Excised leaf tissues of alfalfa were inoculated with Sclerotinia trifoliorum to determine whether leaf-inoculation techniques may be used to screen for heritable and effective resistance to the pathogen. Leaf tissues were placed on water agar in petri plates, inoculated with mycelia of S. trifoliorum, and incubated at 17°C. Host responses were evaluated according to the rate and extent of necrosis that developed during 1 week. One thousand plants of cultivar Delta were screened for resistance by successive inoculation of excised unifoliate leaves, first trifoliate leaflets, and disks of tissue from later leaves. The five plants considered most resistant were intercrossed, and progeny were compared with those of five plants selected for susceptibility, five selected at random, and the parent cultivar. Progeny of resistant plants had significantly (P = 0.05) less severe disease than did progenies of susceptible and random plants and the parent cultivar when evaluated by inoculation of unifoliate leaves, first trifoliate leaflets, leaf disks, excised stems, and whole plants. In field experiments with natural infection during two growing seasons, progeny of resistant plants had less severe disease and produced significantly (P = 0.05) higher yields of forage through the first or second harvests than did progenies of susceptible and random plants and the parent cultivar. These results establish that inoculation of excised leaf tissue is an effective and efficient means to screen for resistance to S. trifoliorum in alfalfa.
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Richard M. Pratt (1996) studied this question.