Asian Soybean Rust - Fungicide Application Technology.

The potential for a rapid appearance of Asian soybean rust has many growers re-evaluating sprayers.  Because the requirements of fungicide application differ from those of commonly-used systemic herbicides such as glyphosate, nozzles will need to be changed and the adequacy of pumps, booms, and other plumbing system components will need to be checked. Due to the lack of experience with foliar fungicide application in the midwest, current recommendations are based on fungicide research in South America. This article was written in collaboration with Mark Hanna, Iowa State University. 

SBR develops deep in the canopy
Effective fungicides are available to manage this pathogen and fungicides will likely be the primary tool used to control Asian soybean rust (SBR) until resistant varieties are available. However, just applying fungicides is not enough. Proper application technology when using fungicides is extremely important, in particular timing and coverage. Effective management of soybean rust with fungicides will depend on placing the fungicides as deeply into the canopy as possible.

Soybean is most sensitive to SBR from flowering (R1) to the end of the seed filling period (R6) with rust pustules first showing up underneath the lowest leaves in the canopy. The fungus then moves into the middle then upper leaves as the plant matures. Because it develops quickly, deep in the soybean canopy, effective management with a fungicide requires good canopy penetration and coverage. Adequate coverage is achieved through a combination of spray volume, nozzle selection, and pressure.

Fungicides require smaller droplet size than herbicides for adequate coverage

Most herbicides used today are systemic, meaning the chemical can move throughout the whole weed. Total plant coverage is not necessary to eradicate a weed; get enough spray on the plant and the plant will translocate the active ingredient throughout. This allows the applicator to use a nozzle that produces large droplets, which helps decrease the chance of drift.

Fungicides, however, are mostly contact or locally systemic. Contact fungicides remain on the surface and protect only the area that is covered. A locally systemic fungicide is absorbed into the plant tissue and will move locally, but not nearly as far as a systemic herbicide. Therefore, smaller droplets are required to get good leaf coverage. Smaller droplets are produced by using nozzles with smaller orifices and/or higher pressures than used for herbicides. Using medium-to-fine droplet sizes and application rates of 15 to 20 gal/ac or more is generally recommended. Following label instructions will be a key.  

Nozzle selection

Matching droplet size to calibrated application rate will dictate the selection of an appropriate nozzle.  Many herbicide sprayers have been set up with newer tip styles (e.g. air induction), lower operating pressures, or larger tips to produce coarser droplets in the 350–450 micron range. Although this remains appropriate and desirable for systemic herbicide spraying because it reduces drift, research indicates that leaf coverage with foliar fungicide is best when using smaller droplets in the 200–300 micron range. Droplets below 200 μm in size are prone to drift.

Check spray tip specifications from the manufacturer for tips that produce droplets in the small-medium to large-fine range according to American Society of Agricultural Engineers standard S-572.Use of two nozzle tips or orifices at each boom outlet may be helpful for several reasons. Dividing a given flow in two at each boom outlet requires smaller tip/orifice sizes which produce smaller droplets. Smaller orifices require higher operating pressure to release a specific flow, which also reduces droplet size. Finally, spraying at different angles with tips into the canopy allows leaves to receive spray from different directions. 

To select an appropriate nozzle, evaluate droplet size classification for nozzle tips and select a size and operating pressure that will deliver the required gal/ac application rate at the desired speed and an appropriate pressure. For most sprayers, standard flat fan or twin type nozzles of the appropriate size can generate the desired volumes, pressure, and droplet sizes. Extended range flat fan nozzles should be suitable within the ideal pressure range, but a drop in pressure may result in creation of undesirably larger droplets. Check to make sure the pressure and nozzle size you will be using produces droplets in the medium-fine range.

Although higher than normal pressure ranges are suggested (and may be used), droplet size is a key. The appropriate pressure is that which delivers a small-medium to large-fine droplet at the required application rate. Higher pressure by itself will not simply “drive” small droplets into the canopy as small droplets rapidly slow close to the nozzle due to air drag. Droplets less than 200 microns tend to drift in a cloud above the canopy. Keep in mind that warm weather conditions during application will evaporate water rapidly from these small droplets, causing them to move away from the application site and be ineffective. 

Pump and boom

In many cases sprayers that successfully operate at 20 to 30 psi, will require operating pressure of 30 to 50 psi or more. Use of abrasive material (e.g. ammonium sulfate) over time may wear pump components. Check the sprayer and pump well before application in case refurbishing or replacement is needed for the pump or other components to operate at higher pressures.   

Boom height may be more critical for fungicides than for herbicide spraying.  The boom should be high enough so that nozzle patterns achieve proper overlap (30–50% for flat fan) at the target height. The boom should be not much higher than this, however, to minimize the travel distance of smaller spray droplets into the plant canopy. Target height on a closed crop canopy for full overlapping coverage on the upper leaves is the upper canopy surface (considerably higher than near or at the soil surface for herbicide application to smaller weeds). Check manufacturer catalogs for proper height.  Note the use of 110-in. tips on 20-in. spacing allows lower boom height (16–18 in.). In earlier growth stages, before the canopy is very developed, it may be desirable to consider multiple directed nozzles mounted between rows.

Summary: good coverage is essential  

Fungicides must be applied correctly to have the greatest chance in controlling soybean rust.  Because soybean rust tends to develop initially in the lower and mid-canopy, thorough coverage of foliage, including penetration of spray into the canopy, is essential. Fungicides are best applied at higher rate, and with smaller nozzles operated at higher pressures than for herbicides. Besides label information, current available research indicates that nozzles should be selected to produce medium-fine droplets at 15–20+ gal/ac application rate. Components, including the pump, should be checked for good operation at the desired operating pressure.

The bottom line for soybean rust control is coverage, coverage, and more coverage!

More rust topics

Asian soybean rust - overview
The effect of Asian soybean rust on soybean yield and grain quality
Impact of cultural practices on Asian soybean rust