Fight against soybean rust starts with calibrating your sprayer
Erdal Ozkan
  

Asian soybean rust has officially arrived in the United States. It has been detected n nine southern states, all the way up to Missouri. It may arrive in Ohio this year. There are no soybean varieties currently available that have high levels of resistance to soybean rust. This leaves us with only one option: be prepared to do as good a job as possible spraying fungicides registered to control this disease.

Fungicides manufactured to control soybean rust work. However, success will largely depend on proper application. Proper application starts with selecting the right equipment, specifically nozzles, and spraying the right amount of fungicide uniformly across the field before the disease is detected.

Pesticide manufacturers have invested heavily to determine the most effective and economical application rate for the fungicides labeled for soybean rust. However, it is up to the applicator to make sure the recommended amount is applied.

Why calibrate
Calibration is the best way to find out if the sprayer is ready to delver the desired rate. And do it early. The window of opportunity to spray for soybean rust is much narrower than for most pests. You will not want take time to calibrate after the rust h as arrived.

Results of many "Sprayer Calibration Clinics" in Ohio show that only a third of applicators are applying chemicals at a rate within 5% (plus or minus) of the intended rated, which is the accuracy level recommended by USDA and EPA. Of the two-thirds who are missing the mar, about half aare under spraying and the other half are over spraying. In one extreme case, the applicator would have been over spraying by 75% had he used the nozzles he just purchased and installed on the boom. Other states show similar accuracy levels.

Calibration takes about 30 to 60 minutes and requires on three "tools" : a watch, a measuring tape and a jar graduated in ounces. Several calibration methods are available but the one described below is simple, straightforward and does not require the memorization of equations.

Calibration Method
When the spray tank is clean, fill it at least half full with water since the amount of water in the tank may affect travel speed. Measure the distance between nozzles in inches and then drive a designated distance depending on the nozzle spacing. For a nozzle spacing of 20 inches, drive 204 feet at normal spraying speed. Drive 136 feet for 30-inch spacing; 113 feet for 36-inch spacing; and 102 feet for 40-inch spacing. The run the parked sprayer at the same pressure level for the same amount of time it took you to drive the designated distance, and collect the output of each nozzle. Calculate the average nozzle output by dividing the total output from all nozzles by the number of nozzles tested. The average nozzle output in ounces equals the gallons per acre applied. For example, if you catch 15 ounces the rate is 15 gallons per acre.

The next step is to minimize the application error. If the difference between your intended application rate and the actual rate is greater than ±5% of your intended rate you should make some adjustment. For example, if your intended application rate is 20 gpa, the calibrated rate should be between 19 and 21. For small changes in the application rate try adjusting the pressure. For larger changes either adjust the travel speed or replace nozzles with the appropriate size, You need to repeeat the calibration process until your application error is no greater than ±5% .

This calibration method is explained in detail in Ohio State University Extension (OSUE) Fact Sheet AEX-520, available from your County Extension office, or from the Ohioline website: http://ohioline.osu.edu/aex-fact/0520.

Just spraying the right amount of fungicide on each acre is not enough to achieve effective control of soybean rust. Uniform deposition on the spray target is as important as the total amount deposited. Each nozzle type produces a unique spray pattern. Some nozzles require precise overlapping of patterns from adjacent nozzles. Check the nozzle catalog to find out the appropriate boom height for your nozzle spacing that will produce uniform spraying across the boom.

Calibrate frequently
Sprayers should be calibrated several times a year. Changes in operating conditions and the type of chemical used my require a new calibration. A Nebraska survey showed that the more often a sprayer was calibrated, the more accurate the application rate. If you hear on the news this summer that soybean rust has reached Kentucky or Indiana, you should calibrate again and prepare to spray.

    

Erdal Ozkan, professor and Extension agricultural engineeer, can be reached at 614-292-3006 or ozkan.2@osu.edu.

This column is provided by the OSU Department of Food, Agricultural, and Biological Engineering.