Spatial patterns of spear rot in oil palm plantations in Surinam

As the aetiology of spear rot of oil palm is unknown, indirect methods were applied to study its putative infectiousness by analysing data from commer cial oil palm plantations in Surinam. Geostatistics and gradient analysis w ere used to examine the spatial variation of spear rot disease in 13 blocks at two plantations. In two blocks, which had low spear rot incidence initi ally, the variogram indicated that affected trees were not spatially relate d, suggesting that infection came from various distant sources. Later, the semivariances in one of these two blocks and in seven others, calculated fo r successive dates, showed a linear increase with distance. The variograms for four blocks showed a nonlinear increase in variance. Over the years, th e variograms suggested that the variation in spear rot was anisotropic, wit h more spatial dependence in a westerly direction. Classical analysis of di sease gradients over time confirmed that there was a preferential direction of disease spread. The data are compatible with the following hypotheses: (1) spear rot is an infectious disease; (2) the causal agent of spear rot i s vector-borne, the vector being displaceable by wind; and (3) spear rot ap pears in two distinct phases, phase 1 being characterized by few randomly s cattered trees, phase 2 by focal spread of disease starting from such scatt ered trees. The trigger of the change from phase 1 to phase 2 remains unkno wn.