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Evaluation of a Method Used to Estimate Loss in Yield of Potatoes Caused by Late Blight. W. C. James, Plant Pathologist, Research Stations, Ottawa, Ontario, Canada Department of Agriculture; L. C. Callbeck(2), W. A. Hodgson(3), and C. S. Shih(4). (2)Plant Pathologist, Canada Department of Agriculture, Charlottetown, Prince Edward Island; (3) Plant Pathologist, Canada Department of Agriculture Fredericton, New Brunswick; and (4)Statistician, Canada Department of Agriculture, Statistical Research Service, Ottawa, Ontario. Phytopathology 61:1471-1476. Accepted for publication 15 April 1971. DOI: 10.1094/Phyto-61-1471.

Principles of a disease assessment method, developed in England and Wales to estimate loss in potato tuber yield caused by late blight, Phytophthora infestans, were evaluated in eastern Canada. Field trials were conducted at three locations in 1969 and two in 1970, using the cultivars Green Mountain and Katahdin. The fungicide Difolatan 4 Flowable [N-([1,1,2,2 -tetrachloroethyl]-sulfenyl)-cis-4-cyclohexene-1,2-dicarboximide] which did not affect tuber yield except by controlling late blight, was used to influence three different epidemics by operating various spray schedules. All the disease progress curves were not satisfactorily defined when the data were transformed using log10 x/1 x and regression lines fitted. The disease assessment method assumes that tuber production stops when defoliation by late blight reaches 75%. The loss is calculated by ascertaining the time when this point is reached and using this datum in conjunction with a tuber development curve for Green Mountain. Actual losses, 42, 33, 52, 17, and 26% derived by weighing, were in poor agreement with the estimated losses, 23, 10, 16, 3, and 0%, respectively, computed by the method. Possible reasons for the divergence in results are discussed.