THE INTERACTION OF HIGHER PLANTS AND SOIL MICRO-ORGANISMS: II. STUDY OF THE MICROBIAL POPULATION OF THE RHIZOSPHERE IN RELATION TO RESISTANCE OF PLANTS TO SOIL-BORNE DISEASES

Abstract

Comparative studies of flax varieties resistant and susceptible respectively to wilt, and of tobacco varieties resistant and susceptible to black root rot, showed higher numbers of micro-organisms in the rhizosphere of the susceptible than of corresponding resistant plants. Though plants of the same variety showed considerable variation in rhizosphere population under field and greenhouse conditions, the general trend remained the same. The abundance of micro-organisms in the rhizosphere of plants of the same variety grown in plots receiving different fertilizer treatment showed relatively little difference, even though the soils varied greatly in productivity.Numbers of micro-organisms in the rhizosphere of flax were greater when the water content of the soil was maintained at 30%, than when held at 60%, of total moisture-holding capacity. However, the microbial population in the soil distant from the roots was lower in the drier soil.Differential counts of fungi and actinomycetes indicated that the number of colonies developing from spores or conidia comprised a small proportion of the total count. Sporulation of fungi was more profuse in soil distant from the plant than in the rhizosphere.The contact slide method indicated a greater number of micro-organisms in the rhizosphere than in soil distant from the roots and showed differences between the rhizosphere of resistant and susceptible varieties which agreed with results from the plating method.