Submitted to: North American Alfalfa Improvement Conference
Publication Type: Proceedings
Publication Acceptance Date: August 3, 1998
Publication Date: N/A
Interpretive Summary: Sclerotinia trifoliorum and Sclerotium rolfsii are soilborne, disease- causing fungi that infect and kill alfalfa plants in the southeastern USA. Sclerotinia kills young plants during the first winter after seeding, whereas S. rolfsii kills plants of all ages during summer months. The best method for controlling these and other fungal diseases of alfalfa is to identify the most resistant individual plants and intercross them to produce resistant populations. Usually resistance to different fungal diseases in plants is unrelated and highly specific -- plants selected for resistance to one disease usually do not show improved resistance to other diseases. In this study, and exceptional instance was found in that populations of alfalfa selected for resistance to Sclerotinia were also found to manifest increased resistance to S. rolfsii, an unrelated pathogen. The common resistance to the two pathogens was revealed in leaf tissue assays, which are based on rates of parasitism of excised leaf tissues in Petri plates by the two fungi. Seven different populations that had been selected for resistance to Sclerotinia also demonstrated increased resistance to S. rolfsii in leaf assays. For two populations, this resistance was further demonstrated when plants 5-7 weeks old were inoculated with S. rolfsii; both populations had significantly greater survival of plants than the parent cultivar. These results indicate that selection for resistance to Sclerotinia should give improved persistence of alfalfa throughout the year, including in the summertime when Sclerotinia is inactive, as a result of increased resistance to S. rolfsii. Results also show for the first time that genes for resistance to S. rolfsii are present in alfalfa.
Technical Abstract: Sclerotium rolfsii and Sclerotinia trifoliorum are taxonomically unrelated fungal pathogens that cause disease under very different environmental conditions in the southeastern USA. However, both fungi cause a coarse rotting of host tissues, and both also secrete oxalic acid which injures host cells in advance of fungal growth. These similarities in parasitism suggested that resistance to S. trifoliorum and S. rolfsii in alfalfa might be related; this study evaluated that hypothesis. Resistance to S. rolfsii in alfalfa was evaluated by leaf tissue assays and whole-plant inoculations. In repeated leaf-tissue assays of STR and MSR, two populations previously developed for resistance to S. trifoliorum from cultivar Delta, both populations expressed resistance to S. rolfsii in comparison to Delta. Similar results were obtained in whole-plant inoculation experiments where resistance was evaluated by plant survival; best differential results between entries were obtained with plants 5-7 weeks old. Additional populations with resistance to S. trifoliorum were developed from 5 alfalfa cultivars by selection with leaf-inoculation techniques and intercrossing. Random populations were developed from the same cultivars by selecting similar numbers of plants at random. Resistance of the resistant populations to S. trifoliorum, in comparison to parent cultivars and random populations, was demonstrated by both leaf assays and whole-plant inoculations. In repeated leaf-tissue assays, all five populations also expressed resistance to S. rolfsii in comparison to the parent cultivars, random populations, or both. These results indicate that selection for resistance to S. trifoliorum in alfalfa also confers increased resistance to S. rolfsii.