Investigations of the microbiological reaction to the use of soil fumigants

Abstract

A John Innes type potting soil was fumigated with chloropicrin, methyl bromide and MBC 33; a 66:33 mixture of the two fumigants. There were two clear-cut patterns of fungal recolonisation. Trichoderma viride (Pers) ex Fr. was the principal recolonist following chloropicrin and MBC 33 treatment and Penicillium spp. following methyl bromide treatment. Removal of residual chloropicrin allowed a Penicillium recolonisation in chloropicrin and MBC33 soils. In the presence of residual chloropicrin, recolonisation by Penicillium spp. was prevented. All isolates of Gliocladium virens Miller Giddens and Foster and some of T. viride selected from amongst recolonists of the fumigated soils were capable of producing antibiotic compounds in culture. They were inhibitory, and in some cases lethal, towards bacteria, actinomycetes and fungi including Phytophthora nicotianae var parasitica (Dastur) Waterh. The independent addition of isolates of G. virens and T. viride to fumigated John Innes type potting soil into which P. parasitica was subsequently inoculated resulted in fewer survivors amongst tomato test plants than in the presence of pathogen alone. That is, Gliocladium and Trichoderma isolates not only failed to control P. parasitica in soil, they actually enhanced pathogenicity toward tomato test plants. Culture filtrates of G. virens and inhibitory T. viride isolates reduced the pathogenic effect of P. parasitica in soil toward tomato test plants. The work described in this thesis indicates that Gliocladium and Trichoderma influence the activity of other microorganisms in soil but their effects would appear to be unrelated to their ability to produce antibiotics in vitro.