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POSTERS: Crop loss assessment

Untangling the effect of late season soybean diseases on yield
Heng-An Lin - University of Illinois At Urbana-Champaign. Maria Villamil- University of Illinois - Crop Sciences Dept, Santiago Mideros Mora- University of Illinois

Brown spot, caused by Septoria glycines is a prevalent foliar disease in soybean production areas. Application of fungicides after flowering reduces the disease severity at the end of the season, yet the yield responses are not consistent among locations and years. To start determining the effects of late-season foliar diseases on soybeans, we focused our attention on S. glycines. Our goal was to determine the effects of different levels of Septoria brown spot on yield. Different levels of disease severity were obtained in the field by weekly application of chlorothalonil for three, six, and nine times after disease inoculation. Fungicide treatments had a significant effect on vertical progress and chlorotic area with no statistically significant effect on yield. However, the yield was negatively correlated with vertical progress of the disease. Vertical progress was the best linear predictor of yield. Based on such model, when the vertical progress of brown spot at R6 increased by 10%, the yield decreased by 142.13 kg/ha. A variance component analysis showed that location explained the largest variation, illustrating the significant effect of local environmental conditions on the disease. Power analyses indicated that at least 8 locations are needed to detect an effect of 403.5 kg/ha yield increase. Our results provide useful information to improve the experimental design for future experiments addressing the yield constraint by late-season soybean diseases.