Update:
Fungicide application timing was evaluated in field trials conducted in Carrington, Langdon and Oakes, ND with the fungicide Endura (boscalid; BASF Corp.) applied at 5.5 oz/ac. A severe hail storm that defoliated the soybeans at bloom initiation precluded disease development in Carrington, but the studies conducted at the other locations were successful. In soybeans seeded to 14- and 15-inch row spacing, Sclerotinia control was maximized when fungicides were applied when 85 to 100% of plants were at the R2 growth stage. In soybeans seeded to wide rows (28- or 30-inch), optimal fungicide timing for the control of Sclerotinia could not be rigorously assessed due to high levels of variability in disease pressure across the footprint of the studies.

The results from the fungicide timing studies conducted in Langdon and Oakes in 2016 and previous studies conducted in Carrington, Langdon, Oakes and Williston in 2014 and 2015 strongly suggest that the standard recommendation of applying fungicides at the R1 growth stage when targeting Sclerotinia is sub-optimal. Sclerotinia disease control was improved in 16 out of 16 field trials and soybean yield under Sclerotinia pressure was increased in 12 out of 16 field trials when applications of the fungicide Endura (5.5 or 8.0 oz/ac) were delayed from the R1 growth stage (60-90% of plants with at least one open blossom) to the early R2 growth stage (80 to 98% of plants with at least one open blossom at one of the two top two nodes on the plant). Sclerotinia disease control was increased in 9 of 11 trials and soybean yield under Sclerotinia disease pressure was improved in 10 of 11 trials when fungicide applications were delayed until the full R2 growth stage (100% of plants at R2).

Strategies for improving fungicide deposition within the soybean canopy were tested with field trials conducted in Carrington and Oakes. The effects of nozzle spray pattern, spray droplet size, spray volume, and boom height were tested utilizing standard boom-mounted nozzles, and conventional applications made with boom-mounted nozzles were compared to applications made with the 360 Undercover drop nozzle (360 Yield Center; Morton, IL). Fungicide deposition was quantified with water-sensitive spray cards placed within the soybean canopy and with fungicide residue analyses. End-of-season Sclerotinia disease pressure was too low to differentiate treatments on the basis of disease control in Oakes. A severe hail storm defoliated the soybeans at bloom initiation in Carrington, precluding disease development, and fungicides were not applied in Carrington until the R5 growth stage, when the soybean canopy had recovered.

When fungicides were applied with nozzles mounted directly to the spray boom, fine and medium spray droplets generally resulted in better fungicide deposition to the interior of the canopy than coarse droplets. Lowering the boom such that the recommended boom height was measured from a point 25% below the top of the canopy rather than from the top of the canopy also improved fungicide deposition within the canopy.

The application of fungicides through drop nozzles resulted in sharp increases in fungicide deposition to lower soybean stems, the location where most Sclerotinia infections begin. In Oakes, applying fungicides through drop nozzles resulted in a 15- to 30-fold increase in fungicide coverage, a 25- to 100-fold increase in the number droplets deposited per unit area, and a 15- to 25-fold increase in the estimated volume of fungicide deposited. Equipping the side ports of the drop nozzles with flat-fan nozzles conferred similar levels of fungicide coverage and fungicide deposition as equipping the drop nozzles with hollow-cone nozzles. In Carrington, applying fungicides through drop nozzles resulted in up to a 20-fold increase in fungicide residua; in the lower stems 48 hours after fungicides were applied and resulted in up to a 100- to 140-fold increase in fungicide coverage and fungicide deposition in the lower to middle of the stems. Equipping the side ports of the drop nozzles with flat-fan nozzles performed similarly to using hollow-cone nozzles, and flat-fan and hollow-cone nozzles were more effective than twin-jet nozzles.

View uploaded report

Project Title:
Optimizing fungicide applications for management of Sclerotinia in soybeans

Executive Report:

Management of Sclerotinia stem rot (white mold) with fungicides is difficult; the disease develops in the interior of dense crop canopies, where achieving satisfactory fungicide deposition is challenging. The goal of this study is to improve Sclerotinia disease control in soybeans by optimizing fungicide application timing and by identifying application methods that improve fungicide deposition within the soybean canopy.

Field studies to evaluate fungicide application timing were established at the NDSU Carrington and Langdon Research Extension Centers and at the NDSU Robert Titus Research Farm in Oakes. Fungicide application timing was tested in soybeans seeded to narrow, intermediate, and wide row spacing. Fungicides were applied at 40 psi with XR80015 nozzles in 15 gal/ac. The fungicide Endura (5.5 oz/ac) was applied at the R1 growth stage, early R2 growth stage, full R2 growth stage, and early R3 growth stage and compared to a non-treated control. To create conditions favorable for white mold, supplemental overhead irrigation was applied. A severe hail storm on July 9 in Carrington defoliated the soybeans at bloom initiation, precluding disease development, but studies at the other locations were successful.

Field studies to evaluate fungicide deposition methods were established in Carrington and Oakes in soybeans seeded to 21-inch rows. In Oakes, applications were made at the R2 growth stage; supplemental overhead irrigation was applied, but disease pressure was low. In Carrington, applications were made at the R5 growth stage when the soybean canopy had recovered from the July 9 hail storm. Water-sensitive spray cards were utilized to assess fungicide deposition patterns within the soybean canopy, and fungicide residue levels in the interior canopy were quantified 2 days after application.

The results strongly suggest that the current recommendation of applying fungicides at the R1 growth stage when targeting Sclerotinia in soybeans is not optimal. Across multi-location field trials conducted in 2014, 2015 and 2016, delaying applications of the fungicide Endura until the R2 growth stage sharply improved disease control, increased soybean yield under Sclerotinia pressure, and reduced contamination of the grain with sclerotia (resting structures of the Sclerotinia fungus).

Applications of fungicides through the 360 Undercover drop nozzle sharply increased fungicide deposition to the interior of the soybean canopy. Relative to conventional applications with boom-mounted nozzles, applications through drop nozzles resulted in up to a 20-fold increase in fungicide deposition to the lower soybean stems where most Sclerotinia infections begin. In fungicide applications with boom-mounted nozzles, deposition to the interior of the canopy was optimized when spray droplet size was medium, not coarse. The impact of the improvements in fungicide deposition on Sclerotinia control and soybean yield could not be quantified due to low disease pressure in Oakes and a hail storm in Carrington, and follow-up studies are in progress in 2017.

This study has facilitated the development of new fungicide application timing recommendations that sharply improve Sclerotinia disease control in soybeans, and it has identified fungicide application methods that have the potential to facilitate additional sharp improvements in disease control.