THE EFFECT OF INORGANIC MANURES, MOISTURE AND INOCULUM ON THE INCIDENCE OF ROOT DISEASE CAUSED BY RHIZOCTONIA SOLANI KUHN IN CULTIVATED SOIL

An account is given of experiments designed to study the effect of inorganic manures, both singly and in combination, soil moisture and available inoculum on the development of root disease in glasshouse soils. Growth of Rhizoctonia solani Kuhn in sterilized soil was increased by applications of balanced fertilizer, by moderate doses of potassium and by the highest rate of phosphates applied. It was decreased by the higher applications of nitrogen and the highest rate of potassium. In unsterilized soil growth was poor in all treatments. Fresh weights of lettuce seedlings were increased by moderate applications of balanced fertilizer but decreased by the higher levels of this and of the individual ingredients. Most disease was found in the balanced (basic) and nitrogen series: potassium and phosphatic amendments had comparatively little effect. Mycelium of R. solani grew best, and produced most disease, in the relatively drier soils. It was found to need a minimum supply of nutriment (food base) to establish itself in soil but high concentrations of inoculum reduced pathogenicity. Its survival in artificially inoculated soils was shown to be little affected by soil fertility. A description is given of a method to demonstrate the persistence and rate of spread of a parasite under relatively natural conditions.