STUDIES ON FUSARIUM WILT OF BANANAS: III. INFLUENCE OF SOIL FUNGITOXINS ON BEHAVIOR OF F. OXYSPORUM f. CUBENSE IN SOIL EXTRACTS AND DIFFUSATES

Abstract

Aqueous extracts from soil obtained (1) by mixing equal volumes of soil and distilled water and filtering until clear, (2) by displacing the soil solution with ethyl alcohol, and (3) by soil perfusion were inhibitory to germination, hyphal growth, sporulation, and chlamydospore formation by Fusarium oxysporum f. cubense. In general, acid loam extracts were less fungitoxic than alkaline clay loam extracts and differences were not related to extract pH. Fungitoxins were reduced by filtering extracts through charcoal, by evaporating or dialyzing the extracts, and by air-drying soil before extracting. Fungitoxins were greatly reduced by filtration through sintered-glass or Seitz filters. Heat sterilization or fumigation with ethylene oxide destroyed most of the fungitoxic effect. However, differences in growth and sporulation between heat-sterilized loam and clay loam extracts and diffusates were often apparent. Fusarium germination, hyphal growth, sporulation, and chlamydospore formation on agar films were inhibited by fungitoxins diffusing through cellulose membranes and by addition of non-sterile soil to 1.5% water agar.High concentrations of nutrients masked and low concentrations reduced effects of fungitoxins in soil extracts and diffusates. The addition of dextrose, glutamic acid, yeast extract, banana sap, or banana roots to extracts stimulated growth and sporulation. Yeast extract or glutamic acid stimulated sporulation at concentrations of 10 p.p.m., particularly where the bacterial population in the extract had been reduced by filtering through sintered glass.Fungitoxins and nutrients interacting, often appeared to act independently on hyphal growth, sporulation, and chlamydospore formation, stimulating or inhibiting one but not the others. Both sporulation and chlamydospore formation, in general, were more sensitive to fungitoxins and nutrients than was hyphal growth. In addition, morphogenic responses of the yellow and non-yellow Fusarium clones to fungitoxins and nutrients were markedly different. Both germination and hyphal growth were greater with the yellow than the non-yellow clone in all extracts. In general, chlamydospores were produced more frequently and in greater numbers in agar films containing diffusates from acid loam than alkaline clay loam. Chlamydospores were formed most rapidly and in greatest abundance on sterile soil-amended or plain agar after 5 to 6 days. In aqueous extracts chlamydospores, if produced, formed after 3 to 5 days. Conidia, if produced, formed within 18 to 42 hours regardless of substrates.The data indicated that fungitoxins in extracts and diffusates were associated with the bacterial soil flora since measures that reduced or altered this flora were reflected in a response by Fusarium.