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Interplot Interference in Field Experiments with Late Blight of Potato (Phytophthora infestans). W. C. James, Plant Pathologist, Ottawa Research Station, Ottawa, Ontario; C. S. Shih(2), L. C. Callbeck(3), and W. A. Hodgson(4). (2)Statistician, Statistical Research Service, Ottawa, Ontario; (3)(4)Plant Pathologists, Research Station, Charlottetown, Prince Edward Island, Fredericton, New Brunswick, Canada Department of Agriculture. Phytopathology 63:1269-1275. Accepted for publication 5 April 1973. DOI: 10.1094/Phyto-63-1269.

Results from 10 field experiments with potatoes (Solanum tuberosum) infected with late blight disease, caused by Phytophthora infestans, were used to measure interplot interference. The merits of the experimental design, and four methods of measuring interference, are discussed. Measuring interference in terms of percentage loss in tuber yield, rather than percentage disease, was considered the most relevant. If interference in a treatment caused an increase in disease, interference was said to be positive. Conversely, negative interference was said to be present in a treatment, if disease was decreased. The presence of an induced epidemic in an experiment exerted a positive interference on the adjacent unsprayed plots. The results were not conclusive as to whether various fungicides spray schedules could suppress positive interference. The “representational error” that may arise in the results of variety trials for assessing horizontal resistance to blight, when interference is present is discussed.

Additional keywords: epidemiology.