First Report of Brown Root Rot on Alfalfa Caused by Phoma sclerotioides in the Continental United States.

Abstract

Phoma sclerotioides G. Preuss ex Sacc. (previously named Plenodomus meliloti Dearn. & G.B. Sanford) is associated with root rot and extensive winterkill of leguminous forage crops, such as clover (Trifolium and Melilotus spp.), sainfoin (Onobrychis viciifolia), and alfalfa (Medicago sativa). Winterkill and root rot of irrigated alfalfa were observed for the first time in a field of cv. Multiplier in western Wyoming during the spring of 1996. Dark brown to black, sunken, rotting lesions were noted on upper secondary roots and taproots of dead and living diseased plants. Superficial and embedded beaked pycnidia and pycnosclerotia were observed near root lesions. A Phoma sp. isolated from a diseased plant in Farson, WY, was maintained on potato dextrose and half-strength V8-juice agars. Beaked pycnidia, typical of P. sclerotioides, were observed in culture when grown at 10°C for 2 months. A pathogenicity test was performed on cv. Multiplier. Two barley seeds colonized by a Phoma sp. derived from a Wyoming isolate were positioned on taproots of healthy, greenhouse-grown, 5-month-old plants ≈2.5 cm below the crown and were covered with a small piece of sterile cotton. Three replicate samples (24 plants inoculated and 24 plants uninoculated per replicate) were winter-hardened for 4 weeks (15.6°C/10°C, day/night, for 2 weeks, followed by 10°C/7.2°C, day/night, for 2 weeks) and placed outside during January 1998 in Laramie, WY, for a 4-month winter exposure period. Plants were rated for disease during June 1998. A disease severity rating of 1 to 5 was assigned to each experimental unit, where 1 = no disease and 5 = dead plant. The percentage of diseased plants at each severity rating for all inoculated plants was 1 = 19%, 2 = 33%, 3 = 31%, 4 = 13%, and 5 = 4%. Mycelium typical of P. sclerotioides was found on 99% of inoculated plant roots whether or not they had pycnidia. Pycnidia were found on the lower stems and petioles of some inoculated plants. Three percent of control plants also developed brown root rot (BRR) symptoms (taproot lesions or discoloration) by June 1998. The percentage of diseased plants at each severity rating for all uninoculated plants was 1 = 96%, 2 = 4%, and 3 through 5 = 0%. Aboveground propagule placement likely contributed to the spread of BRR by raindrop splash and wind-driven plant debris to adjacent alfalfa. Most inoculated plants had immature pycnidia or protopycnidia (94%), whereas 6.9% of the plants also had fully mature, beaked pycnidia. Pure fungal cultures were obtained from several diseased roots and compared with the original Wyoming Phoma sp. culture and a Canada isolate of P. sclerotioides (ATCC no. 56515) (2): colony, pycnidial, and conidial morphologies were identical, completing Koch's postulates. This is the first report of BRR on alfalfa in the continental United States.