Soybean Rust’s Greatest Foe is Aerial Application
Treat Crops via Aerial Application
By Alan McCracken

Several articles and resources have already been written about the disease caused by fungal spores Phakosproa pachyrhiz, which originated in China in the early 1900’s and was first found in Brazil and Paraguay in the year 2000, later spreading to Argentina. Experts believe that wind carried the Asian Soybean Rust spores to the United States from South America in the fall of 2004 via hurricanes that slammed the Gulf Coast.

Soybean crop losses can be very dramatic if not treated correctly. Brazil suffered major crop losses in 2003. A total of 6.5% of soybean crops were lost with an estimated cost of $1.2 billion, which increased to a loss around $2 billion in 2004.

Advantages of aerial application compared to ground equipment.
Aerial applicators in Brazil have demonstrated their ability in controlling rust. Contrary to the opinions of many operators of ground rigs, aerial application has been demonstrated to be the most effective means of control for the following reasons:

• Speed of operation and timeliness of application. Time and speed are critical because if the disease is present and it rains for several days, the disease will continue to develop and spread to neighboring fields.
• Ground rigs are unable to operate until the fields dry after rain
• Ground rigs often become bogged down due to the wet conditions that favor the disease. In such cases the farmers come begging for an aircraft.
• Less use of water per active ingredient per acre.
• Aerial application costs per acre are lower than ground rigs.
• There is no crop damage using aerial. The wheels on ground rigs can easily damage 3-5% of the crop.
• There is no contamination of non-infested areas using aerial; however, ground equipment can trigger a release of spores when driving through a rust-infested field.

Brazilian aerial applications of soybean rust were most effective using the following guidelines:
• Adjusting the equipment to produce medium to small droplet sizes for improved penetration.
• Using low volume application with vegetable oil or a crop oil that assists in the spreading of the droplets on the leaves of the plants. This has also been reported to increase the interval between spraying by several days.
• Applying with wind for better penetration (3 to 8 mph wind speeds). This is essential to obtain adequate penetration when the canopy is dense and closed.
• Observing conditions of temperature and humidity and adjusting droplet sizes accordingly. Skilled operators can start the day with low volumes and smaller droplets even with dew on the leaves to avoid “run-off.’ Then increase droplet size and spray volume as the temperature rises to offset losses due to evaporation.

In Brazil a technique is used for vegetable oils, such as soybean or cottonseed, to be added to the spray solution with a suitable emulsifier to enable production of a stable creamy solution. For application, rotary atomizers are preferred for more uniform droplet size and also to atomize the oil + water mixtures that have higher surface tensions. Conventional hollow cone or flat fan nozzles may also be used, however small orifices are required and higher spray pressures to produce the smaller uniform droplets. The higher airspeeds of the turbine aircraft also permit usage of other types of nozzles since the shear effect of the higher airspeed generally breaks up any larger droplets. Evidence of larger spray droplets is usually indicated by phytotoxicity, meaning the larger spray droplets, which include oil, act like a magnifying glass and cause scorch spots on the leaves.

Conclusion
Although rust disease can be devastating to soybean crops, experience last year in both Brazil and Argentina have proven that the disease can be contained when early scouting is combined with the aerial application of preventative sprays at the first indication of the disease.

Alan McCracken, International Manager – Agribusiness is an agricultural engineer and independent consultant in the area of aircraft application of agrochemicals. He is currently involved in helping with the control of Asian Soybean Rust in Brazil and Argentina. He provides the following services: assistance in testing application parameters for chemicals, application techniques to improve performance of products and operating efficiency and trouble shooting to identify why chemicals did not perform as expected. For more information, you can contact Mr. McCracken at McCrackenAlan@aol.com.