Clippings – June 6, 2005 BROWN ROOT ROT IN ALFALFA IDENTIFIED IN NORTHWEST MINNESOTA Reprinted with permission from Philip Glogoza, Editor, Cropping Issues in Northwest Minnesota. By Deborah Samac, USDA-ARS - U of M Plant Pathologist; Charla Hollingsworth, U of M Plant Pathologist; Paul Peterson, U of M Agronomist; Fred Gray, U of Wyoming Plant Pathologist; Hans Kandel, Regional Extension Educator Brown Root Rot (BRR) was first described in North America on yellow sweetclover (Melilotus) and alfalfa in Canada in 1933. It can be a serious disease of sweetclover in Canada. Brown root rot is caused by a cold-loving fungus, Phoma sclerotioides. The pathogen can also infect other forage legume crops including alsike clover, bird’s-foot trefoil, sainfoin and non-leguminous plants such as grasses. The fungus produces survival structures (pycnidia) that infest soil and may function to spread the disease. It is thought to be a native of the Canadian prairie. Symptoms on diseased plants consist of dark colored lesions of the tap and lateral roots. Initial lesions are often localized but can spread and eventually girdle the roots of susceptible plant hosts and cause the tissues to rot. Nitrogen-fixing bacteria nodules can also be rotted. If lesions occur on the upper tap root there is less healthy tissue for storage of carbohydrates, and diseased plants are more susceptible to winterkill. BRR lesions will worsen with each winter until the plant is killed. Dead plants are noticed in the spring and are considered ‘winterkilled’ since they were alive the preceding fall. Distribution of BRR in North America includes Alaska, the Yukon, the Northwest Territories, the four western provinces of Canada, and Nova Scotia. Since 1996, it has been found in Wyoming, Montana and Idaho. More recently, it was identified in Wisconsin, Minnesota, and New York. Control of BRR consists of the following practices; a) Crop Rotation - grow crops such as wheat, oats or barley for 3 years between alfalfa crops and avoid planting other host perennial forage legume crops during the rotation, b) Harvest Management - avoid stressing the stand going into winter by excessive cutting or late fall cutting, c) Grazing Management - avoid overgrazing and grazing in the fall before the ground is frozen, d) Maintain proper soil fertility. Disease resistance has been noted and work is ongoing by plant pathologists and breeders to produce an alfalfa variety with BRR resistance that is adapted to the growing environment in Minnesota. Research Since 1996, BBR has been associated with winter injury and loss of alfalfa stands in the western US when the environment promoted disease development, but the impact of the disease in the Midwest is unknown. Ongoing research at the University of Minnesota aims to determine the distribution of the fungus, evaluate alfalfa varieties for resistance to the disease, develop a rapid soil test for the fungus, and determine if the fungus is also responsible for causing disease on winter wheat. Surveys completed to date have detected the fungus on alfalfa plants in thirteen Minnesota counties (Benton, Cass, Fillmore, Marshall, Otter Tail, Pennington, Polk, Red Lake, Rock, Sherburne, Wabasha, Washington, and Winona). In the fall of 2004 random sampling of 58 alfalfa fields resulted in 8 (or 14%) fields positive for the fungus. To obtain positive identification DNA was extracted from roots of individual alfalfa plants and the presence of the fungus was detected using a specific polymerase chain reaction (PCR) test. A total of 18 fields with winter wheat or perennial ryegrass located in six Minnesota counties were sampled during 2004. The fungus was found on roots of plants from Kittson, Marshall, and Roseau Counties. A test for determining whether the fungus causes disease on winter wheat, by rotting roots, or lives on roots without causing damage, was started in the Fall of 2004 and will be evaluated during the Spring of 2005. As the fungus was identified from Roseau and Kittson counties to Ottertail county in northwest Minnesota it is likely that the fungus will be found in other northwestern Minnesota counties as sampling has only been conducted for two seasons. Experiments to determine varietal resistance in alfalfa will be established in 2005 in fields in northwestern and west central Minnesota where the fungus has been identified. Sampling for analysis If there is winter kill in alfalfa stands other than obvious drown out spots it may be necessary to sample plants from the field. If possible, get the name of the variety of alfalfa and the age of the stand. When sampling, you can take plants from any part of the field, the fungus should be wide- spread and not localized. Diseased plants may or may not have symptoms, depending on the stage of infection. Sampling Procedure • Obtain via the Extension Service a data sheet and fill in the sheet as completely as possible. • Dig 5-10 alfalfa plants randomly across the field. Plants need to have 6-8 inches of root. To be useful, roots need to be viable, not frozen or dried out. • Shake off soil. Clip stems to 1 inch of the crown (1 inch stubble or less). • Wash roots in tap water to remove remaining soil. • Dry roots with paper towels to remove residual water. • Label bag with sample ID. Use your initials and a number for the sample ID. • Place plants in bag and seal the bag. • Place bag(s) and collection information sheet in Priority Mail box. • Seal and label box. • Ship by Priority Mail to: Dr. Deborah Samac, 1991 Upper Buford Circle, 495 Borlaug Hall, St. Paul, MN 55108. Resources A web site with survey results and information on brown root rot: http://www.tc.umn.edu/~medicago/brr.htm For more information about the disease see “Brown Root Rot of Alfalfa” at: http://www.uwyo.edu/plants/publications/brownrootrottimely.pdf .