The effects of fungicides on soil and leaf fungi

Abstract

The effects of the fungicides CAPTAN, DICLORAN, THIRAM, and VERDASAN on the occurrence and physiology of mesophilic and thermophilous soil and leaf-surface fungi were studied. Regular monthly applications of the fungicides reduced the numbers and altered the natural seasonal incidence of fungi in soil and on leaf-surfaces. In soil, Botryotrichum piluliferum, Gliocladium roseum, and Trichoderma viride were absent or greatly reduced in numbers immediately after application, but the numbers recovered on subsequent sampling days. On leaf-surfaces, Aureovasidium pullulans, Drechslera biseptata, and Epicoccum purpurascens were reduced or eliminated by successive applications of fungicides. The soil and phylloplane fungal numbers were reduced most by the dithiocarbamate THIRAM and the organo-mercurial VERDASAN. Chrysosporium pannorum is reported as a phylloplane fungus. It was the major component of VERDASAN-treated soil from which it was isolated throughout the year. It occurred in higher numbers immediately after spraying than on other sampling days. It was capable of radial growth and sporulation at low temperatures and in five concentrations of VERDASAN which it was able to use as a sole carbon source. The ultrastructure of C. pannorum growing in VERDASAN and HgCl₂ was studied. Physiological evidence is provided to show that Trichoderma viride was a recolonizer of fungicide-treated substrates. The abundance of the slow-growing, weakly cellulolytic, and sensitive fungus Cladosporium cladosporioides on fungicide-treated leaves and in soil is ascribed to its inoculum potential at the period of sampling. Thermophilous fungi were found to be a relatively small but consistent component of the mycoflora of living green leaves and of soil. Only 20% of the total phylloplane fungi and a small fraction of the soil mycoflora were thermophilous. Thermophilous fungi showed higher growth rates and more vigorous cellulolytic abilities than did the mesophilic species, but they were more sensitive to fungicides than were the mesophilic fungi. Aspergillus fumigatus, Coprinus delicatulus, Mucor pusillus, and Thermomyces lanuginosus were the principal phylloplane fungi isolated. A. fumigatus and M. pusillus were persistent on fungicide-treated leaves and soil. The occurrence of thermophilous fungi on leaf-surfaces in relatively higher proportions than their reported levels in the air-spora indicated active growth. Growth rates, spore germination, germ tube elongation, amylolytic, and cellulolytic abilities of both mesophilic and thermophilous fungi were all reduced by the fungicides with maximum inhibition shown by THIRAM and VERDASAN. All the species treated were more sensitive to the organo-mercurial fungicide than to inorganic mercuric chloride. The effects of fungicides on higher plant germination and growth and the effects on soil dehydrogenase activity were also studied