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Representational Errors Due to Interplot Interference in Field Experiments with Late Blight of Potato. W. C. James, Plant Pathologist, Ottawa Research Station, Canada Agriculture, Ottawa, Ontario; C. S. Shih, Statistician, Statistical Research Service, Ottawa, Ontario; W. A. Hodgson, Plant Pathologist, Research Station, Fredericton, New Brunswick; and L. C. Callbeck, Plant Pathologist, Research Station, Charlottetown, Prince Edward Island; all with Canada Department of Agriculture.  Phytopathology 66:695-700.

In field experiments the yield loss associated with fungicide treatments may be overestimated (positive interference) if the adjacent plots have more disease, or underestimated (negative interference) if they have less disease.  Results from 14 field experiments with potatoes (Solanum tuberosum) infected with Phytophthora infestans, were used to study the relative importance of positive and negative interference in plots sprayed with Dithane M-45 (coordination product of zinc ion and manganese ethylene bisdithiocarbamate).  An epidemic in some plots resulted in positive interference in fungicide sprayed plots in the same experiment despite the application of five protectant sprays.  Negative interference occurred in sprayed plots adjacent to plots with no disease, and the magnitude of negative interference was greater than the positive interference.  The representational errors between treatments due to interference were equal to or greater than the corresponding experimental errors.

Additional key words: epidemiology, experimental design.