The effect of pH on the biological control activities of a Trichoderma sp. against Fusarium sp. isolated from the commercial onion fields in Sri Lanka

Trichoderma spp. are effective biocontrol agents of plant pathogenic fungi and possess varied mechanisms of control. Chemical and physical factors including pH are known to have an influence on these mechanisms as well as on the growth and sporulation of Trichoderma. Hence, the effect of pH on the biocontrol mechanisms of a Trichoderma sp. isolated from the soils of commercial onion fields of Matale district Sri Lanka was evaluated in this study. The inhibition of growth of a Fusarium sp. isolated from onion seedlings with damping off disease was evaluated using dual culture assay and volatile metabolite plate assay at pH 4, 5, 6 and 7. Both tests indicated that growth inhibition of the Fusarium sp. tested was most effective at pH 6. The rate of sporulation of the Trichoderma sp. in PDA plates with amended pH values (4, 5, 6 and 7) was also estimated and a significantly high (p≤0.05) rate of sporulation (5×107 spores ml-1 ) was observed at pH 6 after 49 days. The effect of pH on the survival of Trichoderma evaluated using pot experiments showed that the pH values 4.6–5.0 facilitated the highest survival rate of 7.9×105 cfu ml-1 . Effect of Trichoderma sp. on the growth of Fusarium sp. at different pH values was evaluated under greenhouse conditions and a significantly low level of Fusarium colonies were isolated from seedlings treated with Trichoderma at pH 4.6. This is the first report of the effect of pH on the activities of Trichoderma spp. isolated from Sri Lankan soils.