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Effects of Water Stress on Colonization of Poplar Stems and Excised Leaf Disks by Septoria musiva

Septoria musiva causes leaf spot and canker diseases of trees in the genus Populus, and is one of the most damaging fungal pathogens of hybrid poplar in eastern North America. The effect of host water stress on Septoria canker development was studied in two separate greenhouse experiments. Hybrid poplar clones NM6, NC11396, and NE308 were stressed by withholding water until predawn water potential fell below −1.0 MPa. Stems were treated by removing a leaf and applying agar plugs that were either colonized by S. musiva (inoculated) or sterile (control) to the wound. Cankers on inoculated water-stressed trees were significantly larger than those on nonstressed trees. A leaf disk assay also was conducted three times with the NM6 and NE308 trees. We cut two disks from each of 120 stressed and 120 well-watered trees, placing them on water agar in 24-well tissue culture plates. A conidial suspension was applied to one disk in each pair and sterile water to the other. Inoculated disks from water-stressed trees developed less necrosis than those from well-watered trees. These results demonstrate that environmental influences on host condition must be considered in evaluating resistance of clones proposed for widespread culture of hybrid poplar.