Identification of Rhizobium isolates possessing antagonistic activity against Fusarium oxysporum f.sp ciceris, the causal agent of Fusarium wilt of chickpea

Using two cultivars (the susceptible ILC482 and the moderately resistant INRAT 87/1) of chickpea (Cicer arietinum), the antagonistic activities of 21 Rhizobium isolates were tested in vitro in dual culture, and in vivo under greenhouse and field conditions against Fusarium oxysporum f.sp. ciceris (Foc) race 0, the causal agent of Fusarium wilt of chickpea. In dual culture, 14 isolates inhibited the mycelial growth of the pathogen more than 30% and the most effective were Rh8, Rh11, Rh16 and PchsSOM, which inhibited fungal growth more than 50%. Among the 14 Rhizobium isolates tested for volatiles, cyanide production and phosphate solubilisation, 8 significantly inhibited fungal growth by producing volatiles, 6 were positive for cyanide production and only three were able to solubilise phosphate. Isolate Rh8 produced the highest levels of volatiles, giving more than 10.7% fungal 'inhibition, and was the only one positive for both cyanide production and phosphate solubilisation. Greenhouse experiments on the same 14 isolates revealed the effectiveness of five: PchDMS, Pch 121, Rh5, Rh17 and Pch43. These reduced the percentage of wilted plants in both susceptible and moderately resistant cultivars. These percentages ranged from 12.5 to 54.6% in the susceptible cultivar ILC482 and from 8.3 to 29.1.% in the moderately resistant cultivar INRAT 87/1. The best disease control was achieved by isolate PchDMS. Despite its effectiveness in vitro, isolate Rh8 was ineffective under greenhouse conditions. Field experiments showed that none of the 14 Rhizobium isolates significantly reduced the percentage of wilted plants of the susceptible cultivar ILC482, although with the moderately resistant cultivar INRAT 87/1 eight of the isolates significantly reduced wilt incidence. Inoculation of seeds with these isolates reduced the percentage of diseased plants from more than 48.6% in infected control plants to less than 20.3% in plants inoculated with the bacteria and infected with the pathogen. The best protection against disease was obtained with isolates Pch43 and Rh4, which reduced the percentage of wilted plants to less than 8%. Besides their beneficial effects on disease control, our studies showed that rhizobia may improve plant growth and yield. These results indicate that Rhizobium isolates could be effective under commercial conditions in reducing the deleterious effects of Fusarium wilt.