Analysis and Mapping of the Spatial Spread of African Cassava Mosaic Virus Using Geostatistics and the Kriging Technique

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Abstract
Theories of regionalized variables and kriging were used to assess the spatial pattern of African cassava mosaic virus (ACMV). A linearlike semivariogram without a range characterizes the ACMV distribution and indicates a strongly spatially dependent structure with limited random variation. Oriented semivariograms reveal a strong anisotropy in relation to the prevailing wind direction. Further features of the semivariogram and comparisons of semivariograms between fields and between surveys provide additional information and support various hypotheses on the pattern of spread. From a sample of limited size, kriging reproduced the main characteristics of the spatial pattern of spread, including higher incidence along the wind-exposed southwest field borders, disease gradients, and other less obvious features. Up to 60% of the total variance was reconstructed from a 7% sample. Kriging was successfully applied to characterize the spatial pattern of spread in cassava fields differing in planting date, size, arrangement, orientation, and method of sampling. This technique was also efficient when the pattern of spread was heterogenous, although more intensive surveys were then required. Practical applications of geostatistics and kriging in epidemiology are discussed.