RHIZOBIAL COMPETITION AND ENHANCING RHIZOBIAL COLONIZATION IN THE LEGUME RHIZOSPHERE USING A SYSTEMIC FUNGICIDE

Abstract

The inoculation of legumes with effective rhizobia or bradyrhizobia represents an inexpensive alternative to the use of chemical nitrogen fertilizers, whose prices have risen due to the high cost of energy involved in their production.  These fertilizers are also pollution hazards.  The process of symbiotic biological nitrogen fixation requires that the host crop be adequately nodulated by the specific root-nodule bacteria effective in nitrogen fixation.  Not all the strains of Rhizobium or Bradyrhizobium that can produce nodules on a given host are able to use N₂ rapidly and efficiently.  Nonetheless, selection of an effective (i.e. N2-fixing) strain is a prerequisite for any crop to be inoculated. A second important characteristic is the competitiveness of the strain.  Unfortunately, effectiveness and competitiveness are generally mutually exclusive and are not dependent upon each other.  Little information exists on the effects of systemic fungicides on symbiotic nitrogen fixation or nodulation.  It has been reported that the systemic fungicide benomyl increased the relative abundance of nodules formed by the inoculated strain, the number of added rhizobia on the root, the total N content, and the percentage N of soybean plants grown in four soils when the seeds were inoculated with a benomyl-resistant strain of Bradyrhizobium japonicum.  It was also found that oxamyl (a basipetally translocated fungicide) applied to the seeds, foliage, or both increased the yield, N content, percentage N, and weight of nodules, pods, and grains along with the number of nodules formed by the inoculated strain when soybean seeds were inoculated with oxamyl-resistant Rhizobium japonicum.