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Differences in Aggressiveness of Sphaeropsis sapinea RAPD Marker Group Isolates on Several Conifers

Seedlings of Scot's pine varieties East Anglia and Austrian Hills, red pine, mugho pine variety Pumileo, Colorado blue spruce, Douglas-fir, and balsam fir were wounded and inoculated with water agar plugs colonized by isolates of the two random amplified polymorphic DNA (RAPD) marker groups (A and B) of Sphaeropsis sapinea. Isolates were obtained from hosts in Michi-gan, Minnesota, and Wisconsin. Symptom severity (distance from the inoculation site at which necrotic needles were present) resulting from inoculations with each group A isolate exceeded that from inoculations with each group B isolate on all hosts except Colorado blue spruce. Hosts varied considerably in their responses to group A isolates. Based on symptom severity, East Anglia Scot's pine was most susceptible and balsam fir was least susceptible when inoculated with group A isolates. The pathogen was recovered from both symptomatic and asymptomatic seedlings inoculated with isolates of either group. Results emphasize the importance of characterizing a RAPD marker group(s) of S. sapinea encountered in the field or used in research; the need for comparative evaluations of resistance among coniferous genera, species, and varieties to S. sapinea of both groups; and the potential for asymptomatic persistence of S. sapinea from both groups in or on several coniferous hosts.