Fungicide + Insecticide Applications for Increasing Winter Wheat Grain Yield in Oklahoma.

Winter wheat producers in Oklahoma are tank-mixing an insecticide when making foliar fungicide applications, but little is known about this practice. Therefore, the objective of this study was to examine effect of a fungicide + insecticide application at full flag leaf emergence (GS 9) on grain yield and profitability. Trials were established at two locations (Chickasha and Lahoma) in 2016 using a randomized complete block design with treatments arranged in a 2 × 4 factorial. Treatments consisted of two varieties, one susceptible to leaf and stripe rust (Ruby Lee) and the other resistant (Gallagher at Chickasha and Doublestop CL Plus at Lahoma), and four pesticide treatments [non-treated control, fungicide-only (F), insecticide-only (I), fungicide + insecticide (F+I)]. Leaf rust was the primary fungal disease observed at both locations. While insect levels were low overall, barley yellow dwarf (BYD) was observed at both locations. A weak variety × pesticide treatment interaction was found at Chickasha (P = 0.07). The F and F+I treatments applied to Ruby Lee yielded 290 and 360 kg ha^-1 (27 and 17 $ ha^-1) greater than the non-treated control, respectively, but no difference was found between the F and F+I treatments. Only the F+I treatment resulted in a significant increase (300 kg ha^-1; 7 $ ha^-1) compared to the non-treated control for the variety Gallagher. A variety × pesticide treatment was not observed at Lahoma, but the variety and pesticide treatment main effects were significant. Only the F treament resulted in a significant increase in yield (460 kg ha^-1; 51 $ ha^-1) compared to the non-treated control. Preliminary results from the first year of this study indicated a positive yield response with a fungicide + insecticide tankmix was not consistent. A properly timed fungicide application had a more consistent positive increase in yield and profitability.