Spatial pattern analysis of witches’ broom disease of cacao at a landscape scale in Rondônia, Brazil
Keywords:Witches' broom, Spatial autocorrelation, Rondônia
AbstractApproximately 60 000 ha of cacao [Theobroma cacao (L.)] are planted in the state of Rondônia, Brazil. The losses in this important cash crop due to witches' broom [Crinipellis pemiciosa (Stahel) Singer], an endemic fungal disease, are estimated at U.S. $12 million annually. A thorough understanding of witches' broom epidemiology is essential for the development of more effective disease control procedures and higher cacao yields. The spatial pattern of witches' broom was examined at a scale intermediate to the field and regional scales (landscape scale) to evaluate spatial interaction between fields for a municipality in Rondônia. The spatial pattern of disease intensity levels between fields was evaluated using join count statistics, in combination with a distance-based measure of local spatial autocorrelation, G?(d). Most of the municipality exhibited no positive spatial autocorrelation, indicating that plantations exhibiting the highest disease intensity levels were not clustered. The random nature of the spatial pattern was hypothesized to be the result of a combination of the architecture of the individual lots, disease control (phytosanitary pruning), and the separation of plantations by distances greater than can be traversed by viable inocula.