Populus nigra, commonly used in hybrid poplar breeding programs in the north-central United States, is susceptible to Septoria stem canker, caused by Sphaerulina musiva. In this study, two experiments were conducted to (i) characterize the variation in resistance of 47 genotypes of P. nigra collected from seven locations in Europe in terms of cankers per centimeter and disease severity score; (ii) determine whether location, isolate, genotype, or their interactions significantly affect cankers per centimeter and disease severity score; and (iii) examine the correlation of disease severity score between single-isolate and bulk-isolate inoculations. The majority of the variation in resistance for cankers per centimeter was explained by location (72%; P < 0.001) followed by genotype(location) (28%; P < 0.001). In contrast, the majority of the variation in disease severity score was explained by genotype–location (51%; P < 0.001) followed by location (26%; P = 0.025). The disease severity score model also indicated the presence of a significant isolate effect (P = 0.034) and genotype(location) × isolate interaction (P = 0.004). The correlation coefficients for disease severity score indicated a significant range of correlations (r = 0.871 to r = 0.952) when correlating single-isolate to bulk-isolate inoculations.
Bibliographical noteFunding Information:
This study was supported by the United States Department of Agriculture National Institute of Food and Agriculture USDA-NIFA-RIPM 2012-34103-19771 grant. Work on collection and establishment of field tests was supported by the United State Department of Energy, Sun Grant Initiative Regional Biomass Feedstock Partnership through a subcontract from South Dakota State University. A Graduate Research Assistantship was provided to K. L. Dunnell from North Dakota State University-Agriculture Experiment Station. We thank J. Walla, A. Nepal, R. Qin, and L. Rasmussen for technical assistance.