Response surface methodology study of the combined effects of temperature, pH, and aw on the growth rate of Trichoderma asperellum

Aims: To evaluate the influence of environmental parameters (water activity a(w), temperature, and pH) on the radial growth rate of Trichoderma asperellum (strains PR10, PR11, PR12, and 659-7), an antagonist of Phytophthora megakarya, the causal agent of cocoa black pod disease. Methods and Results: The radial growth of four strains of T. asperellum was monitored for 30 days on modified PDA medium. Six levels of a(w) (0.995, 0.980, 0.960, 0.930, 0.910, and 0.880) were combined with three values of pH (4.5, 6.5, and 8.5) and three incubation temperatures (20, 25, and 30 degrees C). Whatever the strain, mycelial growth rate was optimal at a(w) between 0.995 and 0.980, independently of the temperature and pH. Each strain appeared to be very sensitive to a(w) reduction. In addition, all four strains were able to grow at all temperatures and pH values (4.5-8.5) tested, highest growth rate being observed at 30 degrees C and at pH 4.5-6.5. The use of response surface methodology to model the combined effects of a(w), temperature, and pH on the radial growth rate of the T. asperellum strains confirmed the observed results. In our model, growth of the T. asperellum strains showed a greater dependence on a(w) than on temperature or pH under in vitro conditions. Conclusions: a(w) is a crucial environmental factor. Low a(w) can prevent growth of T. asperellum strains under some conditions. The observed and predicted radial growth rate of strain PR11 showed its greater capacity to support low a(w) (0.93) as compared with other tested strains at 20 degrees C. This is in agreement with its better protective level when applied in medium-scale trials on cocoa plantations. Significance and Impact of the Study: This study should contribute towards improving the biocontrol efficacy of T. asperellum strains used against P. megakarya. Integrated into a broader study of the impact of environmental factors on the biocontrol agent-pathogen system, this work should help to build a more rational control strategy, possibly involving the use of a compatible adjuvant protecting T. asperellum against desiccation.