Screening Onion Lines for Resistance to Sclerotium cepivorumBerk. using Field and Onion Scale Assessments

Abstract

Resistance to Sclerotium cepivorum was investigated over 3 years at field sites with known histories of white rot in the Holland Marsh, Ontario, Canada. Onion lines from three sources (Petoseed, Asgrow Ltd., and Univ. of Wisconsin), including commercial cultivars, were direct-seeded (1995) or hand-transplanted (1994 and 1996) and the bulbs were assessed for white rot incidence at harvest. The incidence of white rot in 1994 was low (0% to 2.6%) and not significantly different among lines and cultivars. In 1995, white rot incidence was moderate at sites 1 and 2 (maximum 21.5% and 24%), but low at site 3 (0% to 6.3%). In 1996, white rot incidence ranged from 0.8% to 41.1% at site 1, but was not observed at sites 2 and 3. The results of the 1995 and 1996 assessment suggested that the breeding lines could be divided into two major groups with high (Univ. of Wisconsin) or low (Asgrow Ltd. and Petoseed) resistance to the fungus. Scale segments of harvested bulbs from the 1995 field trial and 35 commercial cultivars were inoculated with mycelial plugs of two isolates of Sclerotium cepivorum. The resulting lesions were measured. Significant differences in lesion diameters among onion lines (9.1–22.4 mm) and cultivars (10.5–26.75 mm) were found within isolates. There was a significant, high, and positive correlation between diameters of lesions formed by the two isolates on the 23 lines (r2 = 0.76, P = 0.05) and 35 cultivars (r2 = 0.62, P = 0.005). Both techniques demonstrated a wide range of resistance to white rot. This suggests a strong potential for increasing resistance through breeding.