Pathogenicity of 14 isolates of Phoma sclerotioides pathogenic to alfalfa

Fourteen isolates of Phoma sclerotioides were compared for pathogenicity in producing root rot in alfalfa. Thirteen isolates were obtained from diseased alfalfa plants collected in Wyoming and one was from Canada (ATCC No. 56515). Barley grain inocula of each of the 14 isolates were placed adjacent to the upper root of 4-6 month old potted alfalfa plants. Following inoculation, plants were placed outside the greenhouse for a winter exposure period. In the spring, plants were given a Disease Severity Rating (DSR) for Brown root rot on a scale of 1-5 (1 = healthy, 5 = severe root rot, plant dead). Surviving plants were placed outside for a second winter and again rated for Brown root rot the following spring. The entire experiment was repeated once. Final DSRs for experiments 1 and 2 were 4.83 and 4.53, while final mortality was 91% and 59.5%. Although all 14 isolates were pathogenic to alfalfa, some appeared to be more pathogenic than others. WY isolate No. 2, being used in our breeding program, ranked 5th with a DSR of 4.17. This isolate is available from the American Type Culture Collection, No. MYA-295.