Control of Soilborne Pathogenic Fungi in Fields of Sweet Onion

Abstract

Populations of soil fungi from fields planted to sweet onion were assayed on selective media. In pathogenicity tests, Rhizoctonia solani AG-4, Pythium irregulare, and Phoma terrestris were the fungi most virulent on onion seedlings. Plots were fumigated with methyl bromide (MBR), chloropicrin (CP), MBR + CP (67% + 33%), metam sodium, 1,3,-dichloropropene (1,3-D), or 1,3-D + 17% CP in four field experiments in 2 years. Sweet onion was transplanted or direct seeded in October or November and harvested in April or May. MBR + CP and CP were effective in reducing populations of Phoma terrestris, Pythium spp., Fusarium spp., and R. solani AG-4 in soil. Metam-sodium and 1,3-D + 17% CP were less efficacious, and MBR and 1,3-D were ineffective. There were no differences in the percentage of bulbs with decay at harvest or after curing among treatments. Increased yield of marketable bulbs was associated with control of soilborne pathogenic fungi. In fields continuously cropped to onion, decreased yield was primarily associated with control of pink-root induced by Phoma terrestris, and P. terrestris was identified in soil from 74% of the fields assayed.