Structural characteristics of cocoa-based agroforestry systems can mitigate citrus Phytophthora foot rot disease intensity

Tropical agroforestry systems are recognized as an attractive way to reconcile productivity and ecosystem services. However, to optimize ecosystem services, existing agrosystems structure must be improved to enhance positive interactions within them. In this study, we evaluated the combined effect of shade rate and citrus tree's spatial structure in cocoa-based agroforestry systems (CBAS) on Phytophthora foot rot disease (PFRD). A set of 33 CBAS of 2500 m2 was selected in five citrus production basins in the humid rainforest zones of Cameroon. The citrus spatial structure was analyzed by the Ripley method, and static simulations, with shadow overlapping were run for each plot, using ShadeMotion software. This allowed to quantified shade rate received by citrus trees. Subsequently, citrus trees were divided into nine treatments based on the shade rate (dense shade, light shade or no shade) and their spatial structure (regular, aggregated or random) in the CBAS. PFRD intensity was quantified by measuring PFRD lesion extent relative to the citrus crown circumference. Citrus trees with a regular spatial structure, and located under dense or light shade showed low PFRD intensity. On the other hand, citrus trees with an aggregated spatial structure, regardless of their shade intensity situation, as well as those with a random spatial structure and no shade, showed high PFRD intensity compared to all other treatments. These results show that appropriate shade rate and spatial structure management in CBAS can significantly contribute to PFRD control and thus promote citrus production in complex agroforests. For the first time, it is demonstrated that optimizing the combined effect of shade rate and spatial structure of associated trees would allow the design of optimal models of resilient agroforestry systems, capable of self-regulating pests and reducing chemical pesticide use.