Updated July 24, 2019:

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Executive Report:
Obtaining consistent control of Sclerotinia (white mold) in soybeans with fungicides is complicated by the difficulty of achieving satisfactory fungicide deposition to the lower canopy, where most white mold infections begin.

White mold infects soybeans primarily through dead blossoms, and soybeans are most susceptible to white mold at the R2, R3 and R4 growth stages when soybeans are in full bloom and dead blossoms are abundant in the interior of the canopy. When conditions favor white mold as soybeans enter bloom, applying fungicides at the R2 growth stage optimizes white mold control. Achieving satisfactory fungicide deposition to the lower soybean canopy at the R2 growth stage can be difficult; soybeans in North Dakota are often at or near canopy closure and 24 to 27 inches tall at R2.

Fungicide deposition to the lower canopy is typically optimized with larger spray droplets that have velocity to penetrate the canopy but still confer acceptable coverage. To evaluate the impact of spray droplet size on white mold control, the fungicide Endura (boscalid; BASF Corp.) was applied at 5.5 oz/ac with Spraying Systems TeeJet extended range (XR) flat-fan nozzles at 30 or 50 psi to emit fine to coarse droplets. Spray volume was 15 gallons/acre, and applications were made with a tractor-mounted boom equipped with a pulse-width modulation system to permit a constant driving speed across all treatments. In testing conducted in soybeans grown under overhead irrigation in Carrington and Oakes, medium spray droplets optimized white mold control when the soybean canopy was open (average canopy closure < 90%) when fungicides were applied and coarse spray droplets optimized white mold control when the soybean canopy was at or near closure (average canopy closure > 90%) when fungicides were applied (Figure 1). In 2019, these studies are being expanded to evaluate nozzles from two manufacturers.

To evaluate the potential use of drop nozzles for delivering fungicides to the lower canopy, the ‘360 Undercover’ drop nozzle (360 Yield Center; Morton, IL) was tested with a tractor-mounted boom in soybeans seeded to 21-inch rows. The gain in white mold control associated with applying fungicides through drop nozzles was greatest when applications were made to a soybean canopy that was at or near closure (90-100% canopy closure), and the use of drop nozzles facilitated strong, consistent disease control with a broad range of fungicides, including low-cost applications of the off-patent fungicide Topsin