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Tan Spot Effects on Yield and Yield Components Relative to Growth Stage in Winter Wheat. A. Shabeer, Graduate Student, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506. W. W. Bockus, Associate Professor, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506. Plant Dis. 72:599-602. Accepted for publication 3 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0599.

Losses in yield and yield components caused by tan spot were measured at different growth stages of wheat in the field and greenhouse. In greenhouse experiments, plants were kept uninoculated or were exposed to a single inoculation period of Pyrenophora tritici-repentis at one of five different growth stages. Yield, 100-kernel wt, grain number per head, and number of heads per plant were determined. The highest yield losses were recorded for inoculations at the boot and flowering stages, indicating that plants were most susceptible physiologically to losses at those stages. Losses were a result of a significant reduction in kernel wt and number of grains per head and not of reduced number of heads per plant. In the field, tan spot epidemics were terminated at each of five growth stages by starting a fungicide spray program (47 day interval) that continued until crop maturity. About 17% of the total yield loss from tan spot occurred from early season infections by ascospores. Furthermore, about half of the total yield loss had occurred by the boot stage. Thus, tan spot activity before boot is important, because multiple infection periods at that time cause significant loss even though the plants may not be as physiologically prone to loss as at later growth stages. Fungicide spray programs to control tan spot in winter wheat should begin sooner to prevent early season disease.