Host and Soil Fumigation Effects on Spore Population Densities of Species of Endogonaceous Mycorrhizal Fungi

Abstract

Field experiments were conducted to determine the effects of fallow, growing pepper, tomato, or the green manure crop sorghum-sudangrass on spore population densities of mycorrhizal fungi. Two crops, pepper and tomato, were grown in soil either fumigated or not with 67% methyl bromide-33% chloropicrin. Glomus macrocarpum comprised over half the total spore population. Spores of G. microcarpum, G. constrictum, and G. caledonium were next most prevalent. Spores or sporocarps of G. fasciculatum, four species of Acaulospora, two of Gigaspora, and Sclerocystis sinuosa occurred sporadically or in low numbers. Growing sorghum-sudangrass, tomato, and pepper (except in nonfumigated soil) increased total spores/kg soil, while fallow reduced them. Populations of Glomus caledonium spores increased 40-fold with sorghum-sudangrass. Populations of spores of G. macrocarpum (except in nonfumigated pepper plots) and G. microcarpum were less with fallow than with any of the crops at the end of the season. Twelve days after fumigation, spores of mycorrhizal fungi were still present but in reduced numbers. Increases in spore population densities during the growing season and measurements of viable mycorrhizal fungus propagules by the Most Probable Number technique at the end of the growing season demonstrated that either soil fumigation did not kill all propagules or the fungi were reintroduced.