As many of our beans move into reproductive stages, now is the time to begin scouting for disease and thinking about fungicide applications. If, when and how often to apply fungicides are questions all soybean growers face. To help answer these questions the NCSPA sponsored research with a group of crop consultants in eastern N.C. The group evaluated four different fungicide products at three different timings across five locations.
Disease severity varied across locations, but at locations with moderate frogeye leaf spot (FLS) pressure the disease became prevalent in R3-R5. Compared with the untreated check (UTC), all but two of the fungicide treatments significantly improved yield. However, there was no significant difference between the different fungicide products. When comparing yield among treatment timings of the same product, no statistical differences were observed between timings, but the R3+R5 application tended to have the highest yields for each product.
After statistical and economic analyses, the optimal fungicide regime was Quadris Top SB applied at R3, earning the grower $40.56 per acre above the UTC. Runner-up was sequential application of Quadris Top SB applied at R3 and R5, earning the grower $40.12 per acre above the UTC. In contrast, the highest negative loss-on-investment of -$33.50 resulted from application of Headline followed by Headline plus Tebustar.
It is widely believed that application of fungicides containing strobilurin can increase green stem percentage at harvest. This study supports that such a relationship does exist and that prolonged greenness can contribute to increased yield. Across all treatment timings, plots treated with Quadris Top SB were on average 5.9-7.4% greener at R7 than plots treated with any other fungicide. This observation corresponds with highest green stem ratings observed in 2015 occurring in plots treated with Quadris Top at R3.
This study does not support the practice of multiple fungicide applications. Although substantial profit was observed following two applications of Quadris Top SB at R3 and R5, results show this is not always the case and may be a risky investment. A strong relationship was seen between FLS control and increased yield. That is, the greatest yield increases from fungicide treatments were observed in locations that had the greatest disease pressure. If conditions are conducive to disease development then fungicides are probably a smart investment, but if disease pressure is minimal the application is not likely to be profitable.