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Ecology and Epidemiology

Disease Gradients of Fusiform Rust on Oak Seedlings Exposed to a Natural Source of Aeciospore Inoculum. Robert A. Schmidt,, Professor of forest pathology, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville 32611; William A. Carey(2), and Charles A. Hollis(3). (2)(3)Graduate research assistant, and associate professor of forest physiology, respectively, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville 32611; (2)Present address: Graduate assistant, Scholl of Forestry and Environmental Studies, Duke University, Durham, NC 27706; (3)Present address: Section leader, Pine Management, International Paper Company, Southlands Experimental Forest, Bainbridge, GA 31717. Phytopathology 72:1485-1489. Accepted for publication 16 April 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1485.

Disease gradients on oak seedlings resulting from infection by aeciospores of Cronartium quercuum f. sp. fusiforme were quantified from the number of sori (uredia and telia) per square centimeter of oak leaf surface area occurring at increasing distances from an abundant source of sporulating pine galls. Averaged data from six gradients—two each during early (10 February–3 March), middle (7–19 March), and late (19 March–1 April) phases of aecial sporulation—indicated that 12.4, 5.6, 5.9, 4.2, 1.8, and 0.9 sori per square centimeter of oak leaf surface area (y) occurred at distances from the source (x) of 3.1, 15.2, 30.5, 61.0, 91.4, and 152.4 m, respectively. These data fit a decreasing exponential function, y=9.78 e^–0.016x; r² = 0.94. Transformation of y yielded the linear equation, log_10y = 0.99 –0.007x; r² = 0.94. The number of sori occurring on oak was reduced ~90% in the distance of 3.1 to 152.4 m. Spore traps located at 15.2, 30.5, and 91.4 m (x) from the aeciospore source yielded relative catches of 537, 378, and 212 spores (y), respectively. These data fit the power function y = 2,212x^–0.52; r² = 0.99 and extrapolation suggests that the numbers of aeciospores decreased ~87% over the distance 3.1 to 152.4 m. These data indicate that the preponderance of aeciospores are deposited near the source and suggest that local sources of aeciospore inoculum are most important in the fusiform rust epidemic.