Armillaria Root Rot - Field Guide to Diseases and Insect Pests of Idaho and Montana Forests

Armillaria Root Rot

Armillaria ostoyae (Romagn.) Herink

From: Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, USDA Forest Service Northern Region, Publication Number R1-89-54

Hosts. – Primary hosts are Douglas-fir and grand fir although all conifers may be attacked, particularly at ages less than 30 years.

Distribution. – Range of hosts in Idaho and Montana, especially west of the Continental Divide.

Damage. – Decays roots and kills cambium in roots and root collar. Causes mortality in groups and scattered, individual trees (fig. 59). Infected trees are often attacked by bark beetles.

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Figure 59 - Scattered Root Disease mortality.	Figure 64 - Fresh and older, dark resinous at the base of an Armillaria Root Rot-infected Douglas-Fir.
Identification. – Trees infected with Armillaria have typical root disease crown symptoms. Resinosus often is extensive on and throughout the bark of the root collar (fig. 64). The most diagnostic feature is the thick, fan-shaped mat of white mycelium (fig. 65) in the cambium of roots and root crown. Other signs of the fungus include rhizomorphs on the outside of infected roots and honey-colored mushrooms. The mushrooms may be produced at the base of infected trees during late summer or early autumn. The decay is white to yellow and stringy or spongy, sometimes with fine, black lines traversing the decayed wood (zone lines).	Image Unavailable
	Figure 65 - Cutting away the bark reveals white fan-shaped felts of Armillaria mycelium in the cambium at the root collar.

Similar damages. – Laminated root rot causes basal resinosus as well but thick, white mycelium fans are not produced in this disease.

Link to Table 2 – Comparison of Common Root Diseases

Link to Images in Forestry Images

References.

Anonymous. 1982. For. Insect & disease identification and management. USDA For. Serv., Northern Region; Idaho Dept. of Lands, Insect and Disease Control; Montana Dept. of State Lands, Division of Forestry. 192 p.

Bega, R. V. 1978. Diseases of Pacific Coast conifers. USDA For. Serv. Ag. Hndbk. No. 521, 206 p.

Boyce, J. S. 1961. Forest Pathology. McGraw-Hill Book Co., New York, NY. 572 p.

Hepting, G. E. 1971. Diseases of forest and shade trees of the United States. USDA For. Serv. Ag. Hndbk. No. 386, 658 p.

Kimmey, J. W. 1964. Heartrots of western hemlock. USDA For. Serv., For. Pest Leaflet 90, 7 p.

Williams, R. W., C. G. Shaw, III, P. M. Wargo, and W. H. Sites. 1986. Armillaria root disease. USDA For. Serv., For. Ins. & Dis. Leaflet 78, 8 p.

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