Importance. -- Butt rot, the decay at the base of living trees, is the result of the invasion by one of a number of decay fungi which enter the trees through wounds. Fire wounds are the most typical type. Data indicate that butt rot affects 29 percent of the white oaks and 39 percent of the red oaks on loess and alluvial sites in the Midsouth. It is the most serious cause of cull.

Identifying the Fungi. -- Numerous fungi can cause butt rot; however, five are responsible for about one-half of the identified cases. The following descriptions will help to identify the most common fungi.

Hedgehog Fungus Rot. -- Hericium erinaceus conks are 4 to 10 inches (10 to 25 cm), globular, and occur singly or in clusters. They are white, but yellow with age, and have tooth-like projections pointing downward. This fungus is found mostly during the fall in butt hollows or where other openings in the tree have developed (figure 52).

Polyporus Fungus Rot. -- Tyromyces fissilis, produces shelf-like, white, succulent conks 3 to 8 inches (7.6 to 20.3 cm) wide, that yellow with age (figure 53). The lower surface is made up of small pores. They usually appear during the fall or winter.

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Varnish Fungus Rot. -- Ganoderma lucidum produces conks 3 to 10 inches (7.6 to 25.4 cm) in size which usually appear during the summer near the soil line (figure 54). The conks have a shiny, reddish, hard upper surface; a short, stout stalk; and pores on the lower surface. The consistency is tough and woody.

Sulfur Fungus Rot. -- Laetiporus sulphureus has conks 2 to 12 inches (5 to 30 cm) wide. They are soft, fleshy, moist, bright orange-red on the upper surface and red-yellow on the lower pore surface. The conks become hard, brittle, and white with age, They appear alone or in clusters, usually during the fall (figure 55).

Oyster Fungus Rot. -- Pleurotus ostreatus forms shelf-like conks which are white to light grey. They are soft and fleshy and may have a short stalk. Gill structures radiate from the point of attachment on the lower surface (figure 56). Conks appear on living trees and slash during most of the year except dry periods.

Identifying the Injury (figure 57). -- Conks, old wounds, hollows, abnormal swellings or butt bulge indicate butt rot. Decayed wood may be soft or brittle, and brown to white. The decay core may be small or include the entire heartwood. The core extends vertically from less than an inch to several feet. Affected trees are weak and subject to breakage.

Biology. -- Following tree wounding, bacteria and non-decay fungi flourish on the exposed woody tissues, creating conditions for establishment of decay fungi. Windborne microscopic spores released for a few days to several weeks from conks on infected trees germinate on wounds and penetrate the tree. The decaying stage of the rot fungi follows and conks will be produced. The rate of decay varies with the tree species, fungus, and wound size. Decay is most extensive when wounds are large; decay usually does not develop in wounds less than 2 inches (5 cm) wide. Regardless of wound size, wood volume loss is minimal when wounds are less than 4 years old.

Control. -- Because all infections occur through bark wounds, injury prevention is the primary approach to control. Severely decayed trees should be deadened. Consider early salvage for infected trees that have value because the lower, most valuable portion of the log is being decayed, with an increased susceptibility to insect attack, windthrow and degrade from stain. Repair valuable urban trees by removing the decay, treating the cavity with a fungicide and filling it with a suitable material.