VIEW ARTICLE

Ecology and Epidemiology

Nature of Suppression of Pythium splendens in a Pasture Soil in South Kohala, Hawaii. C. W. Kao, Graduate research assistant, Department of Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720; W. H. Ko, professor, Department of Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720. Phytopathology 73:1284-1289. Accepted for publication 19 April 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1284.

The effect of a pasture soil suppressive to Pythium splendens in the South Kohala district on the island of Hawaii on germination or growth of various microorganisms ranged from inhibitory to stimulatory compared with that of conducive soil from Hilo, HI. The soil was suppressive to damping-off caused by P. splendens and P. aphanidermatum, but not to damping-off caused by Phytophthora palmivora and P. capsici and it inhibited germination of spores of P. splendens and P. aphanidermatum, but not those of P. palmivora and P. capsici. Sterilization by autoclaving or γ-irradiation completely nullified the inhibitory effect of suppressive soil. Inhibition of germination of sporangia of P. splendens was restored to sterilized suppressive soil by reinfestation by adding suspensions of either suppressive or conducive soil. Reinfestation of sterilized conducive soil with suppressive or conducive soil. Reinfestation of sterilized conducive soil with suppressive soil suspension did not convert it to inhibitory soil. Inhibition of sporangial germination also was restored in autoclaved suppressive soil by reinfestation with actinomycetes, bacteria, or fungi. In mixtures of suppressive and conducive soil, the inhibitory effect of suppressive soil varied inversely with the proportion of conducive soil. Germination inhibition of mixed soil decreased after incubation for 14 and 28 days. Amendment of suppressive soil with 333 ppm of rose bengal, 6,667 ppm of streptomycin, or 3,333 ppm of benomyl partially nullified its inhibition of sporangium germination. The solution obtained from suppressive soil amended with rose bengal or streptomycin was inhibitory to bacteria and actinomycetes but not fungi, whereas the solution obtained from benomyl-amended soil was inhibitory to fungi, but not bacteria and actinomycetes. Neither volatile nor nonvolatile inhibitors were detected in suppressive soil. Suppressive soil remained inhibitory to germination when its pH was adjusted from 6.8 to 5.4, and conducive soil remained conducive to germination when its pH was adjusted from 5.4 to 6.8. Results of this study suggest that a combination of unspecific microbiological factors and unknown abiotic factors is responsible for the microbiological suppressiveness of this pasture soil from South Kohala, HI.