EFFECT OF CROP DEBRIS, PLANT ROOTS, AND CROP SEQUENCE ON THE MICROBIAL FLORA OF THE SOIL IN RELATION TO ROOT ROT IN CEREAL CROPS

Abstract

The decomposition of wheat, oat, or barley straw in soil caused a marked increase in the microflora, as indicated by plate counts. The oat straw compost yielded significantly more colonies of fungi than either the wheat or barley straw composts. The population of bacteria and actinomycetes was increased to about the same degree by all three of the composts.In another laboratory experiment, wheat, oats, barley, and beans were planted in various rotations in pots of soil from field plots known to be heavily infested with the root-rotting pathogens Helminthosporium sativum P. K. and B., Ophiobolus graminis Sacc., and Fusarium spp. Fungi, mostly Penicillium spp. and Mucor spp., were up to 15 times more abundant from the rhizosphere of wheat roots than from the rhizosphere of oat, barley, or bean roots, regardless of crop sequence. It is assumed that the higher counts of saprophytic fungi obtained from the rhizosphere of wheat seedlings were directly correlated with the greater amount of dead root tissue on this crop, since disease on the wheat seedlings was more severe than on the other hosts. In a duplicate experiment in fallow soil, the rhizospheres of wheat, oats, barley, and beans yielded about equal numbers of fungi.