Update:
Methods:
Soybeans were planted on June 2 and 3 in Carrington and on June 3 in Oakes. The relatively late planting date was due to the cold, wet spring. Because of concerns about delayed canopy closure associated with the relatively late planting date, soybeans were seeded to narrow (14-inch) rows. The experiments were established as a randomized complete block design with a split-plot arrangement, with the number of fungicide applications (one versus two) as the main factor and fungicide spray volume as the sub-factor. The study in Carrington was conducted with 9 replicates, and the study in Oakes was conducted with 6 replicates. White mold disease pressure is often highly variable over short distances, and a large number of experimental replicates was utilized to maximize the likelihood of each treatment being evaluated the same number of times in areas of high versus low disease pressure. Treatment plots were 25 feet long and 5 feet wide (consisting of four rows centered within the 5-foot width) at seeding and approx. 19 feet long and 5 feet wide at harvest. To avoid edge effects, alleys were not established until shortly before harvest, after soybeans had reached maturity. To facilitate overspray of treatments and capture any fungicide drift, treatment plots were separated by 5-foot wide non-harvested filler plots on one side and 10-foot wide non-harvested filler plots on the other side. A tractor with a 5-foot wheelbase was used to apply fungicide treatments, and this tractor was driven in the center of the 10-foot wide filler plots. Treatments were applied to a block of multiple soybean varieties concurrently: three varieties (1.0-1.2 maturity) in Oakes and four varieties (0.6-0.9 maturity) in Carrington. The number of varieties assessed in Oakes represents an increase from the two varieties proposed in the original grant. Testing was conducted on the following varieties in Carrington: Xitavo ‘XO0602E’ (0.6 maturity), Asgrow ‘AG06X8’ (0.6 maturity), Xitavo ‘XO0731E’ (0.7 maturity), and Asgrow ‘AG09Xf0’ (0.9 maturity). Testing was conducted on the following varieties in Oakes: Xitavo ‘XO1041E’, Asgrow ‘AG11X8’, and Xitavo ‘XO1212E’. The Asgrow varieties were Extend-type soybeans, and the Xitavo varieties were Enlist-type soybeans. Fertility and weed management were conducted in accordance with recommended best practices. Fungicides were applied with a PTO-driven tractor-mounted sprayer equipped with a pulse-width modulation system (Capstan AG, Topeka, KS). Pulse width was modified as needed to maintain a constant driving speed, the same nozzles, and the same application pressure across spray volume treatments, with pulse width manually adjusted and set on the basis of the measured spray output. Nozzles and application pressures were selected to calibrate the droplet size relative to canopy closure, with medium droplets used when the canopy closure averaged 80-90% across the footprint of the study and coarse droplets used when canopy closure averaged > 90%. TeeJet (Spraying Systems Co.; Wheaton, IL) extended-range flat-fan nozzles were utilized due to their broad commercial usage and the familiarity of North Dakota producers with these nozzles. Fungicide application details are presented in Table 1 of the attached PDF report. In Carrington, fungicides were applied July 25 at the early R2 growth stage (79 to 91% of plants at the R2 growth stage, depending on the variety) and August 1 at early R3 growth stage. In Oakes, fungicides were applied July 21 at the full R2 growth stage (100% of plants at the R2 growth stage across all three soybean varieties) and on August 1 at the R3 growth stage. Studies were established on fields with a history of white mold, and supplemental irrigation was applied to facilitate white mold pressure. Carrington, supplemental irrigation was applied via low-output rotating micro-sprinklers with a 20-foot spray radius established in a 20-foot offset grid pattern. Irrigation commenced at late vegetative growth and continued through the R5 growth stage, with irrigation delivered as needed to maintain the top half-inch of the soil moist (to facilitate production of apothecia and spores by the Sclerotinia pathogen) beginning at late vegetative growth and as needed to create conditions favorable for white mold. In Oakes, supplemental overhead irrigation was applied via a linear irrigator as needed to optimize soybean yield potential. The Oakes site has sandy soils, and irrigation was applied frequently. White mold was assessed on October 13-14 in Carrington and October 20-21 in Oakes when soybeans were at maturity. All plants in the third row (counted from the side of the plot closest to the tractor driving pass) of each four-row plot were individually assessed for white mold severity on a 0 to 5 scale representing the percentage of the plant impacted by Sclerotinia stem rot: 0 = 0%, 1 = 1-25%, 2 = 26-50%, 3 = 51-75%, 4 = 76-99%, 5 = 100%. Soybeans were harvested October 13-14 in Carrington and October 21 in Oakes.
