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Shade Tree Spatial Structure and Pod Production Explain Frosty Pod Rot Intensity in Cacao Agroforests, Costa Rica

First author: Supagro, UMR System, 2 Place Viala, Bt.27, 34060 Montpellier, France: first, second, and fourth authors: CIRAD, UPR Bioagresseurs, F-34398, Montpellier, France; second author: IICA-PROMECAFE, AP. 55, 2200 Coronado, San José, Costa Rica; second and third authors: CATIE, 7170, Cartago, Turrialba, 30501, Costa Rica: and third and fifth authors: CIRAD, UMR System, 2 Place Viala, Bt.27, 34060 Montpellier, France.

Vegetation composition and plant spatial structure affect disease intensity through resource and microclimatic variation effects. The aim of this study was to evaluate the independent effect and relative importance of host composition and plant spatial structure variables in explaining disease intensity at the plot scale. For that purpose, frosty pod rot intensity, a disease caused by Moniliophthora roreri on cacao pods, was monitored in 36 cacao agroforests in Costa Rica in order to assess the vegetation composition and spatial structure variables conducive to the disease. Hierarchical partitioning was used to identify the most causal factors. Firstly, pod production, cacao tree density and shade tree spatial structure had significant independent effects on disease intensity. In our case study, the amount of susceptible tissue was the most relevant host composition variable for explaining disease intensity by resource dilution. Indeed, cacao tree density probably affected disease intensity more by the creation of self-shading rather than by host dilution. Lastly, only regularly distributed forest trees, and not aggregated or randomly distributed forest trees, reduced disease intensity in comparison to plots with a low forest tree density. A regular spatial structure is probably crucial to the creation of moderate and uniform shade as recommended for frosty pod rot management. As pod production is an important service expected from these agroforests, shade tree spatial structure may be a lever for integrated management of frosty pod rot in cacao agroforests.

Additional keywords: biodiversity and ecosystem functioning, fungal pathogen, host composition, plant community, plant patterns, Ripley's function, Theobroma cacao.