Managing foliar soybean diseases and fungicide applications
Fungicide groups having a preventative effect on disease include the strobilurins (Group 11) and SDHIs (Group 7). Ideally, these products should be applied prior to disease development. Triazoles (Group 3) and chlorothalonil (Group M5) products have curative effects on diseases, halting or slowing existing disease development. Thiophanatemethyl (Group 1) is described as having both preventative and curative effects.
The strobilurins (Group 11), SDHIs (Group 7), and thiophanatemethyl (Group 1) have sitespecific modesofaction and should be used conservatively. These products can select for resistant pathogen populations very quickly. In fact, widespread resistance to strobilurin fungicides and thiophanatemethyl has been documented in the Cercospora leaf blight pathogen throughout soybeanproducing areas in Louisiana.
Concurrently, a decline in efficacy of strobilurins and thiophanatemethyl on Cercospora leaf blight has been observed over the past 1015 years. Additionally, strobilurin resistance has been confirmed in the frogeye leaf spot pathogen in several parishes throughout the state. Additional research is needed to determine the extent of this resistance.
Triazoles have a less specific modeofaction, but resistance is still possible and will occur much slower when compared to strobilurins. There have been no documented cases of resistance to chlorothalonil in soybean pathogens. Appropriate application timing also is an important consideration with fungicide applications to soybean. An automatic fungicide application at or near R1 is not currently recommended.
Ongoing research at LSU AgCenter may or may not result in modification of these recommendations in the future. More information is needed to determine if an R1 application timing is effective for producers statewide. According to results from many years of LSU research trials, fungicides have proven most efficacious when applied near the R3 stage. A second application may be considered at R5 as well.
Residual activity of fungicides and the timing between soybean growth stages are also an important consideration. Residual efficacy of fungicides varies, but the average residual of fungicides is 2 to 3 weeks.
However, residual activity does not simply cease after 2 to 3 weeks as if a light switch was thrown to the OFF position. Rather, we would expect a decline in activity until the concentration of the fungicide is no longer sufficient to suppress growth of the pathogen within the leaf. The analogy would be a light bulb getting dimmer with time until there is insufficient light to read. We would also expect that initial concentrations of the fungicide in the leaf and the rate of decline in activity (residual activity) would be a function of rate of application.
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