ABSTRACT
A set of isolates of Septoria musiva differed in aggressiveness in hybrid poplar leaf disk and stem assays and culture growth in vitro. Clone × isolate interactions were observed in one of the stem assay experiments, but not in the leaf disk assay experiments. Random amplified polymorphic DNA (RAPD) analyses were performed using 52 isolates of S. musiva collected from hybrid poplars and a native poplar species in Minnesota, Wisconsin, Iowa, South Dakota, and North Dakota. There was a large degree of genetic similarity, although each isolate had a unique RAPD pattern. No relationships among isolates were found for molecular genetic distance and host clone, parentage, or taxonomic classification section; location or date of collection; or the previously determined level of field resistance of the host clones to Septoria canker. Results of the stem and leaf disk assays indicate that it may not be necessary to choose the most aggressive isolate for disease resistance screening. It may be more useful to select isolates that will discriminate the greatest variation in levels of disease resistance among the clones that are being screened.
