STUDIES ON THE GROWTH IN SOIL AND THE PARASITIC ACTION OF CERTAIN RHIZOCTONIA SOLANI ISOLATES FROM WHEAT

Abstract

An adaptation of the Rossi and Cholodny glass slide technique was found to be an effective means of measuring the growth of Rhizoctonia Solani in soil. After a 6 day and a 12 day period, the extent of growth of 11 isolates of this fungus was, for each growth period, less in a vertical than in a radial direction. Certain isolates grew faster than others. A comparison of the radial growth of a faster and of a slower growing isolate at soil depth of 2, 4, and 6 in. showed that the extent of growth decreased with depth, being significantly greater for both isolates at the 2 in. than at the 6 in. level.In pathogenicity tests on wheat with 10 of these isolates, the disease rating for each isolate was greater in natural than in steam sterilized soil, and in soil with a proportion of inoculum to soil of one to six than of one to three. The addition of cellulosic organic material, grass- or straw-meal, to unsterilized soil was effective in reducing the parasitic action of all isolates. Two distinct types of injury were observed: the one, a severe form of root injury, resulting in reduced plant growth; the other, a girdling of the coleoptile or lower stem tissue, usually unaccompanied by adverse effects on plant growth. The first type was produced by two slow growing isolates of English origin, the second by faster growing isolates of Canadian origin. On the basis of these differences, it is suggested that the root injuring isolates be regarded as a variety of R. Solani Kühn.