Reports on Plant Diseases

In living trees, most of the decay is confined to the older, central wood (heartwood) of roots, trunks, or branches. Once the tree is cut, however, the outer wood or sapwood is also colonized by the wood-decay fungi, as are the wood products made from the tree, if moisture and temperature conditions are favorable for growth of the fungi. When deep wounds or cuts are present, discoloration and decay often spread into the outer wood, and the entire tree, especially if it is a hardwood, loses its economic value.

Symptoms

Trees extensively invaded with a wood-rotting fungus may show a gradual decline in vigor. Twigs and then branches die back with trees becoming structurally weak and more susceptible to ice and wind damage.Most wood-rotting fungi produce fruiting structures or sporophores of the bracket (shelf) or hoof type–called conks (Figures 1, 2, 3, and 4, Plates 1 and 2) – or the mushroom (toadstool) type (Figure 5; Plate 2). Bracket or hoof conks may be corky, leathery, woody, punky, or fleshy in texture. Clusters of mushrooms may form at a trunk base or at wounds. Fruiting structures of decay fungi are unreliable predictors of decay because they commonly do not appear until decay is well advanced.

Decay fungi may cause the colonized wood to become watersoaked, spongy, stringy, crumbly, or flaky. Affected wood may also be discolored (Figure 1 and 1a), usually brown, white, yellow, or some shade of red, for several feet or more above and below where a conk or cluster of mushrooms appears.

The development of wood rots varies somewhat with the fungus involved, the woody host that it invades, and the site on which the tree is growing. Thus, some fungi cause mainly root and butt rot, others a top rot, and still others a trunk and/or branch rot. Top rot decay fungi attack the heartwood in the upper part of the tree. These fungi seldom progress very far into the roots and therefore rarely spread from one tree to another via roots or from the tree stump to the next generation of stump sprouts or root suckers. Root and butt rot fungi colonize the lower stem and roots and can cause serious problems in forest stands generated from sprouts and suckers. Rot in the parent tree and its invasion of cut stumps may serve to infect the new stand with root rot fungi. Fire, logging, lawn mower, and construction scars frequently provide entry points for root- and butt-rotting fungi.

Some species of decay fungi cause root and butt rot in one species of tree and trunk rot in another species. A few fungi cause both top rot and root or butt rot in the same tree species. Fungi that can decay living sapwood and heartwood and cause cankers are called canker rots.

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The sporophores produce basidiospores at the hymenial surface of gills or pores (Figures 5 and 6; Plates 1 and 2) during part or most of the year, and the spores are carried by air currents, rain, insects, birds and other animals, or other agents to nearby tree wounds. A single large conk may shed up to 100 billion basidiospores in a single day.

Typically, the spores are randomly disseminated by the wind. When a spore comes in contact with a wound in a tree and conditions are suitable (proper temperature, the presence of moisture and nutrients, and the lack of inhibitors produced by nondecay organisms), the spore germinates and forms a germ tube that expands into a hypha. The hypha branches and grows into the wood fiber and vessel cells to form a mycelium. Food is obtained by enzymatic digestion of the cell walls. Wood rotting fungi may also enter woody plants as mycelium.

A tree is commonly injured many times during its lifetime. The disease cycle described above may be repeated after each new wound is formed, thus involving more and more wood in the natural and more or less continuous process of discoloration and decay. The end result is one or more cylindrical compartments of discolored and decayed wood that may extend over much of the height of the tree.

Domesticated tree varieties differ greatly in their susceptibility to heartwood decay. Table 1 groups these trees into resistant or very resistant, moderately resistant, and slightly or nonresistant.

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Control

1. Select and grow only species and varieties or cultivars of shade, ornamental and fruit trees and shrubs that are well adapted to the area. Plant only vigorous, disease-free nursery stock. Grow somewhat tender species in sheltered locations. Plant at the proper depth in a large hole, well spaced apart, in fertile, well-drained soil of the proper soil reaction (pH).

2. When feasible, keep woody plants vigorous through (a) proper applications of fertilizer in mid- to late-autumn or early spring; (b) thorough soaking of the soil to a 12-inch depth every 10 to 14 days during extended hot, dry periods; and (c) wrapping the trunks of newly transplanted, thin barked trees with sisalkraft paper, special tree-wrapping paper, or other appropriate material prior to winter.

3. Prune periodically to remove all dead, dying, interfering, and broken branches so that they are nearly flush with a major branch or main stem; leave the "collar" that surrounds the base of the branch. Prune broken stems below the damaged portion so that water will drain off and not collect on the wound surface. The severed ends of roots should be made blunt rather than left jagged. Pruning is best done during the dormant season when the weather is dry. Pruning in late spring often leads to separation of wood and bark around pruning wounds.

4. Avoid burning of trash near trees and shrubs.

5. Make as few changes as possible in the soil grade or drainage patterns in the vicinity of trees. Avoid compacting soil over the roots.

6. Follow cultural practices suggested by Extension horticulturists and foresters at the University of Illinois at Urbana. Your local Extension office and a professional arborist or forester can also provide valuable help on general tree care.

7. Control insect borers by spraying the trunk and major branches with a suggested insecticide following recommendations of University of Illinois Extension entomologists. Many wood boring insects infest trees previously weakened by drought, temperature extremes, various diseases, and so forth.

8. Avoid all unnecessary bark wounds. When bark and wood injuries do occur, treat them promptly. Cut away all loose or discolored bark. Remove splintered wood. Clean, shape, and smooth the wound into a streamlined oval or vertical ellipse. Then swab the surface liberally with an antiseptic such as 70 percent alcohol or shellac. The use of a commercial tree wound dressing (tree paint) is of questionable value since it does NOT check the invasion of wood by decay fungi. The barrier zone of cells formed by the cambium effectively confines the decay within the tissues present at the time the tree was wounded. The use of tree wound dressings is largely cosmetic and their usefulness in preventing wood decay is questionable.

9. Reduce losses in forests, plantations, and farm woodlots by (a) eliminating, as much as possible, the introduction of wood-rotting fungi into healthy stands by early pruning of lower branches; (b) conducting logging and thinning operations to minimize breakage of branches and the creation of major wounds (top breaking, stripping of bark, butt and trunk damage from heavy equipment) to the stems and roots of the remaining trees (such operations should take place during the dry season or winter to avoid much of the mechanical damage to the root systems of the living trees that remain; (c) harvesting trees before they become overly mature and thus increasingly susceptible to wood-rotting fungi; and (d) not letting livestock graze in farm woodlots. Livestock damage trees through soil compaction, butt damage, and root wounds caused by sharp hoofs. All trees that are dead, hazardous, diseased, or pest ridden should be removed.

10. Control discoloration and decay in lumber and other wood products by drying the wood in a kiln or by treating with a recommended wood-preserving fungicide. Wood likely to be in contact with soil or moist surface should be treated with a wood preservative suggested by the Extension forester, University of Illinois at Urbana.

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Table 1.

a From U.S. Department of Agriculture Handbook No. 72.

b The southern and eastern pines and bald cypress are now mostly second growth with a large proportion of sapwood. Consequently, substantial quantities of heartwood lumber of these species are not available.

c These trees have exceptionally high decay resistance.

For further information concerning diseases of woody ornamentals, contact Nancy R. Pataky, Extension Specialist and Director of the Plant Disease Clinic, Department of Crop Sciences, University of Illinois at Urbana.

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