The impact of soil treatments on black pod rot (caused by Phytophthora megakarya) of cacao in Cameroon

In Cameroon, Phytophthora megakarya is the causal agent of black pod rot of cacao (Theobroma cacao L). Primary inoculum of P. megakarya is located in the soil. With the onset of the rainy season, this primary inoculum is activated and through rain splash cacao pods become infected. From there repeated cycles of pod infection, sporulation, spore dispersal and pod infection, result in losses that can reach up to 70-80% 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 possibly be reduced considerably. Biological control of cacao black pod disease, using Trichoderma asperellum PR11, has proven successful in Cameroon. Moreover, T. asperellum PR11 has also been proven to be useful against soilborne inoculum of Pythium myriotylum and Phytophthora ramorum. The objective of this study therefore, was to use soil applications of T. asperellum PR11 to reduce black pod disease due to P. megakarya. This study was undertaken under farmers' field condition in the Centre region of Cameroon. Trichoderma asperellum PRR11, together with a water and fungicide (Ridomil Gold 66 WP) control, were applied on a three weeks basis over two consecutive production seasons. Weekly data collection consisted of counting healthy and diseased pods. For each treatment six replicate plots, each containing 16 trees, were used. Soil samples were taken at the beginning and end of the second season to determine the impact of the treatments on P. megakarya primary inoculum levels as well as the presence of fungal antagonists of P. megakarya. Although soil applications did not significantly reduce the total number of diseased pods, nor overall pod rot rate, soil applications did change disease dynamics by reducing the speed in which the disease progressed. Moreover, soil applications did impact the presence of fungal antagonists and most importantly significantly reduced the amount of P. megakarya primary inoculum present in the soil. Given the effectiveness of T. asperellum in controlling black pod rot, combined aerial and soil applications should have additive effects when combined simultaneously for the control of black pod disease in cacao.