Abstract
The present paper deals with the results of the writer's investigations on the relation of potass um in the soil to the severity of “Helminthosporiose” of rice plant. The susceptibility of seedlings as well as the adult leaves of the host plant was tested by artificial inoculations of the causal fungus. The seedlings were planted in the water culture using Kasugai's solution, in which the potassium amounts were changed as 0mg, 10mg and 40mg per litre. The inoculation experiments to the adult leaves were also carried out, using a water culture technique as well as the potexperiments. The results showed that with solutions or soil in which the nitrogen and phosphorus levels were held constant, the disease-severity decreased as the potassium level was increased. The effect of periods of potassium-deficiency on the susceptibility was also tested. The results showed that the more deficient or thelongeri in deficient conditions of potassium thesoil or the solution is, on which the plants grow, the more susceptible they become. However, the effect of deficiency seems to become inconspicuous after the end of growing stages.
Considering the results of inoculation experiments, the writer came to the conclusion that the pathogenecity of conidia produced on the diseased rice plants, grown in the solution lacking or containinga small amount of potassium fertilizer, seems to be stronger than those produced in the condition of high level of potassium. In this experiment, the diseased adult leaves and seeds of different samples were cut off at the same time and washed by a brush and kept in a wet chamber at 28°C. Gathering the newly formed conidia, the writer made suspentions having a similar concentration and sprayed them on the seedlings grown in the same conditions. The pathogenecity of these conidia was justified by comparing the number of diseased spots per unit area of leaves.
