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Symptoms and signs

Armillaria root disease is usually easy to diagnose conclusively if one can cut into the tree roots or base. A pulaski (Fig. 2) is a good tool for diagnosis of root diseases of trees, because it has a digging/scraping side and a chopping side.

Figure 2

Crown symptoms

Branch dieback and crown thinning are common symptoms of Armillaria root disease, but they are also symptoms of other root diseases and other tree problems. On some conifers, chlorosis (yellowing) or reddening of foliage or heavier-than-normal production of cones (a stress crop of cones) may also appear.

Basal symptoms

On some species, the fungus may grow up from the roots in the inner bark and cause basal cankers above the infected roots. On resinous conifers, resinosis (Fig. 3), an exudation of resin, accompanies and can occur proximal to infection of roots, the root collar and the stem base. When resinosis occurs, it is the most diagnostic of symptoms for Armillaria root disease. In some cases, decayed roots may be apparent or decay in the inner wood of the stem base (butt rot; Fig. 4) may be exposed.


Figure 3	Figure 4

Armillaria species cause a white rot of wood (Fig. 5), as opposed to brown rot. In white rot, both lignin and polysaccharides (cellulose, hemicelluloses) are ultimately degraded. Wood often has a bleached, whitish appearance because the brownish lignin is removed. During decay, there is undecayed, intact cellulose present that gives the wood a fibrous texture; i.e., the orientation of the grain is always apparent. In brown rot, in contrast, cellulose and hemicellulose throughout the wood are depolymerized in early stages, and the lignin is left relatively unchanged. As a result, wood loses its fibrous structure in the early stages of brown rot, tends to shrink and crumble easily, and becomes browner. Wood decayed by Armillaria species is often spongy or stringy, and may be wet. Black lines called zone lines are usually seen in the decayed wood. These lines are actually curved planes in the wood, sometimes called pseudosclerotial plates, composed of thickened, dark fungal cells. They may play a role in protection of Armillaria from unfavorable conditions or other fungi that attempt to invade its territory, including other individuals of the same species. Actively decaying wood may be luminescent, producing a faint glow that is visible in the dark (see Significance).

Figure 5

Signs

Three signs of Armillaria root disease may be seen in the field. Any one of them, correctly identified, permits a conclusive diagnosis.

Mycelial fans (Fig. 6) are nearly always present in infected and recently killed trees. These are white mats of fungal mycelium, between the inner bark and wood. Although other fungi may form mycelial fans, those of Armillaria species are generally more substantial, have a mushroom odor, often have a fan-like pattern, and may be peeled off in small pieces.

Figure 6

Rhizomorphs (Figs. 7, 8) are commonly associated with infection. Depending on the species of Armillaria, they may be few, small, fragile and hard to find or abundant and robust. They are cylindrical (in soil) or flattened (under bark), 1-5 mm (0.04 - 0.20 in.) diameter, reddish-brown to black, branched, and have a cream-colored tip when actively growing. The inner tissue is whitish mycelium, but it may become tan with age. Rhizomorphs are often attached to infected roots, but they may also be attached to the surface of uninfected roots. Although other fungi produce mycelial cords and rhizomorphs, I am aware of no other fungus with rhizomorphs that would be confused with those of Armillaria species. They are much more likely to be confused with roots: even experienced workers sometimes have to examine them carefully to identify them as rhizomorphs!


Figure 7	Figure 8

Mushrooms (Fig. 1) may be produced in late summer or autumn. Fruiting is erratic; mushrooms may be absent in some years and very abundant in others. Although mushroom morphology varies with the species, they are generally in clusters near or on the base of trees. Caps are honey-brown, usually with small tufts of dark hairs, gills are whitish with notched attachment, spore prints are white, and stems are white to brown, usually with an irregular, mottled appearance. Most species have a partial veil that results in a more or less delicate annulus (ring) on the stem. Other differences among species in mushroom morphology are often subtle and variable. The mushrooms are edible, although you should not eat wild mushrooms unless you are with an experienced collector.

Figure 1

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