Fight soybean rust by calibrating applicator properly

Soybean rust has become a pressing agricultural issue. While it remains uncertain whether Virginia farmers will have to deal with this devastating disease this growing season, steps should be taken to prepare for disease control.

Soybean rust has been detected in nine southern states and has over wintered in Florida. According to Robert Grisso, Virginia Cooperative Extension specialist at Virginia Tech, there are no current soybean varieties that have high levels of resistance to soybean rust. Producers are left with one option: be prepared to do a good job spraying fungicides that are registered to control this disease.

The disease has severely damaged soybeans in South America, reducing yields in some fields by as much as 80 percent. Virginia produces approximately $65 million of soybeans annually, making it the state’s second most valuable crop behind tobacco.

Scientists in Virginia Tech’s College of Agriculture and Life Sciences and Virginia Cooperative Extension have formed a task force to monitor soybean rust and assist producers in combating this new disease. Additional information on the disease, proper spraying equipment, including aerial spraying, and spraying techniques can be found at: http://www.vaes.vt.edu/tidewater/soybean/soybeanup/0503/0503.html.

According to Grisso, now is the best time for producers to check their sprayer and calibrate it properly. The window of opportunity to spray for soybean rust is much narrower than for most pests and there may be little time to calibrate after the rust has arrived.

Successful fungicide control depends largely on proper application, starting with selecting the correct nozzles, and spraying the optimal amount of fungicide uniformly across the field.

Grisso said that pesticide manufacturers have determined the most effective and economical application rate for fungicides labeled for soybean rust. However, the applicator must be sure the recommended amount is applied.

Calibration is the best way to assure delivering the desired rate, said Grisso. Results of many "Sprayer Calibration Clinics" show that only a third of applicators are applying chemicals at a rate within five percent (plus or minus) of the intended rate. Calibration of sprayers takes 30 to 60 minutes and requires only three "tools": a stopwatch, a measuring tape, and a jar graduated in ounces. Several calibration methods are available but Grisso described one, known as the “ounce calibration method,” that is simple, straightforward, and does not require cumbersome equations.

Fill a clean spray tank at least half-full with water. The amount of water 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. For 30-inch spacing, drive 136 feet; for 36-inch spacing, drive 113 feet; and for 40-inch spacing, drive 102 feet.

Run the parked sprayer at the same pressure level for the same amount of time it took 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 20 ounces the rate is 20 gallons per acre (GPA).

The next step is to minimize the application error, Grisso said. If the difference between the intended application rate and the actual rate is greater or less than five percent, you should make adjustments. For example, if the intended application rate is 20 GPA, the calibrated rate should be between 19 and 21. Adjust the pressure for small changes in the application rate, Grisso advised. For larger changes, either adjust the travel speed or replace nozzles with the appropriate size. The calibration process should be repeated until the application error is within the five percent range.

The “ounce” calibration method is explained in detail in Virginia Cooperative Extension (VCE) Fact Sheet 442-453, available from your local Extension Office, or from the VCE web site: http://www.ext.vt.edu/pubs/bse/442-453/442-453.html.

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

Studies have shown that the more frequency that a sprayer is calibrated, the more accurate the application is. Changes in operating conditions and the type of chemical used may require a new calibration, Grisso said. Sprayers should be checked and recalibrated several times a year.