The impact of fungicides with contrasting toxicities to rhizobia on the growth and nodulation of pulses in southern Australia

Context Pulse crops generally need inoculation with compatible rhizobia to optimise nodulation and nitrogen (N) fixation; however, seed-applied fungicide, often applied in chickpea, may compromise rhizobial survival. Aims The fungicide P-Pickel T (PPT; thiram and thiabendazole) is highly toxic to rhizobia and nodulation, but this has not been validated outside a controlled environment. Other fungicides containing metalaxyl with low toxicity to rhizobia could be used under certain disease environments to improve root health and nodulation. Methods Field experiments were conducted to provide evidence of the toxic effect of thiram-based fungicides in chickpea, and to test whether the fungicides Apron XL, Uniform and Vibrance applied on-seed and in-furrow could improve nodulation of chickpea and lentil. These experiments were conducted in four environments over 3 years throughout southern Australia. Key results Different inoculant types and inoculation methods showed that PPT application with peat inoculant markedly reduced nodulation in both lower (<300 mm) to medium (>400 mm) rainfall sites, where N fixation was lowered by up to 20% (chickpea N derived from N fixation, %Ndfa) and by 60% (mg N fixed/plant). Granular inoculant improved nodulation compared with using a seed-applied inoculant. Nodulation was increased by nearly 50% at two sites with Apron XL, Uniform and Vibrance. Conclusions PPT should not be used in contact with rhizobial inoculants in chickpea and farmers should be encouraged to separate the fungicide by using an in-furrow inoculant. Implications Fungicides with low to no toxicity to rhizobia may promote nodulation and provide an alternative fungicide for use on pulses.