RESPONSES TO SELECTION FOR RESISTANCE TO SCLEROTINIA-TRIFOLIORUM IN ALFALFA BY STEM INOCULATIONS

Plants of alfalfa cultivar Delta were screened for resistance to Scler otinia trifoliorum by inoculating tips of intact or excised stems and measuring the extent of necrosis that developed basipetally after 2 wk . Four of 404 plants that exhibited low levels of necrosis in repeated tests were cloned and polycrossed. Seed of these selections were harv ested by parental source, and the four half-sib families were evaluate d for resistance to S. trifoliorum by both stem and whole-plant inocul ations. Three of four families were significantly (P = 0.05) more resi stant than the parental cultivar when tested with stem inoculations. T he same three families also were significantly (P = 0.05) more resista nt than the parental cultivar when tested with whole-plant inoculation s. The fourth half-sib family gave intermediate responses with both in oculation methods. Plants of the three most resistant families were co mbined and transplanted at two field sites, along with the parental cu ltivar, into naturally infested soils during two winter seasons. In ea ch of four experiments, significantly (P = 0.05) less natural disease developed in plants of the progeny than in the parental cultivar. Thes e results demonstrate that resistance to S. trifoliorum in alfalfa, as identified by the stem inoculation technique, is heritable and is als o expressed as whole-plant resistance to controlled inoculations and t o natural infection in the field.