Effect of Continuous Plant Culture and Soil Fumigation on Soilborne Plant Pathogens and on Growth of Tomato Transplants

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Abstract
A 4 yr field test was conducted in southern Georgia [USA] to study population changes of selected plant pathogenic fungi [Pythium spp., Fusarium spp. and Rhizoctonia solani] and nematodes associated with repeated transplant culture and to determine the feasibility of using general purpose soil fumigants in tomato [Lycopersicon esculentum] transplant production. There was no evidence that repeated tomato transplant culture consistently increased populations of soilborne plant pathogens except that root galling caused by Meloidogyne incognita increased during the first 3 yr of the test. Annual fumigation with some chemicals significantly increased plant vigor and yield of marketable plants during 3 of the 4 yr. The best chemical treatments also decreased populations of potential soil-borne pathogens and increased survival of tomato seedlings grown in the greenhouse in treated soil from field plots. Methyl bromide was most effective in increasing vigor and yield, reducing populations of potential plant pathogens and increasing survival of tomato seedlings in greenhouse bioassay tests. Methyl bromide applied once did not significantly increase plant vigor or yield or decrease populations of fungal plant pathogens during the 3 yr after treatment. Metham decreased populations of potential pathogens, increased survival of seedlings in greenhouse bioassay tests, and improved yield during 2 yr. Spring applications of DD-MENCS [methyl isothiocyanate, 1,2-dichloropropane-1,3-dichloropropene and related hydrocarbons] and spring and winter applications of sodium azide increased vigor and yields during some years, but were less effective than methyl bromide and metham in reducing populations of soil organisms and in increasing seedling survival in bioassay tests. Reinfestation of fumigated fields and chemical phytotoxicity apparently are major problems that must be overcome to improve the usefulness of general purpose fumigants.