Revisiting flag leaf-based foliar fungicide application thresholds for Stagonospora nodorum blotch management in soft red winter wheat
K. T. Willyerd (1), C. Bradley (2), S. Conley (3), P. Esker (3), L. Madden (1), K. Wise (4), P. PAUL (1)
(1) Ohio State University, Wooster, OH, U.S.A.; (2) University of Illinois, Urbana, IL, U.S.A.; (3) University of Wisconsin, Madison, WI, U.S.A.; (4) Purdue University, West Lafayette, IN, U.S.A.
Phytopathology 101:S191

Field trials were established in Illinois, Indiana, Ohio, and Wisconsin during the 2009/10 season to evaluate associations among disease severity at different crop growth stages and leaf positions and grain yield in an effort to establish fungicide decision thresholds. A randomized complete block design was used with two soft red winter wheat cultivars, one susceptible and one resistant to Stagonospora nodorum blotch (SNB), serving as whole plot. Plots were inoculated at either Feekes 6 or 9 (sub-plot) using four inoculum densities (sub-sub-plot): an uninoculated check, 125,000, 250,000 and 500,000 conidia/ml. SNB severity was assessed at weekly intervals at three positions in the wheat canopy: on the flag leaf and the two leaves below the flag. In general, SNB increased over time and decreased from the lower to upper canopy. Based on preliminary results from linear mixed model covariance analyses, disease severity throughout the canopy and at various growth stages had significant (P < 0.05) negative linear relationships with yield. Lower canopy disease severity had a positive linear relationship with upper canopy severity in many cases. This suggested that disease severity on lower leaves in the canopy could potentially be used to make early foliar fungicide application decisions to minimize the risk of severe damage to the flag leaf. Preliminary disease-yield models and fungicide decision thresholds based on these models will be discussed.