A Nonwounding Greenhouse Screening Protocol for Prediction of Field Resistance of Hybrid Poplar to Septoria Canker

Populus spp. and their hybrids are short-rotation woody crops which supply fiber to a diversity of industries in North America. The potential of hybrid poplar trees has been limited by the fungal pathogen Septoria musiva, the cause of leaf spot and stem canker of Populus spp. An inoculation protocol that does not rely on stem wounding to achieve infection was recently developed to screen poplar clones for resistance to Septoria canker. Prior to this study, the relationship between results obtained using this inoculation protocol and long-term field resistance of clones was unknown. Young ramets of 14 clones of hybrid poplar that were previously assigned to long-term canker damage categories (low, intermediate, and high) were inoculated with a conidial suspension of three isolates of S. musiva under greenhouse conditions. Three weeks post inoculation, lesion number, lesions per centimeter of stem length, area of stem that was necrotic, and proportion of stem area that was necrotic were measured. Logistic regression with lesion number and proportion necrotic area correctly predicted long-term disease damage categories for 11 of 14 clones tested, including the most resistant (NM6) and the most susceptible (NC11505) clones, demonstrating that this screening protocol is a promising method for prediction of long-term disease impact of the most resistant clones.