STUDIES ON FUSARIUM WILT OF BANANAS: I. THE BEHAVIOR OF F. OXYSPORUM f. CUBENSE IN DIFFERENT SOILS

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Abstract
The behavior in soil of Fusarium oxysporum f. cubense (E.F.S.) Sny. and Hans., the cause of Fusarium wilt or Panama disease of bananas, was studied, using soil dilution plates, soil plates, and buried slide techniques. Soil dilution plates were useful in detecting gross population changes above 500–1000 spores per gram of soil. After six to eight months, initially large populations of Fusarium in artificially infested soil had fallen to below 500 spores per gram of soil but could be detected after two years in or around infested glass cloth strips buried in banana plantations. Apparent differences between population counts in loam and sandy loam were attributed to some defect of the soil dilution plate technique.The soil plate technique consisted in adding 1 gm. of artificially infested soil to a culture dish, moistening the soil with a suitable nutrient, and observing growth and sporulation microscopically or by estimating colony increases on soil dilution plates. An external source of nutrient was essential for germination and sporulation. Among the nitrogen sources tested, 1% solutions of tryptophane and glutamic acid stimulated the abundant production of spores whereas inorganic sources of nitrogen were ineffective. Sporulation was greater in soils dried before plating than in soils maintained at 15–35% saturation. Sporulation was greater in acid sandy loam than in alkaline loam and was abundant below pH 7.0 in all soils. Sporulation in soil plates was sparse or absent below pH 3.5 in acidified loam or above pH 8.8 in sandy loam. The optimum pH for sporulation in acidified loam was about 5.0 and in sandy loam, from about 5.0–7.0. In general, rate of disease spread in soils when they are first planted was correlated with soil texture, pH, and amount of growth and sporulation on soil plates and buried slides. Thus, the soil, rather than the host, appears to be exerting the primary influence on the fungus.Fusarium grew and sporulated abundantly in decomposing diseased banana pseudostems, on the surface of cut-open rhizomes, and in adhering soil, but did not sporulate freely in sap or in the xylem in vivo. Fresh banana sap stimulated the abundant production of spores in soil plates. Evidence is presented suggesting that F. oxysporum f. cubense also sporulates in nature in saprophytic association with certain soil-plant components.

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