Disease

Scleroderris canker caused by the fungus, Gremmeniella abietina.

Importance

Scleroderris canker has killed many trees in conifer plantations and forest nurseries in the northern areas of the Lake States. The fungus can be transported on nursery stock to field locations where new pockets of infection develop. Two strains of the fungus are known in North America. The North American strain, present in scattered locations in the northern Lake States and Northeastern States, attacks young trees but does little damage to trees taller than 7 feet (2 m) (fig. 13). The European strain has been found in several New England States where it has killed many trees of all ages in red pine plantations (fig. 14). The European strain is an extremely serious hazard to pine forests. Because of this potential hazard, forest managers in the Lake States need to know how to avoid major losses from this disease.

Figure 13	Figure 14

Biology

Figure 15

In the North American strain, primary infection is by windblown spores (ascospores). These are disseminated during moist weather from April to October; the major spore discharge and infection are in June and July. The spores infect through buds or needles, and infected branch tips usually die by the following summer. A characteristic symptom during May and June is an orange discoloration at the base of otherwise green needles (fig. 15). The fungus may grow down the branch and into the main stem of the tree where a canker commonly forms (fig. 16). The stem of a young seedling is quickly girdled, and the tree is killed.

Figure 16

A few months after an infected branch dies, fruiting bodies (pycnidia) appear near the base of dead needle fasicles. Asexual spores (conidia) ooze out of these pycnidia during wet weather from April to October. These spores are transported by rain splash and wind to nearby branches, spreading the infection in individual trees and onto adjacent trees. Fruiting bodies (apothecia) appear in early summer on branches that have been dead for 1 to 2 years (fig. 17). The apothecia are also commonly found at the base of dead needle fasicles and are often found in association with the pycnidial stage.

Figure 17

Management Guidelines

The best way to prevent the introduction of the dangerous European strain into the Lake States is to make sure that infected nursery stock, Christmas trees, or other infected material is not introduced.

Nurseries

• Do not ship infected nursery stock to the field. This will prevent the establishment of new infection centers.

• Protect red pine nursery stock where there is a potential Scleroderris canker hazard. The fungicide chlorothalonil is registered for nursery use and has proved effective. Apply the fungicide every 10 to 14 days during the major infection period in May or June. Some infection may occur at other times during the growing season, and nursery managers wanting to produce disease-free stock should keep a protec-tive fungicide on seedlings from early spring until the seedlings are covered with snow in late fall.

• Remove infected trees within 2,200 feet (700 m) of nursery beds to reduce chances of stock becoming infected from windblown fungus spores.

• It possible, produce red pine nursery stock in areas where Scieroderris canker is absent.

• Avoid planting red pine seedlings adjacent to infected pine windbreaks.

• Pick red pine planting sites where Scleroderris canker is not present.

• If the planting site is in a Scleroderris canker hazard area, select a species for planting that is immune or has a low susceptibility. Hard pines are highly sus-ceptible, but spruces, cedars, firs, and larches are more resistant to infection.

• Avoid geographic depressions, such as frost pock-ets, where the disease is most damaging.

• Select planting stock from a nursery outside of the range of Scleroderris canker or from a nursery that has an effective fungicide control program.

• Do not plant infected seedlings.

• In high-value stands, prune lower branches on infected and healthy trees.

• Do not ship infected red pine Christmas trees into areas where Scieroderris canker is not present.

• Abide by all quarantine laws designed to prevent the introduction of the European strain into areas where it is not present.

Dorworth, C.E. 1979. Stand reduction of red pine by Gremmeniella abietina. Canadian Journal of Forest Research. 9: 316-322.

Skilling, D.D. 1984. How to prevent conifer nursery and plantation damage by Scleroderris canker. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 6 p.

Skilling, D.D.; O'Brien, J T. 1979. How to identify Scleroderris canker. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 6p.

Skilling, D.D.; O'Brien, J T. 1979. Scleroderns canker of northern conifers. For. Insect and Dis. Leafl. 130. Washington DC: U.S. Department of Agriculture, Forest Service. 7p.

Skilling, D.D.; Kienzler, M. 1983. A serological procedure for identifying strains of Gremmeniella abietina. Gen. Tech. Rep. NC-87. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 15p.

Skilling, D.D.; Schneider, B.; Fasking, D. 198& Biology and control of Scleroderris canker in North America. Res. Pap. NC-275. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 19p.