Results:
Under moderate to high white mold disease pressure, increasing fungicide spray volume from 5.0 to 15.0 gal/ac had little or no impact on white mold management across the seven soybean varieties tested. White mold pressure was representative of that observed in commercial production fields. White mold severity ranged from 9 to 43 percent of the canopy diseased, depending on the soybean variety and study location.
Response to fungicide spray volume was similar across upright varieties with delayed canopy closure, bushy varieties with early canopy closure, tall varieties and short varieties . When a single fungicide application was made, increasing fungicide spray volume from 5.0 to 15.0 gal/ac had no impact on white mold incidence (percent of plants diseased), severity, or severity index (percent of the canopy diseased) or soybean yield. When two sequential fungicide applications were made 9 or 11 days apart, a weak trend of slightly improved disease control and increased yield was observed as spray volume increased from 5.0 to 15.0 gal/ac, but statistical separation across spray volume treatments was only observed for the disease reduction conferred by 5.0 vs. 12.5 gal/ac. Data figures illustrating key results are provided in the attached PDF report.
The use of fungicide spray volumes greater than 10 gal/ac is a widely recommended for white mold management in soybeans, but the economic return to increasing spray volume is not well understood.
Research conducted:
Field trials were conducted in Carrington and Oakes on four varieties of 0.6-0.9 maturity (Carrington) and three varieties of 1.0-1.2 maturity (Oakes). Row spacing was 14 inches. Endura (5.5 oz/ac) was applied at R2 or twice (R2 + 9 or 11 days later) in 5.0, 7.5, 10.0, 12.5, and 15.0 gal/ac. Applications were made with a tractor-mounted, PTO-driven sprayer equipped with a pulse-width modulation system (Capstan AG, Topeka, KS). TeeJet XR11006 nozzles at 35 psi (medium droplets) were used when canopy closure averaged 80-90%; XR11008 nozzles at 30 psi (coarse droplets) were used when canopy closure was >90%. Pulse width was manually set on the basis of measured spray output and modified as needed to maintain a constant driving speed or 9.5, 10.0, 10.5 or 11.2 mph across spray volume treatments. Testing was conducted with six (Oakes) or nine (Carrington) experimental replicates.
Why the research is important:
The use of higher fungicide spray volumes increases the cost of fungicide applications, and quantifying the impact of spray volume on fungicide efficacy allows profit-maximizing decision making.
Findings:
Increasing fungicide spray volume from 5.0 to 15.0 gal/ac had very little impact on white mold management (Figure 1). Response to fungicide spray volume was similar across upright varieties with delayed canopy closure, bushy varieties with early canopy closure, tall varieties and short varieties. When a single fungicide application was made, increasing fungicide spray volume from 5.0 to 15.0 gal/ac had no impact on white mold management or yield. When two sequential fungicide applications were made 9 or 11 days apart, a weak trend of slightly improved disease control and increased yield was observed as spray volume increased from 5.0 to 15.0 gal/ac, but differences were small and highly variable across varieties and statistical separation was not observed across spray volume treatments.
Benefits/recommendations:
The results suggest that there may be little benefit to increasing fungicide spray volume above 10 gal/ac for white mold management in soybeans and that it might be possible to reduce spray volumes below 10 gal/ac without a significant reduction in fungicide efficacy. These results are surprising, and follow-up testing is in progress to assess the replicability of these findings.