SOIL CONDITIONS AND THE TAKE‐ALL DISEASE OF WHEAT

Abstract

SUMMARYA study has been made of the decline in viability of Ophiobolus graminis as resting mycelium in artXcialIy infected wheat straw buried in the soil. The pieces of straw were buried in variously treated soils set up in glass tumblers, and were examined at intervals for the presence of still viable Ophiobolus mycelium by means of a wheat seem test.The experimental results suggest that the disappearance of Ophiobolus from the straws was due to natural decomposition by the other soil microorganisms since, in its resting phase, the fungus tolerated adverse physical conditions of the soil better than conditions optimum for microbiological activity. Decline in viability of the fungus appeared to be indefinitely postponed in air‐dry soil, in soil at 2–3°C., and under sterile conditions in the culture flask; it was less rapid in a waterlogged soil than in one maintained at medium moisture content. The soil conditions least favourable for the advance of Ophiobolus along the host roots in its parasitic phase (Garrett, 1936) may best preserve it during its resting phase because they are also unfavourable for general microbiological activity.Loss of viability was hastened by the addition of energy materials poor or entirely lacking in nitrogen, such as glucose, starch, and rye‐grass meal, to the soil; it was more rapid in a partially sterilized and reinoculated soil than in an untreated soil. These results may be attributed to the rise in numbers and activity of soil micro‐organisms following upon the treatments. The rate of decline of the fungus varied with soil type, being more rapid in rich and heavy soils than in poor, light soils. Rate of decline was apparently not directly affected by soil reaction, nor appreciably by moisture content of the soil over the range 30–80% saturation. Decline of the fungus was more rapid under conditions of fluctuating soil moisture and improved aeration in unglazed flower‐pots than under more uniform conditions in glass tumblers; it was slowest in soil maintained under still and uniform conditions in a small closed incubator.The most rapid disappearance of the Ophiobolus resting mycelium, therefore, seems to have been secured by conditions favouring maximum microbiological activity in the soil. The decline in viability of the fungus did not necessarily proceed parallel with gross decomposition of the infected straw as a whole. It was delayed by the addition of dried blood, containing 13% nitrogen, to the soil, whereas this treatment accelerated decomposition of the straw. Decline of the fungus was accelerated by addition of rye‐grass meal, which delayed decomposition of the straw by taking up the available nitrogen. It is suggested that the Ophiobolus mycelium may itself serve as a source of nitrogen for the decomposition of the straw, and that the rapidity of its disappearance may be related directly to the degree of nitrogen scarcity in the soil and straw medium.I have much pleasure in thanking Dr A. G. Norman for various useful suggestions, and for all the nitrogen determinations. I am especially indebted to Miss L. Cunow and to Miss M. M. Browne for help in the carrying out of the experiments.