Roberto N. Barbosa, James L. Griffin and Clayton Hollier
Farmers must rely on fungicides to protect their crops from certain diseases. One of the most serious of these diseases is Asian soybean rust, which is spread by the fungus Phakopsora pachyrhizi. Were it not for fungicides, this disease might severely damage Louisiana’s soybean crop. Because of their high cost, fungicides must be applied judiciously and effectively. This requires correct timing and placement. Some of the recommended fungicides are protective instead of curative, such as the fungicides to ward off Asian soybean rust. They must be applied to the crop before the infection takes hold. If the fungicide is not there to protect the plant, either because it was not applied at the right time or because the application did not yield a good coverage, there won’t be much control.
Volume of application and method of application, aerial or ground, play important roles in the distribution of the fungicide throughout the soybean canopy. Reaching the lower parts of the plant is critical to providing protection against diseases that often start from the ground up, such as Asian soybean rust. Proper application volume depends on spray droplet size and nozzle type.
LSU AgCenter researchers are investigating spray coverage quality when using aerial or ground application and application how much water to use to ensure good fungicide distribution. During the 2007 growing season, several trials in Louisiana analyzed the effects of different volumes and methods of application in the spray deposition in the soybean canopy. In field trials, a harmless tracer known as tartrazine was added to the spray in lieu of the fungicide. The amount of tracer applied per acre was held constant, even though volume of application changed. Small cards, measuring 2 by 3 inches, were distributed throughout the plant. Deposits of tartrazine in the cards were detected in the laboratory using colorimetry. Because the amount of tracer collected in cards depends on how the spray was applied to the crop and how much water was used in the application, researchers can determine the efficiency of the application process by analyzing tracer concentration.
Results in Ground Application
Application rates of 5, 10 and 15 gallons per acre were tested using different ground sprayers – a tractor-mounted experimental unit and a high clearance self-propelled unit. The amount of tracer used was 21 ounces per acre. Hydraulic nozzles with single or dual spray technologies (twin) were tested for deposition efficiency (Figures 1, 2). System pressure was varied between treatments to accommodate specific application rates in question.
The results show that increasing application rates increases tracer deposition in the canopy as measured by the colorimetry method. However, statistical tests do not differentiate between 10 and 15 gallons per acre. Canopy penetration was expressed by percent spray of total volume that reached the bottom, middle and top of the canopy. Reaching the bottom canopy was not improved by increasing the volume of application. There were no differences in deposition from single or dual spray nozzles at any volume of application. Single spray nozzles deposited 68 percent of the total tracer amount in the top of the canopy, on average. Dual spray nozzles deposited 62 percent. The influence of droplet size in aiding canopy penetration was also analyzed. For dual spray nozzles, it is beneficial to choose a nozzle that produces medium-sized droplets because it will aid in the deposition, regardless of the chosen volume of application. More research is needed concerning single spray nozzles.
Results in Aerial Application Application rates of 3, 5, 7 and 10 gallons per acre were tested using an agricultural aircraft model Air Tractor AT-602. The aircraft is equipped with 82 flat fan nozzles pointed straight back with no deflection. Pressure was kept around 30 PSI, and the application’s swath width was calculated at 65 feet. The results for aerial application also show that higher application rates yielded higher deposits of tracer in the canopy. However, no statistical differences were found between 3 and 5 gallons per acre, or between 7 and 10 gallons per acre. Canopy penetration as expressed by percent spray (of total volume) reaching middle and bottom canopy was not consistently improved by increasing volume of application. Of note is that 5 gallons per acre yielded the best tracer distribution in the canopy among all volumes tested.
Increasing application rates to increase product deposition in the soybean canopy, especially in the middle and lower portions of the canopy, is a strategy that may yield good results, regardless if done by air or ground. Higher application rates, however, are not a "silver bullet" and will not compensate for poorly maintained sprayers, clogged nozzles and low quality water used during spray. Higher application rates can also affect operation efficiency and ultimately add to the cost of production. Changes in any process have to be well-planned and well-executed to be successful.
During spraying season, sprayers should be kept clean and nozzles should be checked, cleaned and replaced when worn. Calibration procedures should be performed several times during the season. Aerial applicators should also keep their aircrafts in order. The LSU AgCenter offers calibration clinics for aerial applicators. These clinics help them to ensure their equipment’s top performance. Weather conditions are important to success, and farmers are to spray only when favorable conditions are met.
Roberto N. Barbosa, Assistant Professor, Department of Biological & Agricultural Engineering; James L. Griffin, Lee Mason LSU Alumni Association Professor, School of Plant, Soil & Environmental Sciences; and Clayton Hollier, Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.
(This article was published in the summer 2008 issue of Louisiana Agriculture.)