Sclerotium rolfsii and Sclerotinia trifoliorum are taxonomically unrelated fungal pathogens that cause disease under very different environmental conditions. Nevertheless, these two fungi bear some similarities in modes and mechanisms of pathogenesis. Both cause rapid rotting of leaves, petioles and stems that often results in death of infected plants. The mechanism of pathogenesis by both fungi is believed to involve the secretion of oxalic acid, which weakens or destroys host cells in advance of hyphal growth. From these similarities in parasitism, it appeared that resistance to S. trifoliorum and S. rolfsii in alfalfa might be related. Therefore, this study was conducted to determine whether populations of alfalfa selected for resistance to S. trifoliorum also manifest resistance to S. rolfsii.
Evaluations were performed on two alfalfa populations previously developed for resistance to S. trifoliorum (STR and MSR) and on four new populations. Most evaluations of resistance to S. rolfsii were performed by a leaf tissue assay similar to that developed for S. trifoliorum. Whole plant inoculations also were performed to verify resistance in STR and MSR. In repeated leaf-tissue assays of STR and MSR, both populations expressed resistance to S. rolfsii in comparison to the parent cultivar, Delta. In whole-plant inoculation experiments with S. rolfsii and plants of different ages, where resistance was evaluated by percentage plant survival, both MSR and STR had greater survival than Delta in repeated experiments. However, although MSR is more resistant to S. trifoliorum than is STR, it was suggested from results of these and other experiments that STR may be more resistant to S. rolfsii than is MSR.
Four additional populations with resistance to S.
trifoliorum were developed from the cultivars Magnum-III
Wet, WAMPR, WL-323, and 5472 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
to S. trifoliorum in the populations selected for
resistance, in comparison to parent cultivars and random
populations, was demonstrated by leaf assays and by whole
plant inoculations. In repeated leaf-tissue assays, all
four new populations developed for resistance to S.
trifoliorum also expressed resistance to S. rolfsii in
comparison to the parent cultivars, random populations, or
both.
From these results, it is concluded that selection
for resistance to S. trifoliorum in alfalfa also confers
increased resistance to S. rolfsii. Therefore, populations
selected for resistance to S. trifoliorum during winter and
early spring might also demonstrate improved persistence
during summer months in the southeastern USA because of
increased resistance to S. rolfsii.