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Reaction of Canadian Spring Wheats to Septoria nodorum and the Relationship Between Disease Severity and Yield Components. J. Gilbert, Agriculture Canada, Research Station, 195 Dafoe Road, Winnipeg, Manitoba, Canada R3T 2M9. A. Tekauz, Agriculture Canada, Research Station, 195 Dafoe Road, Winnipeg, Manitoba, Canada R3T 2M9. Plant Dis. 77:398-402. Accepted for publication 7 December 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0398.

Twelve cultivars of wheat were tested two ways: in a controlled environment for reaction to Septoria nodorum based on lesion type at seedling and adult stages of development; and in the field to assess the relationship between disease severity and two yield components, kernels per head and thousand-kernel weight. The four durum wheats were moderately resistant to S. nodorum at both stages; cultivars of other wheat types were moderately susceptible to susceptible at the seedling stage, but more resistant as adults. In the field, kernel number per head generally was not affected by S. nodorum infection. However, significant reductions of 6.815.6% in thousand-kernel weight occurred in half the cultivars in 1989, despite dry conditions which led to only moderate disease development. Higher disease severities were associated with minimal yield reductions in several cultivars; such cultivars may be tolerant to Septoria leaf blotch. In 1990, abundant early-season rainfall resulted in rapid and extensive disease development, and significant yield reductions occurred in all cultivars. The inconsistent relationship between disease severity and yield loss suggest that kernel weight may be a more reliable predictor of cultivar performance than foliar disease severity.