Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/1998
Publication Date: N/A
Interpretive Summary: Sclerotinia trifoliorum is a fungus that causes a serious disease of first-year plantings of alfalfa and clovers in the southeastern USA. Breeding resistant varieties offers the best hope for control. Previous research showed that if leaves are removed from plants and inoculated with S. trifoliorum, resistance to the fungus is expressed and the most resistant plants can be identified. However, the original leaf-inoculation procedures were too difficult to apply rapidly and on a large scale in practical plant breeding programs. In this study, simplified leaf-inoculation procedures for more rapid identification of resistance were developed and evaluated. Leaves of different ages, wounded and nonwounded, were added directly to different kinds of agar on which S. trifoliorum was grown, and the expression of resistance in known resistant and susceptible populations of alfalfa were compared. Numbers of f"escape" leaves, which remained free of disease because infection did not occur, also were recorded. For leaves from both young seedlings and older plants, several combinations of agar and wounding treatments gave good expression of resistance with only low frequencies of escapes. These simplified leaf-inoculation procedures should enable plant breeders to identify resistant plants more rapidly and in larger numbers, and eventually to breed new resistant varieties in a shorter period of time.
Technical Abstract: Previous studies established that excised leaf tissues of alfalfa can be inoculated with S. trifoliorum to select for heritable resistance, but the original procedures were not practical for use in large-scale screening programs. In this study, simplified leaf-inoculation procedures were developed. Cotyledons, unifoliate leaves, and trifoliate leaflets of plants from three predominantly susceptible cultivars and one resistant germplasm were applied, with and without wounding, directly to colony margins of S. trifoliorum on cornmeal agar, V-8 juice agar, and water agar. Leaves were scored according to the rate and extent of development of necrosis. Significant differences between alfalfa populations were expressed in unifoliate leaves and trifoliate leaflets but not in cotyledons. Disease severity in the resistant germplasm (MSR) was less than in the three cultivars on all agar media. Wounding of leaf tissues increased disease severity and greatly reduced the incidence of symptomless leaves, which are indicated to be escapes, but wounding generally did not prevent expression of resistance in MSR. No significant interactions were observed between alfalfa populations and agar media or wounding treatments. Results indicated that screening for resistance to S. trifoliorum in alfalfa may be accomplished rapidly and effectively by applying unwounded or wounded unifoliate leaves directly to colony margins on cornmeal or V-8 juice agars, respectively, and wounded trifoliate leaflets directly to colony margins on either agar medium.