| Three studies examined Septoria musiva, a fungal pathogen causing severe damage in North American Populus plantations.;The second study used the P. x jackii - Septoria leaf spot pathosystem to test the 'hybrid susceptibility' and 'phenological sinks' hypotheses. Sixty-nine clones were sampled along a 400 km transect of the Red Deer River and used in inoculation and common garden experiments. In the inoculation experiment, hybrids exhibited 'dominance of susceptibility', with P. x jackii being as susceptible as the susceptible parent, P. balsamifera, and both being more susceptible than the resistant parent, P. deltoides. The common garden experiment did not support the 'phenological sinks' hypothesis; However, mixed stands of hybrids and pure parent types were more variable in date of first leaf emergence than stands of single parent types.;In the third study, greenhouse inoculation experiments demonstrated the reliability and repeatability of an inoculation method that did not require wounding. In the first study, 16 clones of hybrid poplar were inoculated with three isolates of S. musiva either with or without wounding. Canker severity was strongly correlated between the two methods. Ninety percent of clones with a mean disease severity greater than 4, following wound inoculation, had at least one stem lesion, following spray inoculation. In two separate studies, this method allowed detection of clonal differences in resistance among a total of 98 clones inoculated with three isolates.;The first study investigated the cause of an outbreak of stem cankers in a clonal trial of Populus balsamifera. In a greenhouse experiment, four and five clones of P. balsamifera and Populus deltoides, respectively and one hybrid clone (Northwest) were inoculated with seven S. musiva isolates from Alberta or Quebec. All clones were infected and the mean disease severity of P. balsamifera (x¯ = 4.2) was greater than P. deltoides (x¯ = 2.76) and Northwest ( x¯ = 3.45). Virulence was similar between Alberta and Quebec isolates. Molecular analysis indicated that the outbreak was not caused by a single isolate. In the field, canker age distribution indicated that the outbreak likely resulted from an inoculum buildup rather than a 'wave year'. |
