Studies on the Biology of Brown Root Rot (Phoma sclerotioides) of Alfalfa in Wyoming. Charla R. Hollingsworth¹, Fred A. Gray², Tom E. Heald³ & Dave W. Koch^{4 1,2,4}Department of Plant Sciences, and ³Cooperative Extension Service University of Wyoming, Laramie, WY 82071
Brown root rot (BRR) of alfalfa caused by the soil-borne fungus Plenodomus sclerotioides (now Phoma sclerotioides) was first reported from Alberta and Saskatchewan, Canada (Sanford, 1933). Symptoms consist of brown, slightly sunken, necrotic lesions, which appear on the tap or lateral roots and rootlets of mature plants (Davidson, 1990). Black pycnidia (asexual fruiting bodies) are frequently present on or in necrotic root tissue. Plants with severe root rot may winter-kill or have poor spring growth. If secondary lateral roots are produced, diseased plants may partially recover. Fields with BRR may show severe winter-kill during certain years. BRR was tentatively identified in Wyoming from 3-year old alfalfa stands showing severe winter-kill in the spring of 1996 (Gray, et al., 1997). This disease had not previously been reported from the USA. BRR has since been identified in over 2,500 acres of irrigated alfalfa in southwestern Wyoming. Isolation of diseased plant roots resulted in a pycnidial fungus which was later identified as P. sclerotioides when grown on potato dextrose agar at 10°C (50°F). In vitro growth of P. sclerotioides occurs best on dilute V-8 juice agar. Pycnidia and pycnidiospore production, necessary for positive identification of the fungus, require a 6-8 week growth period at 10°C (50°F). Phoma sclerotioides can be differentiated from P. medicaginis var. medicaginis and Stagonospora meliloti, both pycnidia producing fungal pathogens of alfalfa, by cultural and morphological characteristics. Inoculum, produced on sterile barley, remained viable for 10 months at -14°C (6°F) and for only 5 months at 24°C (75°F). In January of 1998, five-month-old alfalfa plants (cultivar 'Multiplier'), were inoculated and placed outside for an abbreviated winter season. A high frequency of root infection by P. sclerotioides was observed the following spring. Recent observations of a 7-year-old alfalfa cultivar trial located near Eden, Wyoming show a marked difference in alfalfa persistence in the presence of BRR. This suggests field resistance may be present in some USA-developed alfalfa cultivars.
References
Davidson, J. G. N. 1990. Brown root rot. pp 29-31, In, Compendium of alfalfa diseases. 2nd Ed., APS Press.

Gray, F. A., T. E. Heald, C. R. Hollingsworth, and D. W. Koch. 1997. Brown root rot caused by Phoma sclerotioides, a new disease of alfalfa in the Proceedings of the tenth Western Alfalfa Improvement Conference, pp.

Sanford, G. B. 1933. A root rot of sweet clover and related crops caused by Plenodomus meliloti Dearness and Sanford. Can. J. Res. 8:337-348.