Spatial analysis of blackleg-affected seed potato crops in Scotland

Potato blackleg, caused by Pectobacterium and Dickeya species, is one of the most significant bacterial diseases affecting potato production globally. Although it is generally accepted to be a seedborne disease, the processes underlying the spread of disease largely remain unknown. Spatial point pattern analysis was applied to blackleg occurrence in seed potato crops in Scotland during the period of 2010-2013 (approximately 8000 blackleg-affected crops), to assess whether its distribution was random, regular or aggregated, and the spatial scales at which these patterns occurred. Blackleg-affected crops derived from mother stocks with symptoms were omitted from the analyses in order to examine the statistical evidence for horizontal transmission of blackleg. The pair correlation function was used to test for global spatial autocorrelation, and results indicated significant (P<005) clustering of incidence at a wide range of spatial scales. Strength of clustering (degree of aggregation) among blackleg-affected crops was notably larger at spatial scales of 25km or less. A hot- and coldspot analysis was performed to test for local spatial autocorrelation, and statistically significant clusters of high and low values of disease were found across the country. These analyses provide the first quantitative evidence of localized and large-scale spatial clustering of potato blackleg. Understanding the mode(s) of inoculum dispersal will be important for developing new management strategies that minimize host-pathogen contacts in potato and numerous other crops affected by pathogenic Pectobacterium and Dickeya species.