Characteristics of Inhibition of a Taiwanese Soil Suppressive to Fusarium oxysporum f. sp. raphani

Germination of nutrient-amended chlamydospores of the three formae speciales of Fusarium oxysporum tested were inhibited in a suppressive soil collected from central Taiwan. The suppressive soil released a volatile substance when moistened with alkaline solution. The inhibition spectrum of the volatile substance was different from that of the suppressive soil. The inhibitory effect of the suppressive soil was greatly reduced when it was heat-treated for 30 min at 40°C or higher. The inhibitory effect of the heat-treated suppressive soil was restored after infestation with 1% conducive or suppressive soil for 14 days. However, infestation of heat-treated conducive soil even with 1% suppressive soil did not render it suppressive. Amendment of suppressive soil with rose bengal, streptomycin or Rubigan completely or partially reduced the inhibitory effect. Increasing the total population of indigenous microorganisms in conducive soil by amendment with rice germ or soybean meal to about the same level as that in suppressive soil did not render it suppressive. Results suggest that a combination of biotic and abiotic factors is responsible for the inhibitory effect of the suppressive soil.