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Frogeye Leaf Spot Management with Fungicides in Soybean

According to recent surveys, frogeye leaf spot [FLS] has been a significant damaging disease of soybean in several midsouthern states over the last 10 years. However, in the last 5 years, its negative effect on yield in some of these states has diminished. This may be associated with the increasing use of efficacious foliar fungicides that have been applied to control the FLS pathogen as well as the increasing use of soybean varieties that have some level of resistance to the pathogen. There are several foliar fungicides that are labeled for FLS control in soybean, but producers need information about just which ones or their combinations provide the most profitable level of control.

An article titled “Efficacy and Profitability of Fungicides for Managing Frogeye Leaf Spot on Soybean in the United States: A 10-Year Quantitative Summary” provides results from 66 uniform fungicide trials that were conducted in 8 states [including Ark., La., and Miss.] over the 2012-2021 period. The collected data can be used to decide which fungicide product to use in order to realize the greatest control and profit when applied to control FLS. Funding for the research was provided by the United Soybean Board under Project No. 2220-172-0146. Pertinent information from the research that is described in the above-linked article follows.

•    The following fungicides [trade name, fungicide designation in the article, and FRAC code in brackets] were applied at soybean growth stage R3 during the course of the 10-year study. A set of non-treated plots served as the control.

     •    azoxystrobin + difenoconazole [Quadris Top, AZOX + DIFE, FRAC code 11,3]

     •    difenoconazole + pydiflumetofen [Miravis Top, DIFE + PYDI, FRAC code 3,7]

     •    pyraclostrobin [Headline, PYRA, FRAC code 11]

     •    pyraclostrobin + fluxapyroxad + propiconazole [Priaxor + Tilt, PYRA + FLUX + PROP, FRAC code 11,7,3]

     •    tetraconazole [Domark, TTRA, FRAC code 3]

     •    thiophanate-methyl [Topsin, TMET, FRAC code 1]

     •    thiophanate-methyl + tebuconazole [Topsin XTR, TMET + TEBU, FRAC code 1,3]

     •    trifloxystrobin + prothioconazole [Stratego YLD, TFLX + PROT, FRAC code 11, 3]

     •    Untreated control [no fungicide applied].

All of the above fungicides or components of fungicide mixes had FLS listed as a target disease in their labels, but the ratings for their control of FLS range from Poor to Very Good [Crop Protection Network–2023].

•    Each trial at each location used an adapted susceptible soybean variety in order to increase the likelihood of infection from a natural infestation of the FLS pathogen.

•    Disease severity was determined in the upper 1/3 of the crop canopy by estimating the percentage of leaf area showing FLS symptoms at 4 weeks after application of the fungicides [approximately R6 stage].

•    FLS severity in the untreated control was greatest in Miss. [43%] and least in Arkansas [8%].

•    FLS severity was decreased and yield was increased by all fungicide treatments compared with the untreated control.

•    Quadris Top, Miravis Top, Topsin XTR, and Stratego YLD all reduced FLS severity by more than 50%. All other treatments except Headline [11%] reduced FLS severity by at least 40%.

•    The average yield differences between fungicide-treated and nontreated plots ranged from 2.0 to 6.6 bu/acre. Application of DIFE + PYDI [Miravis Top; FRAC code 3,7] and AZOX +DIFE [Quadris Top; FRAC code 11,3] resulted in the largest yield increase above the nontreated control. Both have a “Very Good” efficacy rating against FLS [Crop Protection Network–2023]. The least effective fungicide at protecting yield was PYRA [Headline; FRAC code 11], and it has a “Poor” efficacy rating against FLS.

•    In general, the relationship between fungicide control efficacy and yield was consistent–i.e., the fungicide treatment(s) that resulted in the best FLS control across environments resulted in the most yield protection from an FLS infestation.

•    The fungicide treatment that reduced FLS severity the most and that resulted in the greatest yield response was the premix of DIFE + PYDI–i.e. Miravis Top [FRAC code 3,7].

•    The probabilities of breaking even when FLS severity was low were about 50% regardless of the fungicide treatment. The most effective fungicide treatment–i.e. DIFE + PYDI [Miravis Top]–resulted in the greatest profitability when FLS disease pressure was high.

•    Results from this study lead to the following conclusions.

     •    The best performing fungicide was the premix of DIFE + PYDI–i.e. Miravis Top.

     •    Greater yields and profitability from the best-performing fungicides were observed in trials with greater FLS pressure.

     •    Sole use of either FLS-resistant soybean varieties or the same fungicide(s) at each application pose the same risk–e.g. selection of FLS races that will be resistant to either.

     •    The use of an integrated strategy that includes both FLS-resistant varieties plus fungicides with differing modes of action should help protect soybean plants against the FLS pathogen when disease pressure is high.

     •    These results provide critical information for selecting a fungicide to manage FLS in a susceptible soybean variety.

Labels for each of the fungicides used in the above study can be found here.

Composed by Larry G. Heatherly, Feb. 2024,