Ornamental & Tree Diseases

Plant Disease Dispatch Sheets
December 18, 2000

M-414 Armillaria Root Disease

Armillaria root disease, also known as shoestring root rot, shoestring fungus rot, rhizomorphic root rot, mushroom root rot, toadstool disease, resin flow, and resin glut is caused by the fungus Armillaria ostoyae. This soilborne basidiomycete continues to inflict significant losses in coniferous and hardwood forests, plantations, and amenity plantings worldwide. The more than 40 species of Armillaria endemic to most forest soils serve several roles. As saprophytes, some species play a beneficial role in decomposition. As weak, opportunistic facultative parasites, other species of Armillaria infect trees under stress (trees infected by other fungi, insect infestations, lightening injury, drought, freezing, pollution, and offsite plantings). These species may also cause disease in plantations where stand composition has been changed (for example, on hardwood sites that are artificially regenerated with conifers and/or where extensive stumps remain from former plantations). Armillaria ostoyae, however, is considered to be a virulent pathogen capable of penetrating, infecting, and killing unwounded, unstressed, vigorously growing trees.

Mycelium, mycelial fans, and rhizomorphs produced by A. ostoyae in infected roots and stumps allow the fungus to overwinter and also constitute the most significant forms of inoculum for subsequent infections. Tree-to-tree dissemination occurs by mycelial growth through root grafts and growth of rhizomorphs through soil to adjacent roots. Long distance dissemination of pieces of rhizomorphs on equipment has been documented. The role of basidiospores produced in fleshy basidiocarps at bases of infected trees is still not clearly understood. Entry into host trees occurs through root grafts, wounds on trunks, branches, and roots, and directly through uninjured tissues. Using infected roots as a source of nourishment by A. ostoyae is aided by the secretion of pectolytic and cellulolytic enzymes. Cellular structure and function are altered, causing a number of alterations in roots, trunks, and crowns.

Armillaria inhabited roots decay, reducing or preventing absorption and translocation of water and mineral elements from the soil to above-ground parts. Above-ground symptoms may not be evident in small trees or in trees where root systems are killed rapidly. However, a number of nonspecific symptoms may develop in stems and leaves of older trees, where root decline and death is slower, including reduction in shoot growth, stem dieback, yellowing of foliage, defoliation, and stress-induced reproduction. Visual evidence of Armillaria infection on trunks includes internal and external resinosis and heart rot.

Because many symptoms produced by Armillaria-infected trees are nonspecific and easily confused with those induced by drought stress, mineral deficiencies, beetle infestations, and other root pathogens, confirmation of Armillaria root disease is based on finding characteristic "signs" or evidence of the presence of A. ostoyae. They include white mycelial "fans" that typically form between the bark and wood of infected buttress roots and lower trunks; black to reddish-brown rhizomorphs (shoestring-like aggregations of mycelium) on surfaces of infected roots, under the bark of infected trees, and in the soil near infected trees; and clumps of honey-colored, fleshy basidiocarps at bases of infected trees. Basidiocarps appear in late summer or early fall and persist for only two to three weeks. Their black, shriveled remains can often be found several months later, however.

Management recommendations include the following:

1. Evading the fungus by trenching around infected trees to prevent growth of rhizomorphs to adjacent trees (seed orchards and perhaps in plantations).

2. Evading the fungus by delaying planting for several years on land previously occupied by oaks.

3. Eradicating or reducing fungus populations by removing dead and dying trees during harvesting operations.

4. Eradicating or reducing fungus populations by plowing up and burning stumps of Armillaria-infected trees before replanting site.

5. Eradicating or reducing fungus populations by fumigating infested areas (nurseries and seed orchards).

6. Avoiding wounding residual stand during logging operations.

7. Using silvicultural activity that eliminates or reduces environmental stresses (temperature, moisture, mineral) and maintains or increases stand (tree) vigor.

8. Using silvicultural practices that maintain the natural composition and density of local tree species. Avoid off site plantings.

Vernon Ammon, Ph.D., Professor and Plant Pathologist
Mukund V. Patel, M.S., Extension Plant Pathologist
December 2000

Information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended or other products that may also be suitable and that these products have maintained label clearance.