Chemical and Biological Control of Onion White Rot in Muck and Mineral Soils. R. S. Utkhede, Research Scientist, Agriculture Canada, Research Station, Summerland, BC V0H 1Z0. J. E. Rahe, Associate Professor, Department of Biological Sciences, Simon Fraser University, Burnaby, BC V5A 1S6. Plant Dis. 67:153-155. Accepted for publication 24 June 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-153.

Four isolates of Bacillus subtilis applied as seed treatments singly or in combination with broadcast chemicals significantly reduced white rot of onions in unsterile muck soils in a controlled-environment chamber. Field trials conducted on muck soil that contained high levels of natural inoculum of Sclerotium cepivorum in the 1978 and 1979 growing seasons had established that vinclozolin and iprodione provided significant control of onion white rot when applied as preplant broadcast treatments. Trials in the same seasons had shown that seed inoculation with the BACT-2 strain of B. subtilis also provided significant control of white rot. White rot of onion was completely controlled by vinclozolin and iprodione, and bacterial seed treatment significantly reduced infection on the pickling onion cultivar Silverqueen in mineral soil. Although vinclozolin continued to provide effective control of white rot, iprodione and BACT-2 were generally ineffective in 1980 in muck soil. In a growth-chamber pot trial, bacterial colony-forming units were significantly higher in the onion root zone in BACT-2 seed-treated soils. A significant (P = 0.05) correlation (r = 0.64) occurred between viable sclerotia present in the soil after harvest and the percent infection.