Biocontrol of cocoa pod diseases with mycoparasite mixtures

Five native mycoparasitic strains of Clonostachys rosea and three of Trichoderma spp. were isolated from healthy cocoa tissue or basidiocarps of Crinipellis perniciosa using a baiting technique. They were compared singly or in combination with the commercial biocontrol agent Trichoderma virens (SoilGuard) for their potential to control three cocoa pod diseases: moniliasis, caused by Moniliophthora roreri; witches' broom, caused by C. perniciosa; and black pod, caused by Phytophthora palmivora. All isolates except Trichoderma T-1 inhibited basidiocarp formation of C. perniciosa under controlled conditions. The remaining isolates except Trichoderma T-3 reduced vegetative broom formation in a seedling bioassay. Clonostachys rosea G-3 and Trichoderma strains T-2 and T-3 significantly reduced symptoms caused by M. roreri in a seedling bioassay. Host-range studies identified P. palmivora as most susceptible to mycoparasitism and M. roreri as most resistant; C. perniciosa was intermediate. Different degrees of susceptibility were discovered at the pathogen strain level, especially for P. palmivora. However, broad host-range mycoparasites which attacked all three pathogen species were identified. Under field conditions, all selected treatments except a combination of C. rosea G-2 + G-3 reduced moniliasis, the main disease, significantly by 14.6-24.9% as compared with optimized, cultural control alone. No significant reduction of witches' broom or black pod was achieved but a combination of five C. rosea strains (G-1, G-2, G-3, G-4 + G-5) performed consistently best against all three diseases simultaneously. Yield increased by 16.7% and net returns by 24%. Control of moniliasis and yield were positively correlated to the number of mycoparasites in the inoculum. The results suggest that simultaneous biocontrol of the three major cocoa pod diseases with mycoparasite mixtures is highly promising. Future development strategies are discussed. (C) 2001 Academic Press.