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 commercial oil palm plantations in Surinam. Geostatistics and gradient analysis were 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 initially, the variogram indicated that affected trees were not spatially related, suggesting that infection came from various distant sources. Later, the semivariances in one of these two blocks and in seven others, calculated for successive dates, showed a linear increase with distance. The variograms for four blocks showed a nonlinear increase in variance. Over the years, the variograms suggested that the variation in spear rot was anisotropic, with more spatial dependence in a westerly direction. Classical analysis of disease 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 is vector-borne, the vector being displaceable by wind; and (3) spear rot appears in two distinct phases, phase 1 being characterized by few randomly scattered trees, phase 2 by focal spread of disease starting from such scattered trees. The trigger of the change from phase 1 to phase 2 remains unknown.