Responses to Selection for Resistance to Sclerotinia trifoliorum in Alfalfa by Stem Inoculations. ROBERT G. PRATT, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. DENNIS E. ROWE, USDA-ARS, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. Plant Dis. 78:826-829. Accepted for publication 6 June 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994. DOI: 10.1094/PD-78-0826.

Plants of alfalfa cultivar Delta were screened for resistance to Sclerotinia trifoliorum by inoculating tips of intact or excised stems and measuring the extent of necrosis that developed basipetally after 2 wk. Four of 494 plants that exhibited low levels of necrosis in repeated tests were cloned and polycrossed. Seed of these selections were harvested by parental source, and the four half-sib families were evaluated for resistance to 5. trifoliorum by both stem and whole-plant inoculations. Three of four families were significantly (P = 0.05) more resistant than the parental cultivar when tested with stem inoculations. The same three families also were significantly (P = 0.05) more resistant than the parental cultivar when tested with whole-plant inoculations. The fourth half-sib family gave intermediate responses with both inoculation methods. Plants of the three most resistant families were combined and transplanted at two field sites, along with the parental cultivar, into naturally infested soils during two winter seasons. In each of four experiments, significantly (P = 0.05) less natural disease developed in plants of the progeny than in the parental cultivar. These results demonstrate that resistance to 5. trifoliorum in alfalfa, as identified by the stem inoculation technique, is heritable and is also expressed as whole-plant resistance to controlled inoculations and to natural infection in the field.