Susceptibility of soybeans to white mold increases from the R1 growth stage, when there are relatively few dead blossoms on plants, into the R2 growth stage, when there are multiple dead blossoms per plant. In multi-location research conducted in North Dakota on soybeans, fungicide efficacy against white mold was optimized when conditions favored white mold as soybeans entered bloom by applying at the R2 growth stage unless the canopy closed earlier. This research was conducted with a single fungicide application, and irrigated producers often apply fungicides twice. Research conducted on dry beans indicates optimal application timing is 2 to 3 days earlier when a second fungicide application is made.
Fine spray droplets optimize fungicide coverage but lack the velocity to penetrate dense canopies; as canopy density increases, coarser droplets are needed. Multi-location research conducted in North Dakota in soybeans under white mold pressure showed that the yield gain conferred by fungicides is increased by 50-100% when spray droplet size is calibrated relative to canopy closure. That research was conducted in soybeans seeded to intermediate (21-inch) or narrow (14-inch) rows. Preliminary results from parallel research conducted on kidney beans suggests that coarser droplets may be needed to penetrate canopy in wide (28-inch) rows. Optimal droplet size corresponded to canopy closure when kidney beans were seeded to narrow rows, but coarse droplets were optimal in kidney beans seeded to wide rows even when ground was still showing between rows. The seeding rate was kept equal as row spacing changed, and canopy density within rows was much higher in the kidneys seeded to wide rows.
Fungicide spray droplet size testing in soybeans and dry beans has been conducted with TeeJet and Wilger nozzles, and growers frequently ask about the performance of Hypro’s ‘3D’ nozzles. The ‘3D’ nozzles are advertised to reduce driftable fine droplets by 50-70%, which would be expected to improve fungicide delivery to the interior of the crop canopy.
Growers often report success with the use of low seeding rates to reduce white mold pressure in soybeans. In multi-location research conducted in North Dakota, increasing soybean seeding rate had no impact on white mold management (Figure 4), but very low seeding rates were not tested. Growers utilizing seeding rate as a white mold management tool generally report planting soybeans at 90,000 to 110,000 viable seeds/ac.
Field studies will be established at the NDSU Carrington Research Extension Center and at the NDSU Robert Titus Research Farm south of Oakes at sites with overhead irrigation capabilities and a previous history of white mold. In all studies, fungicides will be applied in 15 gpa with a PTO-driven, tractor mounted sprayer at 6 or 10 mph. Pulse width will be modified as needed to maintain constant spray volume and driving speed across droplet size treatments differing in spray output, with pulse width manually calibrated based on measured spray output. Fungicide application timing will be tested in soybeans seeded at 140,000 viable seeds/ac in narrow (14-inch) and wide (28-inch) rows. Five application timings, each 2-3 days apart, will be tested versus non-treated for a single fungicide application and two sequential applications (12 treatments total). The first application will be tested three times with different droplet sizes corresponding to optimums identified in previous research: TeeJet nozzles delivering fine, medium or coarse droplets or Wilter nozzles delivering medium, coarse and very coarse droplets. The second application will be made 11-12 days after the first with coarse (TeeJet) or very coarse droplets (Wilger). Fungicide droplet size will be tested in soybeans planted to three seeding rates (100,000; 140,000; and 180,000 viable seeds/ac) in each of two row spacings (21- and 28-inch). A single fungicide application will be made when 90-100% of plants have reached the R2 growth stage. A non-treated control will be compared to TeeJet extended-range nozzles and John Deere ‘3D’ nozzles delivering fine, medium and coarse droplets (7 treatments total). Experimental design will be a randomized complete block with a minimum 8 replicates and a split-plot arrangement (fungicide timing, main factor = one vs. two applications, sub-factor = timing; droplet size, main factor = seeding rate, sub-factor = nozzle type and droplet size). Separate studies will be conducted for each row spacing. With a large number of replicates, by random chance every treatment ends up on areas of high disease pressure versus low disease pressure approximately the same number of times, resulting in an accurate representation of treatment effects. Non-harvested filler plots will be established between plots to permit overspray of treatments, capture spray drift, and to facilitate the establishment of a tractor driving pass adjacent to each plot. Non-harvested plots will be established at the ends of each treatment as a transition zone for turning on and off the sprayer. Overhead irrigation will be applied as needed to facilitate white mold disease pressure. At each fungicide application, soybean growth stage and canopy closure will be assessed in each row spacing and seeding rate treatment in each experimental replicate. Canopy closure will be assessed with visual estimates, with closure evaluated on a plot-level basis and on a 12-inch width centered on each row. In the fungicide timing studies, apothecia development will be assessed concurrently with the first four application timings. A 1.0 m2 area will be evaluated in each row spacing and each replicate, with assessments conducted in non-harvested filler plots adjacent to treatment plots. At soybean maturity, plants will be individually assessed for white mold using a 0 to 5 scale corresponding to 0 to 100% of the plant impacted by white mold, with every plant in one row of each plot evaluated. Sclerotinia incidence, severity, and severity index will be calculated for each plot, and soybean yield, contamination of grain with sclerotia (resting structures of the Sclerotinia sclerotiorum, causal agent of white mold), and test weight will be assessed. Yield will be calculated on the basis of the measured plot length and at a standard 13% moisture. Data will be analyzed with analysis of variance in SAS 9.4 (SAS Institute, Cary, NC) with data conformity to model assumptions assessed.