Brown root rot (BRR) of forage legumes [caused by Phoma sclerotioides G. Preuss ex Sacc. (previously Plenodomus meliloti Dearn. and G.B. Sanford)] was reported on the North American continent from Alaska and the Northwest Territories to nearly the Canada/U.S. border prior to 1997 (Davidson, 1990). Brown root rot has been identified on common clover (Trifolium spp.), sweet clover (Melilotus spp.), alsike clover (T. hybridum), and alfalfa (Medicago sativa ssp. sativa) from locales with extended, severe winters (Sanford, 1933). Research was initiated in Wyoming during the spring of 1996 in response to winter mortality of irrigated alfalfa plants in the Green River drainage basin. Alfalfa symptoms included dark, necrotic lesions of the lateral and taproots. Young lesions were generally brown and circular with dark perimeters. Maturing BRR lesions generally expand asymmetrically and eventually girdle the root (Davidson, 1990). Two pathogenicity studies were conducted over the 1998 and 1999 winters in Laramie, Wyoming. Barley seed, colonized by Phoma sclerotioides, were used as a BRR inoculum source. Five-month-old, greenhouse-grown alfalfa (cv. Multi-plier) plants were divided into three replicates with 48 plants per replicate. Twenty-four plants in each replicate were BRR inoculated. Plants were winter-hardened and placed outside for four months of Laramie, Wyoming, winter and spring conditions. During early summer each plant received a disease severity rating from 1 to 5 where 1 = no symptoms and 5 = dead plant. Later that year plants were again exposed to seven months of winter and spring conditions and rated for BRR disease severity. A second pathogenicity study was conducted on sixteen-month-old BRR susceptible alfalfa plants (Multi-plier) and resistant plants (Peace). Seventy-two plants from each cultivar were inoculated with BRR while 72 were uninoculated. Plants were placed outside for seven months of winter and spring conditions. Disease severity ratings were assigned during mid-summer. A chi-square goodness-of-fit test (a < 0.01) indicated an absence of replicate interactions the first year of pathogenicity study 1. Data from the second year of pathogenicity study 1 and pathogenicity study 2 were analyzed with three-dimensional loglinear models due to significant interactions. Inoculated Peace plants had an overall winter mortality rate of 67% compared with a mortality rate of 90% for inoculated Multi-plier plants. Loglinear odds ratios indicated Peace plants were six times more likely to remain symptom-free (DSR = 1) and half as likely to die from the disease (DSR = 5) compared with Multi-plier plants. Results from the comparison indicate Multi-plier may be more susceptible to BRR than Peace. Brown root rot symptoms were identified on many uninoculated plants during the seven-month winter season of pathogenicity tests 1 and 2. This research indicates wind-blown dispersion of fungal fruiting structures on alfalfa stubble may be an effective means of spread by the soil-borne pathogen.

References

Davidson, J.G.N. 1990. Brown root rot. pp 29-31 In Compendium of Alfalfa Diseases. D.L. Stuteville and D.C. Erwin, eds. APS Press. St. Paul, MN.

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

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