Influence of Rhizosphere Micro-organisms on the Virulence of Rhizobium trifolii

Abstract

THE increasing interest which has been shown in the interactions of Rhizobia with other microorganisms of the indigenous population of the soil has been reflected in the growing list of organisms which have been found to be inhibitory or antagonistic to their growth in various media. The reviews by Casas-Campillo¹ and Abdel-Ghaffar and Allen² emphasize the prominent role of Actinomycetes in such antibioses, but indicate that quite a wide range of bacteria may produce significant effects also. It is noteworthy that, though much attention has been paid to antagonistic organisms, relatively little has been reported of micro-organisms stimulating the activity of Rhizobia. Krasilnikov and Korenyako³ found that a number of non-sporing bacteria of the genera Pseudomonas and Achromobacter introduced into the rhizosphere of the leguminous host induced an increase in the numbers of Rhizobia per gram of rhizosphere soil, and that as a consequence the number of nodules per plant showed an increase and the time taken for nodules to appear was shortened. Such bacteria were termed ‘activators’ in contrast to types which were inhibitors, or to others which were without effect. These workers advocate that seed should be inoculated with mixed cultures of Rhizobia plus activators on the basis of their findings of increased herbage yield and seed set of beans, lucerne and lupins.