RESISTANCE, TOLERANCE, AND YIELD OF WESTERN BLACK COTTONWOOD INFECTED BY MELAMPSORA RUST

Potted ramets of 14 western black cottonwood (Populus trichocarpa Torr. & Gray) clones from southern British Columbia were inoculated with Melampsora occidentalis H. Jacks. to produce a range of disease severities, and their size and dry weight were determined after 1 or 2 years. Response to inoculation varied significantly between clones. Clones from drier interior locations were less resistant than those from coastal or moister interior locations. Local- (within leaf) or systemic-induced resistance was not detected. Yield (total dry weight) decreased linearly with disease severity. Percent reduction in yield was greater than the cumulative percent leaf area infected for all clones. Yield losses were substantial: dry weights of ramets with disease severity levels similar to those experienced by natural cottonwood populations were about 75% of controls; heavily infected ramets were < 50% of controls. Stem:root ratios increased rapidly with increasing disease severity in all clones, and at significantly different rates. Severe infection resulted in substantial mortality in the following winter and reduced initial stem volume growth in the following growing season. Tolerance, defined as the relationship between disease severity and yield, varied significantly between clones. The most significant discovery of this study was that tolerance and resistance were correlated, greater tolerance being associated with reduced resistance. The concepts of resistance, tolerance, and disease hazard, as quantified in this study, can be used to predict yield and to select the most appropriate clones for different disease hazard conditions.