USING OMETHOATE INSECTICIDE AND LEGUME INOCULANT ON SEED

Omethoate, an organophosphorus insecticide used for early control of redlegged earth mite on legume seed, was toxic to some Rhizobium strains on direct contact when diffused through agar seeded with these bacteria or mixed in broth cultures containing the bacteria. Omethoate mixed with peat-based legume inoculant and applied to seed of subterranean clover or lucerne significantly (P < 0.05) reduced the number of nodules formed over 3 weeks on seedlings grown in pots of sand, compared with inoculated controls. Most nodules were immature and located on lateral roots, and plant dry matter at 4 weeks was significantly (P < 0.05) less for lucerne. Significantly (P < 0.05) fewer rhizobia were recovered from seeds and seedlings of lucerne when seed was treated with omethoate. When subterranean clover seed was pretreated with omethoate 16 h before inoculation and sown into moist sand, there were no significant differences in the mean numbers of rhizobia, mean numbers of nodules or dry matter compared with inoculated controls. With lucerne, the number of nodules formed following omethoate pretreatment was significantly less than on inoculated controls at day 21 but not at day 30, and the dry matter at day 30 was not adversely affected. Rhizobia numbers were reduced less than by mixing omethoate and inoculant, so more taproot nodulation occurred and more of the nodules were functional (i.e. N2-fixing) at 3 weeks. In a field trial with pea, white clover and murex medic, seed pretreatment with omethoate before inoculation had no effect on nodule number 9-11 weeks after sowing, compared with inoculated controls.