DOTHISTROMA NEEDLE BLIGHT
Kathryn J. Bevacqua
Dothistroma needle blight, caused by the fungus Mycosphaerella pini (imperfect stage Dothistroma septospora), can affect Austrian pine (Pinus nigra), ponderosa pine (P. Ponderosa), and mugo pine (P. Mugo). Austrian pine and other young pines are most seriously affected, while red pine (P. Resinosa) and Scots pine (P. Sylvestris) are generally more resistant.
Photos: Chad Behrendt
Needles infected the previous year that remain on the tree, release spores during cool, rainy weather in the spring. These spores are windblown or rain-splashed to second-year or older needles, where new infections begin. Infected needles that have fallen to the ground are not a significant source of inoculum. Symptoms initially appear as yellow to tan colored spots on the needles [Fig. 1]. These spots enlarge into brown or reddish brown bands that encircle and girdle the needle. Girdling causes death of the needle from the point of infection outward, while the base of the needle often remains green [Figures 2 & 3]. The transition from green to dead areas is abrupt. New growth is not affected until midsummer. In the fall, tiny black fungal fruiting bodies appear in the bands or in dead areas of the needles. These fruiting bodies will release spores the following spring to begin a new disease cycle.
Dothistroma needle blight is sometimes confused with winter injury. Needles killed as a result of winter injury die back evenly, while dieback as a result of disease is uneven. Also, Dothistroma generally affects lower branches, whereas winter injury is most severe on the south side of trees above the snow line. Dothistroma can also be confused with other needle diseases. It is important to determine accurately which needle disease is affecting a tree because the control recommendations may be different. Often, an accurate diagnosis can be made only by examining spores under a microscope.
Several cultural practices, such as proper site selection, watering, mulching, and fertilizing, can reduce the severity of Dothistroma. Since the disease flourishes under wet conditions and high relative humidity, trees should be adequately spaced when planted to ensure good air movement around them. Do not plant trees in low-lying areas with poor drainage; control weeds under the trees. Also, the lower whorl of branches may be removed to help increase air circulation.
Only fungicides containing copper are currently labeled for use. The first application should be made as the new growth emerges, and a second application three to four weeks later.
Koepsell, Paul A. and Jay W. Pscheidt, editors. Pacific Northwest 1994 Plant Disease Control Handbook. Extension services of Oregon State University, Washington State University, and the University of Idaho. 1994
Peterson, Glenn. Dothistroma Needle Blight of Pines, Forest Insect & Disease Leaflet 143. U.S. Department of Agriculture Forest Service. 1982.
Riffle, Jerry W. and Glenn W. Peterson. Diseases of Trees in the Great Plains, General Technical Report RM-129. Fort Collins: USDA, Forest Service, Rocky Mountain Forest and Range Experiment Station. 1986.
Sinclair, Wayne A., Howard H. Lyon, and Warren T. Johnson. Diseases of Trees and Shrubs. Ithaca: Cornell University. 1987.
Chad Behrendt, Crystal Floyd