Stem Decays of Hardwoods in the Plains Numerous decay fungi, numerous hosts Pathogen—Many fungi decay wood in the roots, butts, and stems of hardwoods in the Great Plains of South Dakota, Nebraska, Kansas, and Colorado. Three stem-decay fungi are presented here: Phellinus igniarius, Fomiti- poria (Phellinus) punctata, and Perenniporia fraxinophila (table 1). Hosts—Phellinus igniarius has a wide host range, infecting species in over 20 genera of hardwoods. It is com- mon in birch, but has also been found in ash, black walnut, poplars, buckthorn, and willows. Fomitiporia punctata also has a wide host range. In a survey of North Dakota windbreaks, plantings, and natural stands, it was found on live willow, ash, Prunus, Rhamnus, Caragana, and Syringa. Perenniporia fraxinophila infects primarily ash species, with a few records on other hardwood genera and even junipers. Signs and Symptoms—Conks, the spore-producing fruiting bodies, are evidence of infection, but they are not always present on infected trees. Conks may form anywhere on the stem or branches (fig. 1), but are most common at branch stubs, in cankers, and near cavity open- ings. In addition to conks, symptoms may be evident. Openings leading to internal hollows (cavities) may form, especially at branch stubs or openings created by cavity-nesting birds (fig. 2).Fomitiporia punctata also causes cankers, or patches of killed bark—such a disease caused by wood-decay fungi is called a canker rot. The fungus decays wood inside the stem but grows out along branch traces and kills the cam- bium around branch stubs, which results in death of the overlying bark. These fungi all cause a uniform white rot of the wood. Disease Cycle—For most stem-decay diseases, the precise point of infection is uncertain. Branch stubs and even small dead twigs can Figure 1. Perenniporia fraxinophila on green ash. be infected in some cases, but wounds, especially large ones, greatly Photo: James T. Blodgett, USDA Forest Service. increase the likelihood of stem decay. Spores germinate at the point of infection, and the fungus may grow down a branch stub or into a wound surface to reach the inner wood. The fungus decays the wood for some years and then returns to the surface, often along a branch trace, to produce a conk. Spores are released from the conk and are blown by wind to reach a new infection point. Impact—Decay columns slowly expand, in some cases shrinking sapwood area and causing physiological stress and slower growth. Decay weakens wood, which can lead to structural failure of stems and branches. In devel- oped areas, this creates hazard to people or structures. The softened inner wood also creates habitat suitable for excavation of cavity nests by birds (fig. 2). Fomitiporia punctata is the most important stem-decay fungus on green ash in North Dakota, and it also occurs in Nebraska (fig. 3).Phellinus igniarius Figure 2. Two cavity nest openings on ash. occurs throughout the Great Plains on a variety of hosts (fig. 4). In Ne- Stem-decay fungi create conditions necessary for excavation of cavity nests. Photo: Jim Wor- braska, Perenniporia fraxinophila is the most important stem-decay fungus rall, USDA Forest Service. Forest Health Protection Rocky Mountain Region • 2011 Stem Decays inthePlains ofHardwoods 2 -page Table 1. Descriptions of conks of three stem-decay fungi. Forest HealthProtection Forest Decay fungus Shape Surface features Other features Phellinus igniarius Projects up to 4 3/4 inches (12 cm); Upper surface gray to black, hair less, Conk is hard and woody. Interior is dark reddish brown; upper surface curved down; rarely becoming deeply cracked and crusty. tubes with bits of white tis sue. almost flat. Lower surface is Lower surface pale to dark; cinnamon gener ally flat or angled to purplish brown. Pores on lower slightly upward. surface are fine. Fomitiporia punctata Spreads flat on the surface, but Margin is at first yellowish brown, Interior is dark red dish brown. often with a cap on the upper side. becoming black and cracked. Pore surface is yellowish to grayish brown. Pores are very fine. Perenniporia fraxinophila Varies from projecting up to Upper surface may be reddish brown Interior is buff to pale yellowish brown, corky. Rocky Mountain Region•2011 2 3/4 inches (7 cm) with a dis tinct initially, becoming cracked, crusty, cap to com pletely flat on and grayish black. Pore surface is the surface. ivory to buff; larger pores than the other species. Stem Decays of Hardwoods in the Plains - page 3 Figure 4. Small conk of Phellinus igniarius. Photo: Jim Worrall, USDA For- est Service. Figure 3. Conks of Fomitiporia punctata. Photo: Mike Schomaker, Colorado State Forest Service, Bugwood.org. in green ash, fruiting on about 5.5% of trees in windbreaks (as of 1978) and about 10% of the green ash in wood- lands (as of 1981). Incidence of the latter fungus increased consistently with tree diameter, and infected trees had an average of nine conks. The incidence of infection and amount of decay in infected trees increased with tree age. Management—Wound prevention is the primary preventive approach to stem decays. Wounds from vehicles, machinery, livestock, and fire increase the likelihood of stem decay. Prune early when branches are small. Pruning should be done properly to minimize damage and maximize the ability of callus to cover the wound quickly. Stud- ies have not shown any benefit from wound dressing treatments and, in some cases, have shown negative effects, so they are not recommended. Where hazard to people and property is a consideration, removal of infected trees is recommended. 1. Bedker, P.J.; O’Brien, J.G.; Mielke, M.M. 1995. How to prune trees. NA-FR-01-95. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Area, State and Private Forestry. 12 p. Online: http://www.na.fs.fed.us/Spfo/pubs/ howtos/ht_prune/htprune.pdf. 2. Riffle, J.W.; Conway, K.E. 1986. Phellinus stem decays of hardwoods. In: Riffle, J.W.; Peterson, G.W., tech. coords. Diseases of trees in the Great Plains. Gen. Tech. Rep. RM-129. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 149 p. 3. Riffle, J.W.; Ostrofsky, W.D.; James, R.L. 1981. Fomes fraxinophilus on green ash in Nebraska windbreaks. Plant Disease 65:667-669. 4. Riffle, J.W.; Sharon, E.M.; Harrell, M.O. 1984. Incidence of Fomes fraxinophilus on green ash in Nebraska woodlands. Plant Disease 68(4):322-324. 5. Riffle, J.W.; Walla, J.A. 1986. Perennial wood-rotting fungi that cause stem decays of hardwoods. In: Riffle, J.W.; Peterson, G.W., tech. coords. Diseases of trees in the Great Plains. Gen. Tech. Rep. RM-129. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 149 p. 6. Walla, J.A. 1984. Incidence of Phellinus punctatus on living woody plants in North Dakota. Plant Disease 68:252-253. Forest Health Protection Rocky Mountain Region • 2011.