Assessing synergism of combined applications of Trichoderma harzianum and Pseudomonas fluorescens to control blast and bacterial leaf blight of rice

Combined application of biocontrol agents (BCAs) have been used to exploit their synergistic potential. But the analysis of additive, synergistic and antagonistic effects of BCAs has rarely been explored for the biocontrol of plant diseases. This study was conducted to evaluate in vitro and in vivo biocontrol potential of the combined application of Trichoderma harzianum strain Th3 and Pseudomonas fluorescens strain RRb11, and the fungicide carbendazim against the rice blast (RB) pathogen, Magnaporthe oryzae, and the bacterial leaf blight (BLB) pathogen, Xanthomonas oryzae pv. oryzae . Combined application of T. harzianum Th3 and P. fluorescens RRb11 synergistically reduced severity of RB by 69.5% in comparison to the untreated control, displaying a synergy factor (SF) of 1.29. P. fluorescens RRb 11 alone and in combination with T. harzianum Th3 and the mixture of T. harzianum Th3 + carbendazim was antagonistic (SF = 0.71 and 0.45, respectively). The combined application of T. harzianum Th3 and P. fluorescens RRb 11 also enhanced several rice plant growth and yield parameters. Additive, synergistic or antagonistic effects among the two BCAs were assessed by applying the numerical hypothesis of ‘Bliss Independence’ which suggested that the combined application of the two BCAs was synergistic in reducing RB but antagonistic with respect to BLB. Out of the six combined application interactions for both the diseases only one interaction ( T. harzianum Th3 and P. fluorescens RRb11) was synergistic against RB, which suggested that synergism is a rare event in combined use of BCAs.