Nodulation and Yields of Common Bean are Not Affected Either by Fungicides or by the Method of Inoculation

Though some studies observed negative effects of fungicide seed treatment on yield of common bean (Phaseolus vulgaris L.), few studies have directly evaluated fungicide compatibility with seed inoculation treatments in field conditions. For fungicides that are incompatible, rhizobia inoculation in the planting furrow (in-furrow inoculation) would permit combined use of both management practices since this would avoid direct contact between the chemical compound and the rhizobia. Thus, the aim of this study was to evaluate the compatibility of seed fungicide treatment and rhizobia inoculation techniques to improve N derived from rhizobia symbiosis in common bean. A preliminary in vitro experiment evaluated the survival of Rhizobium tropici after contact with fungicides compared with an untreated control. Two field experiments were conducted in a 6 x 4 factorial arrangement involving the same treatments used for the in vitro survival test, as well as four sources of N [two types of liquid inoculation (inoculation on the seed or in-furrow inoculation), plus two controls without inoculation]. We concluded that captan and difenoconazole fungicides reduced in vitro survival of rhizobia inoculated on the seed. Yield measurements in the field studies indicated that symbiotic nitrogen fixation could replace N fertilization, regardless of the inoculation treatment method and environmental conditions. Difenoconazole is potentially harmful to native bacteria in the field; therefore, in-furrow inoculation may be recommended when the use of difenoconazole is necessary. Moreover, based on the harmful effects observed in vitro, precautions similar to the use of difenoconazole should also be taken when using captan fungicide in field conditions.