Behaviour of the fungus Rhizoctonia solani Kiihn in the soil

Abstract

Rhizoctonia solani was found able to grow as a saprophyte through natural unsterilized soil. Its rate of growth under different soil conditions in glass tumblers was studied by the Rossi‐Cholodny soil‐plate method. Growth was most rapid at the lowest soil‐moisture content tested, viz. 30 % saturation, and was accelerated by forced aeration of the soil. The maximum distance to which mycelial growth could be supported on the food reserves of the agar inoculum alone was some 5 cm., as shown by extent of growth through tubes of moist sand, but in 23 days the fungus grew 21–24 cm through tubes of soil. Removal of the agar disk 2 days after inoculation of the tubes reduced growth through sand by more than half, but through soil by only a small proportion. In soil, Rhizoctonia was able to cause 100% damping‐off of radish seedlings planted at a radial distance of 4 cm. from the agar inoculum, and some 40 % damping‐off at a distance of 9 cm. The depressing effect of additions of 1 % ground‐wheat straw or dried grass to the soil upon growth of the fungus was attributed to (1) the negligible cellulose‐decomposing ability of Rhizoctonia, (2) nitrogen starvation of the mycelium, through rapid utilization of the available soil nitrogen by the cellulose‐decomposing micro‐organisms multiplying upon the fresh organic material, (3) fungistatic action on Rhizoctonia of the respiratory carbon dioxide produced by the cellulose‐decomposers.