February 16, 2005 | By: Laura Skillman

Whether Kentucky farmers will deal with soybean rust, a potentially yield-robbing pest, in the coming growing season is uncertain but there are things they can do to prepare to control the disease.

A diligent scouting program, adequate spray equipment, and timely fungicide application(s) will be vital in controlling rust in soybean fields this year. To help farmers best prepare for an intense spray schedule, specialists with the University of Kentucky College of Agriculture’s Biosystems and Agricultural Engineering Department have collaborated to put together a list of things farmers need to do.

As they prepare to control rust, farmers should think about three main aspects: what type of nozzles should be used; can they operate at the right pressures for proper coverage; and can they get enough water to the sprayer to keep going, said Tim Stombaugh, Extension agricultural engineer.

Spraying fungicides is unfamiliar to many Kentucky growers and applying them is different than applying herbicides or insecticides. Spraying for soybean rust will take some 15 gallons of water per acre - much more water than producers are used to using in grain crops, he said.

Good canopy penetration to reach the lower leaves and coverage are also essential, Stombaugh said.

The key to good coverage with fungicides is having many smaller droplets versus a few large droplets from the nozzle. Medium droplet size is preferred (250 microns ± 100 microns). 

Small droplets do not reach lower canopy, and larger droplets bounce off, or do not cover leaf surfaces.

An ideal distribution of spray droplets consists of 25 percent around 100-150 microns, 50 percent around 200 microns, and 25 percent around 250-300 microns. The actual distribution of different nozzles depends on pressure and volume, but this is a good target to shoot for. Higher pressures than those used for herbicide applications are needed to penetrate the canopy.

Avoid using cone nozzles if possible because droplet sizes are too small to ensure adequate canopy penetration. A double-nozzle configuration improves canopy penetration over a single-nozzle configuration. Utilize manufacturer’s literature to select nozzle size and operating pressure – make certain this combination generates the correct droplet size distribution.

Higher pressures help drive droplets into the canopy, so when possible operate on the upper end of the manufacturer’s recommended range.  Be careful not to sacrifice droplet size. Use water-sensitive paper attached to the lower and middle canopy to check penetration and coverage in the field.

A boom height of 12 to 18 inches provides good coverage. Match boom height to nozzle spacing to provide good coverage and avoid skips in the top canopy.  A combination that works well is a nozzle spacing of 20 inches and a boom height of 12 to 18 inches above the canopy for wide angle nozzles (110 degrees).  Adjust boom height as necessary to improve coverage and penetration.

Timely spraying also is key to reducing potential rust damage. Do maintenance checks on spray equipment so it will be ready to go when needed.  Be sure equipment is large enough to spray all fields within only a few days.

If you need to spray at night to cover your acreage, be sure running lights on the tractor are working properly. Replace bulbs and have spares. If using a light bar or other guidance aid in a large field, spray turn rows and field borders before dark. Also, remember if dew is heavy within a few hours after spraying you may be wasting your time.


Writer: Laura Skillman 270-365-7541 ext. 278
Sources: Tim Stombaugh, 859-257-3000, ext. 214; Sam McNeill, 270-365-7541, ext. 213