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Tissue Culture and Leaf Spot Bioassays as Variables in Regression Models Explaining Hypoxylon mammatum Incidence on Populus tremuloides Clones in the Field. R. R. Bélanger, Graduate students, College of Environmental Science and Forestry, State University of New York, Syracuse 13210, Present address: Dept. de Phytologie, FSAA, Université Laval, Ste-Foy, Québec G1K 7P4; S. P. Falk(2), P. D. Manion(3), and D. H. Griffin(4). (2)(3)(4)Graduate students, and professors, College of Environmental Science and Forestry, State University of New York, Syracuse 13210, (2)Present address: Department of Plant Pathology, The Ohio State University, Wooster 44691. Phytopathology 79:318-321. Accepted for publication 16 September 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-318.

Regression models were used to interpret the relationships among sensitivity of aspen to metabolites produced by Hypoxylon mammatum, disease incidence in the field, and several other clone and site variables. Twenty-nine naturally occurring aspen clones in central New York were intensively surveyed. Dormant buds of 10 clones representing the range of variation in infection were cultured and bioassayed with culture filtrates of H. mammatum to compare with bioassays of leaves collected in the field. Bioassay systems based on tissue culture and field collections demonstrated the occurrence of clonal differences in response to metabolites produced by H. mammatum. High positive correlations among bioassays, with various fungal isolates, supported the conclusion that clonal response to metabolites is genetically controlled. However, low correlations of toxin assays with various measures of disease did not support the theory of a direct relationship between disease incidence and toxin assays. Rather, an interaction of the toxin assay with other clonal and site variables in regression models was required to explain the variation in disease incidence in the field.