Studies on Soil Microflora and Fungistasis

Abstract

Studies were made on the soil microbial populations and composition in an arable soil at one locality in the Wairarapa district of the North Island, New Zealand, during 1978. The microbial population counts per gram of dry soil were found to be : (i) 8.83 x 107 and 2.73 x 107 for bacteria and actinomycetes respectively on soil extract agar and 2.92 x 107 and 9.73 x 106 respectively on nutrient agar; (ii) 1.09 x 108 for actinomycetes on Jensen's agar; (iii) 1.36 x 105 for fungi on V-8 juice agar. The possible reasons for the high population count of actinomycetes per gram of dry soil are discussed. Forty four species of fungi and 41 isolates of actinomycetes were obtained and listed. A computer method was developed to aid the identification of the Streptomyces or Actinomyces species. A new unusual fungus Bimuria novae-zelandiae is described, including scanning electron micrographs of the ascospores and its cultural characteristics are outlined. Bimuria novae-zelandiae is unusual because it has a very thin peridium, 2-spored asci and the endoascus deliquesces to liberate the ascospores. The ascospores are muriform, dark brown and lack gelatinous sheaths. Three other unusual fungi and the unidentified Streptomyces species are also described. Fungistasis was demonstrated and shown to be widespread in the agricultural soils tested. A local isolate of the fungus Cladosporium cladosporioides proved to be very suitable as a test organism because its spores can germinate very well in distilled water. Spores were supported on a half-folded membrane filter in testing for fungistasis, since this proved to be a sensitive and convenient method. The fungistatic effect in air enclosed above soil was influenced by temperature and rainfall in the five soils studied. Warm temperature and extreme low moisture content in the soil enhanced the fungistatic effect in the air enclosed above the soil. Harrowing of the soil and burning of debris on the soil, released the fungistatic effect in the air enclosed above the soil and the fungistatic effect was restored by keeping the soil undisturbed in the laboratory or by growing vegetation on it. Fungi and Streptomyces species produced volatile or non-volatile inhibitors of spore germination at various growth stages. The mobility of microbial metabolites in sterile sand was determined and fungistasis in sand was determined and fungistasis in sand was demonstrated. Fungistatic effect caused by microbial metabolites was released to a certain level by increasing nutrients of glucose in vitro. The two theories of the cause of soil fungistasis are evaluated. The Streptomyces species were found to be the most active group in producing volatile fungistatic factors. A refined method was developed for the concentration of volatile metabolites of growing Streptomyces griseoruber. Volatile components were detected by gas chromatography, but have not yet been characterized. It was found that methane, ethylene, ethane and propane were not involved directly in the fungistatic effect. The principle of soil fungistasis was applied in an attempt to control the pathogen, Drechslera sorokiniana by coating barley seeds with a concentrated culture filtrate of Streptomyces lydicus. In sterile soil eighty-nine percent control was obtained and in natural soil eighty-five percent. The possible control mechanism is discussed.