Trees in the genus Populus and their interspecific hybrids are used across North America for fiber production and as a potential source of biofuel. Plantations of these species are severely impacted by a fungal pathogen, Sphaerulina musiva, the cause of leaf spot and stem canker. An inoculation protocol that does not rely on stem wounding to achieve infection was recently developed. Using this protocol two experiments were conducted to examine infection biology and disease development in the S. musiva-Populus interaction. In the first experiment non-wounded stems of one moderately resistant clone (NM6) and one susceptible clone (NC11505) were inoculated and examined by scanning electron microscopy at six different times (6 h, 12 h, 24 h, 72 h, 1 week, and 3 weeks) post-inoculation. The images indicate that the pathogen appears to enter host tissue through small openings and lenticels and that there are no significant differences in the penetration rate between the moderately resistant (NM6) and susceptible (NC11505) clones at 12 h post-inoculation. In a second experiment a histological comparison of stem cankers for resistant clone DN74 and susceptible clone NC11505 were conducted at three time points (3 weeks, 5 weeks, and 7 weeks) post-inoculation. Distinct differences in disease development were apparent between the resistant and susceptible clones at each time point, with the susceptible clone exhibiting a weak and delayed defense response. These results suggest, that following penetration, the pathogen may be able to interfere with the defense response in the susceptible host.

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.

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