Rhizobial inoculant for iprodione-treated lupin seed: evaluation of an iprodine-resistant Rhizobium lupini

Abstract

Lupin nodulation is reduced when seed of this legume is treated with commercial inoculant (Rhizobium lupini WU425) and the fungicide Rovral (a.i. iprodione). The number of WU425 colonising soil beneath lupin seedlings established without iprodione was greater than where iprodione was used. R. lupini CC606B has greater tolerance of iprodione than WU425; 85% of CC606B survived 15 min in iprodione (Rovral 16 g/L), compared with <1% survival of WU425. CC606B fixed as much nitrogen as WU425 on serradella grown in sand culture, but it nodulated lupin poorly in the field. It was shown that CC606B comprised bacteria that were either positive or negative for nodulation of serradella and lupin. Thus, an isolate of CC606B (CC606B/1), able to form nodules on lupin and serradella and also tolerant of iprodione (Rovral 16 g/L), was selected and compared with WU425 as inoculants of lupin, in field trials with and without iprodione seed treatment. In the treatments without the fungicide, CC606B/1, like its parent, failed to nodulate lupin as extensively as WU425. CC606B/1 was unable to colonise soil beneath lupin crops as well as WU425. With iprodione, nodulation by CC606B/1 was significantly worse than without the fungicide; so the tolerance level of CC606B/1 to iprodione was insufficient. With these treatments it was shown that nodule abundance in late winter was related to the abundance of rhizobia in the soil beneath lupin seedlings in early winter. When there were fewer nodules, less dry matter was produced; but grain yield was not affected by the reductions in nodulation and dry matter. The implications of these changes on N2 fixation and lupin effects on soil N are discussed. Other species and strains of Rhizobium varied in their ability to survive iprodione.