Sphaeropsis shoot blight is a fungal disease that most frequently infects the shoots of Austrian, red, and Scotch pines. However, Sphaeropsis shoot blight is also able to infect ponderosa pine, white pine, spruces, and occasionally true firs. This disease, caused by the fungus Sphaeropsis sapinea, formerly known as Diplodia pinea, infects young, succulent shoots on trees of all ages causing death of the shoots. Repeated infections may eventually lead to death of the tree. Sphaeropsis shoot blight may also infect wounded stems or branches. These infections result in the formation of girdling cankers, which kill the outer portion of the branch or stem. In addition, Sphaeropsis shoot blight is able to infect and kill young seedlings.

Fig. 1  Photo: Chad Behrendt Fig. 2 Photo: Chad Behrendt Fig. 3 Photo: Chad Behrendt

Infection of young, succulent growth occurs in the spring as buds break and needles emerge. During wet periods in the spring, spores released from black fruiting bodies (pycnidia) are rain splashed and windblown to healthy trees where the fungus may directly penetrate the needles, shoots, and second year cones, or enter wounded stems and branches. Previous year's growth, including fully elongated needles, shoots, and bark tissue, is not susceptible to infection by Sphaeropsis shoot blight, unless it has been previously wounded. Infection may also occur later in the growing season as spores released from pycnidia enter wounded stems and branches. Wounds created by hail, pruning, and insects are common entry points for the fungus.

Infected shoots are usually killed before the needles fully expand, causing the new growth to appear stunted, curled, and discolored (orange-brown) (Fig. 1 & 2). In general, Sphaeropsis shoot blight will kill only the current year's growth. However, canker infections may cause substantial branch dieback. Cankers appearing on stems and branches usually contain resin exudation and staining of the sapwood. To distinguish between environmental factors such as drought or winter injury which may be confused with Sphaeropsis shoot blight, observe the pattern and number of shoots infected. Environmental factors usually affect all or many of the shoots, while Sphaeropsis shoot blight infections will be more random (Fig. 3). A quick way to detect Sphaeropsis shoot blight is to look with a hand lens for pycnidia on dead needles (under the fascicle), on shoots, and on the scales of cones. Pycnidia are typically formed in the fall or spring following their initial infection.

Stressed trees are usually more susceptible to Sphaeropsis shoot blight than are healthy, vigorous trees. Factors such as poor site, drought, insect activity, storm damage (hail), or excessive pruning predispose the tree to disease.

Control of Sphaeropsis shoot blight can be difficult since the fungus is able to survive for many years in needles, shoots, and cones remaining in the tree, as well as on the ground. Cultural control measures should include watering, pruning, mulching, and proper fertilization of weakened trees. Since drought is known to predispose trees to Sphaeropsis shoot blight it is important to water trees to alleviate drought stress. Pruning will help reduce the number of new infections and the cosmetic appearance of the tree, but it is of limited value for controlling disease. Fungicides containing thiophanate-methyl (Cleary's 3336) or copper will most likely be required for complete control. Fungicide applications should begin when the new growth starts. It is essential, when spraying, to obtain complete coverage of the new shoots and buds.

When deciding to plant a new tree, try to use disease-free stock, and avoid planting pines on poor sites.

REFERENCES

French, D. W. 1991. Forest and Shade Tree Pathology. University of Minnesota. 276 pp.
Sinclair, Lyon, and Johnson. 1987. Diseases of Trees and Shrubs. Cornell University Press. 574 pp.
Tainter, F. H. and Baker, F. A. 1996. Principles of Forest Pathology. John Wiley and Sons, Inc., New York. 805 pp.

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