters.

Isolation

Isolation of the fungus from the field may be made from decayed wood, basidiomes, fans or rhizomorphs. Surface disinfestation is usually necessary. Rhizomorph growing tips are advantageous in that they grow faster than undifferentiated hyphae and may thus escape contaminants. Modified Weinhold’s medium, which is the defined medium for Armillaria provides good rhizomorph production. Benomyl dichloron streptomycin (BDS) and orthophenylphenate agar (OPP) media are two commonly used media that generally works well for Amillaria species.
 
 
 

Identification

In the absence of mushrooms, field identification of Armillaria root disease is based on the presence of mycelial fans, rhizomorphs and/or decay pattern (white, spongy rot of wood with zone lines or pseudosclerotial plates). Distinguishing among the species of Armillaria in the field is difficult. Species differ to some extent in cultural characteristics; however, such identifications may only be practical in regions with a limited number of Armillaria species. For the present, critical identification of cultures relies on pairings with tester strains. Although various techniques are employed most rely on the difference between cultural mat morphology of haploid single-basidiospore isolates and putatively diploid isolates from decay, fans or rhizomorphs. When sexually compatible single-basidiospore isolates of the same species are placed near each other on malt extract agar (MAE 1-3%) and allowed to grow together for over three weeks, the fluffy white mycelium may flatten, darken and become crustose. If the isolates are of different species, the mycelia remain white or light tan, continue to produce abundant aerial mycelium and do not become crustose.
 
 

Symptoms and Signs

The symptoms of the aboveground parts are similar to those caused by other root rot diseases. The commonest symptoms are reduced growth, yellowish leaves, dieback of twigs and branches, and gradual or sudden death of the tree. Initially the infected trees are scattered but due to the spread of the fungus from its primary infection point, circular areas of diseased trees appear. Diagnostic characteristics appear at decayed areas in the bark, at the root collar, and on the roots. White mycelial fans form between the bark and wood. Because these fungi commonly inhabit roots, their detection is difficult unless characteristic mushrooms are produced around the base of the tree or symptoms become obvious in the crown or on the lower stem.
 
 
 

On large, lightly infected or vigorous trees, crown symptoms develop over a number of years, until the trees die. Conifers, particularly Douglas fir and western larch, frequently produce a larger-than-normal crop of cones, known as stress cones, shortly before they die.

Trees affected by prolonged drought or attacked by rodents, bark beetles, or other fungi, particularly other root pathogens, can produce crown symptoms similar to those caused by Armillaria. Thus, additional evidence, often found on the roots and on the lower stem, is needed to diagnose the disease. On most conifers, the infected portions of the lower stems are somewhat enlarged and exude large amounts of resin. Infected portions of the roots frequently become heavily encrusted with resin, soil, and sometimes, fungal tissue.

If Armillaria is present, removing the bark covering infections will expose the characteristic, white mycelial mats or the rhizomorphs that grow between the wood and the bark. The white mycelial mats are marked by irregular, fanlike striations; hence, they are often referred to as mycelial "fans." The thick mats decompose, leaving impressions on the resin-impregnated inner bark. The mycelium may extend a few feet upward in the phloem and cambium, and in some trees, such as oak, sugar maple, and hemlock, it may cause a white-rot decay. Another characteristic sign of the disease is the formation of rhizomorphs or "shoestrings". Rhizomorphs growing beneath the bark are flat and black to reddish brown in color. They have a compact outer layer of dark mycelium and an inner core of white mycelium. Rhizomorphs also grow through the soil. Mushrooms, the reproductive stage of these fungi, confirm the presence of Armillaria. The short-lived mushrooms may be found growing in clusters at the base of dead or dying tree trunks, stumps or on the ground near infected roots. These honey-colored, speckled mushrooms, about seven or more centimeters tall and with a cap 5-15 cm in diameter are produced sporadically in late summer or autumn, and are most abundant during moist periods.
 
 

Ecology and life cycle

The fungus overwinters and as mycelium or rhizomorphs in diseased trees or in decaying roots. The principal method of tree-to-tree spread of the fungus is through rhizomorphs that grow from infected roots through the soil to the adjacent healthy trees or direct root contact. Pieces of rhizomorphs in plant debris may be carried by cultivating equipment into new areas. The fungus can be spread by basidiospores, but spores generally colonize dead stumps or woody material first and then rhizomorphs radiating from these, attack living roots directly or through wounds. Trees and roots weakened from other causes are much more easily attacked by Armillaria than are vigorous trees. Armillaria mellea derive predominantly carbohydrates from their woody hosts. Among the commonest Armillaria species, A. mellea is considered as one of the most pathogenic. However, the pathogenic species only infect their hosts under environmental conditions that favor development of the fungus or that weakens the host resistance. Conditions allowing abundant development of soil inoculum of Armillaria make hosts more prone to infection, while at constant inoculum density infection is more likely to occur when trees are weakened. Armillaria species can survive for long periods in colonized wood or as rhizomorphs. The opportunistic life style of Armillaria is explained by its ability to persist when there are no food sources, waiting for conditions that allow the weakening of the host.

Morrison, D.J. Armillaria root disease: a guide to disease diagnosis, development and management in British Columbia. BC-X-23.Victoria, BC: Canadian Forestry Service, Pacific Forest Research Centre; 1981. 15p.

Munnecke, D.E., Kolbezen, M.J. Wilbur, W.D. and Ohr, H.D. 1981. Interactions involved in controlling Armillaria mellea. Plant Disease 65(5): 384-389.

Raabe, R.D. Host list of the root rot fungus, Armillaria mellea. Hilgardia. 33(2): 23-88; 1962.

Shaw, C.G., III; Roth, L.F. Control of Armillaria root rot in managed coniferous forests: a literature review. European Journal of Forest Pathology. 8(3): 163-174; 1978.

Wargo, P.M. and Shaw, C.G. III. 1985. Armillaria root rot: the puzzle is being solved. Plant Disease 69(10): 826-832.

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Pathogen profiles

Other related links
NC State University Department of Plant Pathology
PP 318: Forest Pathology By Dr. Larry F. Grand, NCSU.
Armillaria root rot: Royal Botanic Gardens, Sydney, Australia.
American Phytopathological Society
                                                                                                                                                                                                          Posted Spring 2001