VIEW ARTICLE

Ecology and Epidemiology

The Role of Calcium and Microorganisms in Suppression of Cucumber Damping-Off Caused by Pythium splendens in a Hawaiian Soil. 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 76:221-225. Accepted for publication 10 September 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-221.

Amendment of conducive soil with CaCO₃, alfalfa meal, or CaCO₃ plus alfalfa meal reduced damping-off caused in cucumber seedlings by Pythium splendens from 78 to 30, 44, and 11%, respectively. CaSO₄ was as effective as CaCO₃ or Ca(OH)₂ in reducing disease incidence even though it did not increase the soil pH. Growth of cucumber seedlings and the number of roots were greatly increased by amendment of soil with Ca, alfalfa meal, or Ca plus alfalfa meal. Amendment of soil with Ca plus alfalfa meal was the most effective of these treatments. Sporangial germination of P. splendens was most responsive to increased nutrients from cucumber root extract in conducive soil followed by Ca-amended soil and alfalfa meal-amended soil. Germination was least responsive to nutrients in suppressive soil and conducive soil amended with Ca plus alfalfa meal. In the cucumber rhizosphere, sporangia of P. splendens germinated 67% on conducive soil and 23% on suppressive soil. Germination on conducive soil was decreased to 50% with amendment of Ca, 44% with alfalfa meal, and 27% with Ca plus alfalfa meal. Infection occurred on 81% of the inoculated cucumber roots grown in conducive soil, 25% in suppressive soil and 21% in conducive soil amended with Ca. Amendment of conducive soil with alfalfa meal did not decrease the percentage of root infection, nor did it decrease the size of necrotic lesions on roots. Amendment of soil with Ca increased the concentration of N and Ca in cucumber tissues, whereas amendment of soil with Ca plus alfalfa meal increased the concentration of N, K, and Ca. Two weeks after planting in unamended field conducive soil, 59% of cucumber seedlings were killed by P. splendens, whereas only 11, 15, and 8% of seedlings were killed in soil amended with Ca, alfalfa meal, and Ca plus alfalfa meal, respectively. Leaves of cucumber seedlings grown in unamended soil were small and yellowish, while leaves were large and green in amended soil. Soils amended with alfalfa meal and Ca plus alfalfa meal contained 4.0 x 10⁸ and 6.0 x 10⁸ colony-forming units of microorganisms per gram of soil, respectively. Amendment of soil with Ca also increased the total microbial population from 4.6 x 10⁶ to 6.9 x 10⁷ colony-forming units per gram of soil.