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Ecology and Epidemiology

A Conducive Day Concept to Explain the Effect of Low Temperature on the Development of Scleroderris Shoot Blight. M. Marosy, Department of Plant Pathology (Marosy and Patton) and ARS/USDA and Department of Plant Pathology (Upper), University of Wisconsin, Madison 53706, Current address: USDA-Forest Service, Forest Pest Management, 630 Sansome St., San Francisco, CA 94111; R. F. Patton, and C. D. Upper. Department of Plant Pathology (Marosy and Patton) and ARS/USDA and Department of Plant Pathology (Upper), University of Wisconsin, Madison 53706. Phytopathology 79:1293-1301. Accepted for publication 20 June 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/Phyto-79-1293.

The in vitro growth of Gremmeniella abietina at –6 C both in the presence or absence of ice crystals was confirmed. Red pine seedlings artificially inoculated with the North American serotype, exposed to natural field conditions and artificially manipulated field conditions, developed symptoms of Scleroderris shoot blight when exposed to 44 or more days in which the temperature remained between –6 and +5 C or snow completely covered the seedlings or tree parts—a conducive day. Thus, an extended period of relatively mild canopy temperature during the winter appears to favor disease development. The conducive period—a period in which 44 or more conducive days occurred—could either occur in the winter after inoculation, or over the two winters after inoculation. The apparent latent period in the disease cycle may result from the need for winter conditions before symptom development can occur. The occurrence of symptoms primarily on lower branches, and the restriction of the disease to latitudes that receive sustained snow cover in the Lake States are consistent with this observation. Comparison of literature descriptions of outbreaks of the disease to weather records revealed a strong association between conducive periods, usually single conducive winters, and the occurrence of symptoms. The natural range of the disease may be restricted by the need for recurrence of conducive periods within 3 yr to avoid breaking the disease cycle.