VIEW ARTICLE

Ecology and Epidemiology

Suppression of Pythium splendens in a Hawaiian Soil by Calcium and Microorganisms. C. W. Kao, Former graduate research assistant, Department of Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720, Present address: Taiwan Agricultural Chemicals and Toxic Substances Research Institute, Wufeng, Taichung, Taiwan; W. H. Ko, professor, Department of Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720. Phytopathology 76:215-220. Accepted for publication 4 September 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-215.

Germination inhibition of sporangia of Pythium splendens in seven different soil types was associated with high Ca content and high total microbial population. Conducive soils which were low in Ca content and microbial population became suppressive to P. splendens after amendment with 0.6% Ca and mixture of microorganisms, or with Ca and 1% alfalfa meal to increase the population of indigenous microorganisms. Germination of sporangia of P. splendens on a suppressive soil from South Kohala, HI, increased with time of storage, and the increase of germination was associated with the decrease of total microbial population. Germination inhibition was restored to suppressive soil stored for 1 and 1.5 yr, by adjusting the moisture content of soils from -110 to -7 bars to increase the microbial population. Suppressiveness also was restored to soil stored for 3 yr by adding fungi, bacteria, actinomycetes, or their combination. Conducive soils which were high in Ca and low in microbial population were converted to suppressive soil by adding alfalfa meal to increase the microbial population. These results suggest that a combination of high Ca and high microbial population is the cause of suppression in the soil from South Kohala. The inhibition characteristics of the suppressive soil created by amending the conducive soil with Ca and alfalfa meal were similar to those of naturally suppressive soil from South Kohala. Naturally and induced suppressive soils were inhibitory or stimulatory to the same species of microorganisms and the inhibitory effect of both soils was associated with microbial activity. Inhibition of sporangial germination of P. splendens on both soils decreased when sporangia were separated from soil by a polycarbonate membrane. Neither volatile inhibitors nor nonvolatile inhibitors were detected in either soil. Both soils were fungistatic and their inhibitory effect was not transferable. Similarity of inhibition characteristics indicates that the artificially created suppressive soil operates by the same mechanism as the naturally suppressive soil, and supports the hypothesis that a combination of high Ca and high microbial population is responsible for the suppression of P. splendens in the suppressive soil from South Kohala.