Variation in resistance of populus nigra to Sphaerulina musiva in the north-central United States

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.