EFFECTS OF ENVIRONMENTAL-STRESS ON THE DEVELOPMENT OF CYTOSPORA CANKER OF ASPEN

Greenhouse, field, and laboratory studies examined the role of selecte d environmental stresses on the development of Cytospora canker of asp en trees. In greenhouse studies, we examined the resistance of aspen t o Cytospora chrysosperma after exposure to drought, flooding, or defol iation. Drought stressed trees had larger cankers than control trees, whereas flooded trees did not. Water potential of trees was a signific ant covariant that explained variation in canker size. Severely defoli ated 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 pot ential. Peak susceptibility to canker expansion occurred when water po tential dropped below -1.6 MPa. Relative turgidity was not associated with canker size. In vitro growth of C. chrysosperma was affected posi tively by decreasing osmotic- and matrix-based water potential until w ater potentials were lowered to -1.0 MPa. Below -1.0 MPa, fungal growt h was affected negatively.