A new way of managing white mold in soybean

This article was written by Angie Peltier and Dean Malvick and Jeff Nielsen, Michael Leiseth and Joe Wodarek at the UMN Northwest Research & Education Center in Crookston made very significant contributions to getting this Minnesota Soybean Research & Promotion Council-sponsored project done. Thank you also to Marc Cartwright, Pioneer Seeds, for the head's up on the between-the-row commercial sprayer equipment.

Purpose of Study

White mold in soybeans has always been difficult to manage. The fungus that causes this disease produces long-lived survival structures and has a wide host range, causing economic losses in many crops important to NW MN, including soybean, edible beans, sunflower and canola.  Partial resistance in soybean varieties means that in years in which weather favors disease, some yield loss is still likely to occur. Similarly, while there are several protectant fungicides labeled for white mold management, sub-optimal canopy penetration and coverage at the site of infection (flower buds at leaf axils) means that some yield loss likely occurs even with a well-timed application.

While the connection may not initially be apparent, the convergence of recent economic and environmental concerns and the availability of equipment that allows farmers to spoon-feed nitrogen (N) to their crops, paved the way for this soybean white mold management project.  With corn producers feeling both an internal pressure to make sure that every last bit of N at least pays for itself and an external pressure to reduce N lost to the environment, some split their N, applying a baseline in the spring and coming back later on to side-dress the remaining N into a standing crop. It is the equipment that allows this in-season side-dressing to take place (think y-drop applicators) that provides an opportunity to research different fungicide application techniques. 

In an effort to improve fungicide coverage, we compared deposition, coverage and efficacy when fungicides were applied either within the canopy between rows or in the typical over-the-top fashion. Personnel built a spray boom to position multiple nozzles between rows and within the canopy (Figure 1).  Chemical-resistant hose, plumbing and sprayer fixtures and junctions were used to fashion the body onto which to affix the nozzle filters and nozzles. Zip ties were used to connect the nozzle body onto the bottom of a square, hollow steel pipe that would ride within the canopy and between rows.  Plastic skid plates were bent and riveted to the steel pipe so that the pipe and nozzle body could easily glide through the canopy, minimizing potential plant injury. Details regarding the over-the-top and between-the-row sprayer setups can be found in Table 1. Note that while fungicides work best to protect plants when droplet size is small and more plant surfaces are covered, some fungicide labels suggest increasing droplet size for white mold management to ensure sufficient canopy penetration.

Figure 1. Configuration of the tractor-mounted hydraulic-powered plot sprayer used to apply fungicides in this experiment. Note that two different within-the-canopy booms were built to allow application down the center of both 22 (Crookston study site) and 30 inch (Staples study site)-spaced soybean rows. The within-the-canopy nozzle body (black circle/square) rode approximately 12“ from the soil surface and the over-the-top nozzles (white circle/square) rode approximately 8“ above the soybean canopy.

Table 1. Details regarding the nozzle type and details, spray volume, speed, pressure and droplet size of fungicides applied over the top of the canopy and within the canopy. See Figure 1 for a picture of what both look like.

Results

Treatments

To improve the chance of white mold occurring, some plots were infested with the fungus that causes white mold (Sclerotinia sclerotiorum, Ss) and all plots were irrigated weekly after fungicide application. Experimental treatments included an untreated control that was neither infested with Ss nor treated with fungicide, a positive control in which plots were infested with Ss, but not treated with fungicide, and over-the-top and within-the-canopy fungicide treatments that were infested with Ss.

Assessing spray coverage and deposition.

Prior to applying fungicides, short (18”-tall, installed) pieces of metal fencing material were pounded into soybean rows in plots that were to have over-the-top or within-the-canopy applications; and small spring-loaded two-sided alligator-type clips were attached to them at 6” and 12” above the soil line. Just before fungicide application, water-sensitive paper was attached to the clips and oriented to sit within the canopy. After application and time for the water-sensitive paper to dry, personnel put on appropriate PPE and retrieved the papers, placing them into pre-labeled plastic sandwich bags to shield them from moisture or humidity. A scanner and USDA-developed software program called “Deposit Scan” were used to objectively analyze spray coverage and deposition on the water sensitive paper.

Data collected. 

At the beginning flowering (R1) growth stage, 8 oz/A of Endura was applied to the center four rows of six 22 inch-row soybean plots at the Northwest Research and Outreach Center in Crookston and to the center four rows of six 30 inch-row soybean plots at the Central Lakes College Ag and Energy Center in Staples.  Data that was collected from these plots included: fungicide coverage and deposition, white mold incidence and severity and harvest moisture and yield.

Yield. 

Despite doing our best to initiate disease in these experiments, warm temperatures prevailed after treatment, resulting in no disease. Consequently, it was not a surprise that there were no differences observed among treatments for soybean yield (66.7 bu/A average, P = 0.2869) and moisture (12.0% average, P = 0.2307) at the Staples site and yield (29.8 bu/A average, P = 0.9644) and moisture (8.8% average, P = 0.1882) at the Crookston site.

Fungicide coverage. 

The within-the-canopy application resulted in significantly better fungicide coverage within the soybean row 6 inches above the soil line than the over-the-top application in Crookston, but not in Staples (Table 2, Figure 2).  This same trend was observed 12 inches above the soil line (Table 3), with significantly more coverage when applying fungicides within-the-canopy at Crookston and numerically better coverage in Staples compared to over-the-top. We speculate that at the CLC in Staples the thick canopy may have interfered with fungicide penetration at the 6-inch height regardless of application method. More research is needed.

 Table 2. Coverage (%) and deposition (microL/cm²) of fungicides applied over-the-top or within-the-canopy captured by water-sensitive paper placed within the R1 soybean canopy at 6 inches above the soil line in 22 inch rows at the NWROC in Crookston and in 30 inch soybean rows at the CLC in Staples. Treatments means within a column followed by different letters are significantly different from one another.

Figure 2. Water sensitive paper that had been placed 6 inches above the soil line in the soybean row before fungicide was applied using either the traditional over-the-top method (left) or the experimental within-the-canopy method (right). A document scanner and the Deposit Scan software was used to impartially assess spray coverage and fungicide deposition. Note that darker areas indicate where fungicide droplets fell on the water sensitive paper.

 Table 3. Coverage (%) and deposition (microL/cm²) of fungicides applied over-the-top or within-the-canopy captured by water-sensitive paper placed within the R1 soybean canopy at 12 inches above the soil line in 22 inch rows at the NWROC in Crookston and in 30 inch soybean rows at the CLC in Staples. Treatments means within a column followed by different letters are significantly different from one another.