Resistance to white rot infections in bulb onion seed lots

Five seed lots and the self- and open-pollinated progenies of the cultivar ‘Ailsa Craig’ were tested for disease reaction to Sclerotium cepivorum, the causal fungus of onion white rot. Field tests were conducted on S. cepjuorum-infested soil in Burnaby, British Columbia, from 1976 to 1980. No significant differences were observed between ‘Ailsa Craig’ and ‘Autumn Spice’ with respect to disease resistance during the 1979 and 1980 tests, whereas resistance of ‘Ailsa Craig’ had been noted in previous years. Tests indicated that resistance varied with seed lot, thus providing a possible explanation for the 1979 and 1980 results. ‘Ailsa Craig’ selfed progeny selection had significantly fewer infections than its parental seed lot and the local recommended cultivar ‘Autumn Spice’. Resistance to infection by S. cepivorum should be assigned to seed lots or breeding lines and not to presently named cultivars. These results suggest that differences in infection by S. cepivorum occur, and are available for the development of white rot resistant cultivars.