Root and butt rot fungi can substantially reduce the growth of affected trees. The examples discussed here are the most important and common rots that affect sugar maple.

Armillaria root rot. Armillaria root rot, sometimes called shoestring root rot, is one of the most destructive diseases affecting the roots and butts of most tree species, both conifer and broad leaved. Infection by Armillaria can result in rapid mortality, reduced growth, or increased susceptibility to windthrow. Generally, decay is not extensive in living trees, but it spreads rapidly when the tree dies. Armillaria can infect and establish itself in the non-wounded root systems of stressed trees and widespread death of large trees can occur suddenly following severe stresses such as recurring droughts or severe defoliations. When the stress abates, many trees are able to contain the further spread of the fungus while they recover. Infection is most likely to occur where there are nearby infected hardwood stumps to serve as food bases for the fungus. The fungus spreads from diseased to nearby trees through roots or by means of fungal structures called rhizomorphs which penetrate intact back of stressed trees. Rhizomorphs are black and cord-like and grow freely in the soil so long as they remain attached to stumps or roots that serve as the food bases. After infection, the fungus spreads to the cambial zone under the bark and produces a white mat of tissue (Fig. 96). When conditions are favorable, the fungus spreads to other roots and up to the root collar where it may girdle the tree. Usually, at this stage there has been little wood decay. Visible growth reduction and crown symptoms (smaller and lighter colored leaves or branch dieback) will appear only after one-half or more of the root system is killed. When a tree dies, the mat of white fungus tissue beneath the bark is rapidly replaced by rhizomorphs (Fig. 97), and the active wood-decay stage sets in. The root system of dead trees or stumps can supply Armillaria with food energy for up to 10 years, enabling it to produce rhizomorphs and infect nearby stress-weakened trees.

This fungus produces an edible, honey-colored mushroom (Fig. 98). The central stalk is 2 to 4 inches (5 to 10cm) long and generally has a ring on its upper part. The yellowish brown cap is 1 to 4 inches (3 to 10 cm) in diameter and in late summer occurs in clusters of 5 to 10 on dead stumps. Mushrooms also are found on roots of infected trees or directly on the soil, but they are always attached to an underlying infected root.

Management of Armillaria root rot entails keeping stands as vigorous as possible. Under most circumstances, controlling high populations of insect defoliators is warranted. However, little can be done to combat drought, which seems to be one of the major stress factors that increases susceptibility to infection. Mechanical injury to roots as well as soil compaction should be avoided. Experiments have shown that partially burning or debarking and chipping infected stumps accelerates their decay. This rapidly reduces their potential as food bases for the fungus.

Hypoxylon root and butt rot. The root and butt rot fungus Hypoxylon deustum (formerly Ustulina vulgaris) infects trees through mechanical wounds on the butt and large roots. It also spreads from parent stumps to sprouts. While causing some root and butt decay in living trees, the fungus becomes most active as a wood-decay organism in dead or downed trees. It forms large masses of tissue near the original wound or on the cut surface of stumps. These masses of fungal tissue are grayish and leathery at first, but soon become black and brittle (Fig. 99). Minute fruiting bodies eventually develop in this tissue. H. Deustum affects sugar and red maples, ash, basswood, beech, birch, and many oaks.

This fungus is common though not serious in sugarbushes. Minimizing the number of large wounds at the base of trees or on large roots should limit its development in a stand.

Black root rot. Another disease of sugar maple, black root rot (Xylaria polymorpha) is sometimes confused with Hypoxylon root rot. Xylaria causes root decay, especially of stressed trees. It is recognized by a black mycelial sheet that develops on decaying roots. In late summer or early fall, clusters of gray to black finger-like to club-shaped fruiting bodies, 1 to 20 inches (3 to 5 cm) tall, develop on large infected roots or butts (Fig. 100). The appearance of these structures has evoked the common name “dead man’s fingers.” Above ground, symptoms of infection include reduced growth and sparse, chlorotic, and dwarfed foliage. These appear when the infection is severe. A tree may show symptoms only on one side of the crown if only one large root is infected.