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The Relationship Between Stem Rust and Loss in Yield of Spring Wheat. R. W. Romig, Former Research Plant Pathologist, Crops Research Division, ARS, USDA, St. Paul Minnesota, now Geneticist, Northrup King Research Center, Eden Prairie, Minnesota 55343; L. Calpouzos, Associate Professor, Department of Plant Pathology, University of Minnesota, St. Paul 55101. Phytopathology 60:1801-1805. Accepted for publication 17 July 1970. DOI: 10.1094/Phyto-60-1801.

Yield loss was correlated with disease severity from four different stem rust epidemics on a spring wheat variety at one site. Polynomial and logit functions of the disease progress curves were derived. Area under the entire disease progress curve or any part thereof was not consistently proportional to the resulting yield loss. Yield loss was approximately equal to disease severity estimates made at growth stages late milk to early dough for short epidemics and to estimates made at hard dough stage for long epidemics. The best estimate of loss, however, was found with the log_e of disease severity taken when the developing caryopsis had reached three-fourths its final size. The data suggested that a reliable generalized method for estimating spring wheat losses due to stem rust is most likely to be developed by trying to find a critical growth stage of wheat at which to make disease estimates.