The impact of Trichoderma asperellum soil treatments on Phytophthora megakarya, the causal agent of black pod rot of cacao in Cameroon

Many Phytophthora species have a soil borne phase in their natural life cycles even though disease expression often occurs on aerial plant parts. In Cameroon, black pod rot of cacao (Theobroma cacao L.), is caused by P. megakarya. Primary inoculum of P. megakarya is located in the soil and with the onset of the rainy season, through rain splash, cacao pods become infected. From there the repeated cycles of pod infection and sporulation are at the origin of losses that can reach up to 80% when no control measures are in place. Thus, if this soil borne inoculum could be eliminated or prevented from reaching cacao pods, losses from black pod could be reduced considerably. In Cameroon, successful biological control of black pod rot has been obtained with the use of Trichoderma asperellum PR11 applications directed at cacao pods. The objective of this project therefore, was to investigate the possibility of using T. asperellum PR11 in soil applications in order to reduce primary inoculum and/or reduce black pod disease incidence due to P. megakarya. The study was undertaken in a farmer's field near Nkolbisson, Centre region of Cameroon. Trichoderma asperellum PR11 was applied on a monthly basis over two consecutive production seasons. A water only and a fungicide, Ridomil Gold 66 WP (6 % metalaxyl-M & 60 % CuO) treatment were used as controls. Weekly data collection consisted of counting all healthy and diseased pods. Soil samples were taken at the onset and end of the experiment to determine primary inoculum levels in the soil. For each treatment, six replicate plots each containing 16 cocoa trees were used. The absolute number of rotten pods and percentage pod rot was lower for T. asperellum treated plots when compared with the water control yet higher than the fungicide control, albeit these differences were not significant. There was however, a significant difference between treatments with regard to disease progression over time. Moreover, T. asperellum treatments were very efficient in reducing the amount of primary inoculum in the soil, even more so than the fungicide treatment. Given that T. asperellum applications directed at cacao pods have shown efficiency in controlling black pod rot, the use of soil applications of T. asperellum, which slows disease progression and reduces soil inoculum, should therefore lead to additive effects when both control strategies are employed simultaneously. (Resume d'auteur)