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Prophylactic Fungicide Application Timing to Soybeans

A prophylactic application of an appropriate foliar fungicide to soybeans at stage R3 is an accepted practice in the midsouthern US, and is done to assure that the soybean crop is protected against the myriad diseases that often negatively affect soybean yield in the region. Click here for survey results that show the major foliar diseases that affect soybeans in the Midsouth, and the estimated losses associated with each one.

An article titled “Evaluation of Prophylactic Fungicide Application Timing on Soybean Grain Yield, Quality, and Economic Return in Mississippi” by C.A. Floyd et al. that is published in the online journal Crop, Forage and Turfgrass Management (https://doi:10.1002/cft2.20102) presents results from research that was conducted to validate this practice. Details about and results from this research follow. The research was under the direction of Dr. Trent Irby, Extension Soybean Specialist for Mississippi.

•    Irrigated field experiments were conducted at Starkville and Stoneville, Miss., during the 2017 and 2018 growing seasons. Seed of a MG IV variety was planted in all experiments in the first half of May each year. The varieties–different each year–were selected based on optimal ratings against foliar diseases common in Mississippi soybeans.

•    The objective of the research was to determine the soybean growth stage at which the application of a prophylactic fungicide would provide the greatest yield and economic benefit.

•    Three fungicide products [(Quadris (azoxystrobin–Group 11), Quadris Top SBX (azoxystrobin + difenoconazole–Groups 11+3), and Priaxor (fluxapyroxad + pyraclostrobin–Groups 7+11) combined with Domark 230ME (tetraconazole–Group 3)] applied at five application timings (R3, R4, R5, R6, and R3+R5) were the treatments, plus non-treated controls for each application timing, for a total of 20 treatments.

•    Treatments at all locations were monitored for disease presence each year; there was no observable disease presence in the experiments in either year.

•    Measurements of seed yield and seed mass were made each year. Quality of harvested seed was determined and used to assign discounts resulting from overall seed damage. These discounts were incorporated into the economic analyses that were conducted to calculate partial returns to the fungicide application.

•    Fungicide application timing had no effect on soybean seed yield., and there were no significant interactions between fungicide application timing and product.

•    Yield of soybean in these studies was increased a significant 3.8% (62.1 vs. 59.8 bu/acre) and 4.1% (62.3 vs. 59.8 bu/acre) when treated with the multiple-mode-of-action fungicides Quadris Top SBX and Priaxor + Domark 230ME, respectively, compared to the yield of the treatment that received no fungicide. The small yield increases were associated with similar small increases in seed mass.

•    Overall, the single application of Quadris Top SBX resulted in the greatest partial returns because it cost less.

•    The results from these experiments indicate that, at a soybean market price of $8/bu or below, applications of Quadris and Priaxor + Domark 230ME would not be profitable.

•    These data suggest the following. 1) A delay in a prophylactic fungicide application beyond the recommended R3 stage in the absence of disease presence will not negate the positive yield effect realized from that application. 2) The small but significant yield increases resulting from a single prophylactic fungicide application will only be realized if a multiple-mode-of-action fungicide product is used. 3) An application of a prophylactic fungicide at both the R3 and R5 stages was not profitable, and resulted in lower partial returns than the same fungicide product applied once at any of the treatment stages. 4) These results highlight the apparent residual protection against diseases provided by a prophylactic fungicide application to soybeans in an irrigated environment.

The above research was funded by the MSPB as part of Dr. Trent Irby’s SMART project.

Caveats. 1) When yield-limiting diseases are present, a delay in the application of an appropriate fungicide product past the recommended R3 stage could result in significant yield loss. 2) The above results likely are not applicable to soybeans grown without irrigation where yield potential is lower–i.e., in such a situation, yield-limiting disease pressure plus commodity price should be used as a trigger for fungicide application. 3) Annual prophylactic applications of even multiple-mode-of-action fungicides may contribute to resistance development in targeted foliar fungi. Thus, producers may want to consider applying foliar fungicides only where needed to prevent or delay this possible occurrence even though the small and consistent yield and profit increases resulting from the automatic fungicide application practice are not easily ignored, especially when commodity prices are high. 4) Regardless of which approach is used for applying foliar fungicides to soybeans, growers should be aware of which diseases are most likely to affect soybeans in the midsouthern US (click here for survey estimates of soybean yield loss to diseases in the midsouthen US), and which fungicide products are most efficacious against them (click here for an up-to-date chart that shows the efficacy of fungicide products available for use on soybeans).

Composed by Larry G. Heatherly, June 2021,