Abstract
Six bacteria and one fungus isolated from sclerotia of Sclerotium cepivorum, the causal agent of white rot of onions, produced diffusible antibiotics antagonistic to growth of S. cepivorum on potato dextrose agar. Three of the bacterial isolates applied as seed treatments to onions grown in non-sterile muck soil in a controlled environment chamber reduced the proportion of infections by S. cepivorum. Antagonists were further evaluated as seed treatments for field control of white rot on two onion cultivars grown on muck soil containing high levels of natural inoculum. Four of the bacterial isolates provided significant season-long protection on the partially-resistant cultivar Festival, and the best of these also provided significant protection on the susceptible cultivar Autumn Spice. The fungal antagonist has been identified as Penicillium nigricans, and all bacterial isolates appear to be Bacillus subtilis. The levels of protection provided by some of these latter isolates were comparable to those provided by chemical treatments and represent practical potential for field control of white rot.
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