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 fit were wounded and inoculated with water agar plugs colonized by isolate s of the two random amplified polymorphic DNA (RAPD) marker groups (A and B ) of Sphaeropsis sapinea. Isolates were obtained from hosts in Michigan, Mi nnesota, and Wisconsin. Symptom severity (distance from the inoculation sit e at which necrotic needles were present) resulting from inoculations with each group A isolate exceeded that from inoculations with each group B isol ate 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 sym ptomatic and asymptomatic seedlings inoculated with isolates of either grou p. 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 hos ts.