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Epidemiology of Leaf Blotch of Soft Red Winter Wheat Caused by Septoria tritici and Stagonospora nodorum. Gregory Shaner, Professor; Department of Botany and Plant Pathology, Purdue University, 1155 Lilly Hall, West Lafayette, Indiana 47907-1155. George Buechley, Research Associaie, Department of Botany and Plant Pathology, Purdue University, 1155 Lilly Hall, West Lafayette, Indiana 47907-1155. Plant Dis. 79:928-938. Accepted for publication 12 June 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0928.

Septoria tritici and Stagonospora nodorum frequently cause leaf blotch of wheat in the soft red winter wheat region east of the Mississippi River. Since 1955, leaf blotch has been the most destructive disease of wheat in Indiana. Seploria tritici was initially the principal pathogen responsible for this disease. Since 1986, S. nodorum has also been a devastating pathogen, causing both leaf and glume blotch. In order to understand the dynamics of this disease, we have monitored disease development on wheat cultivar Monon in replicated wheat performance trials each year since 1973. Monon has served as a long-term check cultivar in these trials, and it is very susceptible to both Seploria tritici and S. nodorum. The percent leaf area showing symptoms of Septoria leaf blotch was used to quantify disease severity, using a scale that reflected the vertical gradient of symptoms. Plots were usually assessed weekly for disease from the time of flag leaf emergence until crop maturity. During the 19 years of disease observation reported here, we rarely saw symptoms of leaf blotch in the fall. Initial symptoms usually appeared early in the spring on leaves touching or near the soil surface, suggesting that primary infection takes place during autumn, before the onset of cold weather, and that Septoria tritici and S. nodorum overwinter as asymptomatic infections. In most years, disease incidence was 100% by the time of flag leaf emergence; there was considerable variation in severity, but symptoms usually progressed at least to the third leaf below the flag leaf. The time from spike emergence to harvest (average = 49 days) was the main period of disease increase on the upper four leaves of the culm. For many of the years, the increase in severity with time, as recorded on the assessment scale, was approximately linear. In every year but 1982, the model based on plotting severity scale values vs. days relative to spike emergence yielded a reasonably linear relationship between severity and time.