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Effects of Environmental Stress on the Development of Cytospora Canker of Aspen. J. C. Guyon, Former Graduate Student; Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins 80523. W. R. Jacobi, Professor, and G. A. Mclntyre, Professor, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins 80523. Plant Dis. 80:1320- J 326. Accepted for publication 29 August 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1320.

Greenhouse, field, and laboratory studies examined the role of selected environmental stresses on the development of Cytospora canker of aspen trees. In greenhouse studies, we examined the resistance of aspen to Cytospora chrysosperma after exposure to drought, flooding, or defoliation. Drought-stressed trees had larger cankers than control trees, whereas flooded trees did not. Water potential of trees was a significant covariant that explained variation in canker size. Severely defoliated trees (75 to 100%) had larger cankers than nondefoliated control trees or trees with 50% defoliation. Carbohydrate content of roots of defoliated trees was significantly less in 100% defoliated trees than in 75 and 50% defoliated trees. Canker size on field-inoculated aspen and cottonwood (cv. Siouxland) was related inversely to tree water potential. Peak susceptibility to canker expansion occurred when water potential dropped below -1.6 MPa. Relative turgidity was not associated with canker size. In vitro growth of C. chrysosperma was affected positively by decreasing osmotic- and matrix-based water potential until water potentials were lowered to –1.0 MPa. Below –1.0 MPa, fungal growth was affected negatively.