The Relationship Between Bacterial Wilt Resistance and Persistence is Not the Same Among Alfalfa Varieties

Edwin T. Bingham Agronomy Department University of Wisconsin, Madison, WI 53706

Experimental Synthetics produced from four year old field survivors of 'Vernal', 'Wrangler', and 'Oneida' alfalfa were 8-12% higher in bacterial wilt resistance than the respective original varieties. This result is consistent with natural selection that has been important in the history of alfalfa production (3). However, experimental svnthetics produced from field survivors of 'Arrow' and 'Elevation' dug from the same nursery (1) decreased 22 and 8%, respectively in bacterial wilt resistance. This was unexpected, and appears to be a lead in understanding the persistence problem (2,4). It appears that some of the bacterial wilt resistant plants in Arrow and Elevation lacked persistence and were eliminated in the first three years. Many of the survivors in Arrow and Elevation at four years were escapes from bacterial wilt. This set the stage for elimination of plants susceptible to bacterial wilt and may be one of the reasons for the rapid stand loss in these varieties. The relationship between bacterial wilt resistance and persistence is being studied in additional varieties. This research and research aimed at gaining some insight into the genetics of persistence per se, will be described.

Previously, a study of 30 years of Wisconsin alfalfa variety trials revealed that correlations of disease resistance with yield decreased over time indicating that more resistant varieties on average were less persistent (2). An independent study using data from eight Mid west states also found a small and sometimes negative impact of disease resistance ratings on alfalfa productivity in the third year (4). Our goal as we enter the next century is balanced improvement of yield, persistence, and pest resistance.

References 1. Holland, J. B. and E. T. Bingham. 1994. Genetic improvement for yield and fertility of alfalfa cultivars representing different eras of breeding. Crop Sci. 34:953-957. 2. Ipson, R. B., D. A. Rohweder, E. T. Bingham, C. R. Grau and D. J. Undersander. 1991. Central Alfalfa Impr. Conf. 22: 19. 3. Tydal, H. M. and H. L. Westover. 1937. Alfalfa Improvement Yearbook of Agriculture. USDA. P. 1122-1153. 4. Volenec. J. 1993. Selection for disease resistence: Are we improving alfalfa persistence? Central Alfalfa Impr. Conf. 23: 17.