CONTROL OF SOILBORNE PATHOGENIC FUNGI IN FIELDS OF SWEET ONION

Populations of soil fungi from fields planted to sweet onion were assa yed on selective media. In pathogenicity tests, Rhizoctonia solani AG- 4, Pythium irregulare, and Phoma terrestris were the fungi most virule nt on onion seedlings. Plots were fumigated with methyl bromide (MBR), chloropicrin (CP), MBR + CP (67% + 33%), metam sodium, 1, 3,-dichloro propene (1,3-D), or 1,3-D + 17% CP in four field experiments in 2 year s. Sweet onion was transplanted or direct seeded in October or Novembe r and harvested in April or May. MBR + CP and CP were effective in red ucing populations of Phoma terrestris, Pythium spp., Fusarium spp., an d R. solani AG-4 in soil. Metam-sodium and 1,3-D + 17% CP were less ef ficacious, and MBR and 1,3-D were ineffective. There were no differenc es in the percentage of bulbs with decay at harvest or after curing am ong treatments. Increased yield of marketable bulbs was associated wit h control of soilborne pathogenic fungi. In fields continuously croppe d to onion, decreased yield was primarily associated with control of p ink-root induced by Phoma terrestris, and P. terrestris was identified in soil from 74% of the fields assayed.