Spatial and temporal analysis of a Phytophthora megakarya epidemic in a plantation in the centre region of Cameroon
Black pod rot of cocoa caused by Phytophthora megakarya causes significant losses in Cameroon. Studying the spatial and temporal disease dynamics of P. megakarya provides useful information on the mechanisms of dispersal and the physiological and biological factors that are important for its spread and ultimately its management. Therefore, we studied the spatial and temporal development of a P. megakarya epidemic in a plantation in the Centre region of Cameroon over two production seasons. A map was made of a cocoa plantation of 2.61 ha, containing a total of 2536 cocoa trees and 438 neighbor/shade trees. Each cocoa and neighbor tree was assigned x and y coordinates by projecting the plantation map onto an orthonormal grid. From a total of 421 cocoa trees located throughout the plantation, production and pod rot data were collected weekly. Subsequently, for both production seasons, the spatial relation between the numbers of rotten pods harvested from the observed cocoa trees was analyzed by semivariogram analysis. Block kriging was used as an interpolation method. Analysis of the semivariograms revealed a spatial dependence of pod rot distribution. In all cases where the theoretical model fitted well to the actual data the semivariogram was exponential. The estimated dispersion range of P. megakarya varied from 2.7 to 6.9 m in 2006 (mean 5.1, SD 0.56 m) and 3.3 to 6.9 m in 2007 (mean 5.6, SD 0.94 m). Kriging maps revealed the simultaneous appearance of multiple infection points throughout the plantation at distances larger than the dispersion range. Infection hot spots wer located at similar locations for both years. Based on these results, we hypothesize that primary inoculum is the main determinant for the spatial and temporal development of an epidemic at the plantation level and that secondary inoculum is mainly responsible for the within-tree temporal development of an epidemic. Therefore, more attention should be given to reducing primary inoculum levels of P. megakarya in order to improve control efficacy.