Spraying for Soybean Rust
Erdal Ozkan
  

Soybean Rust disease can cause complete defoliation of soybean plants within two weeks if not detected and then treated immediately. There are no soybean varieties currently available resistant to soybean rust. This leaves one option - be prepared to do as good a job as possible applying fungicides registered for protecting against or controlling this disease.

Fungicides manufactured to control soybean rust are effective. However, success will largely depend on being prepared before the disease is detected in your fields.

The single most important factor is to get a thorough coverage with the fungicide. Spraying the right amount is not enough. Getting total coverage with a fungicide is much more challenging than spraying for weeds and insects. With soybean rust, we are concerned with the vertical distribution of the fungicide on soybean leaves as much as uniform horizontal distribution. Penetrating droplets inside the canopy of fully grown plants is not easy.

Sprayer Equipment
Fine to medium spray droplets (mostly in the range of 200-300 micron in size) are recommended by nozzle manufacturers. Large droplets don't provide good coverage and result in wasted chemicals. . Droplets that are too fine are more prone to drift. Air -assisted sprayers are usually better at getting penetration into a dense canopy. Using an application rate above 15 gpa can improve coverage, but that requires more frequent refilling of the sprayer tank.

Specific recommendations

• Choose the appropriate nozzles and operate them at a pressure that will produce a find to medium spray quality. Thorough coverage of the plant from top to bottom is essential.

• Flat-fan nozzles are better than cone nozzles because they produce a much smaller proportion of extremely small, drift-prone droplets.

• Use twin nozzle/pattern technology. Two nozzles, with one angled forward and one backward, work better than single nozzles spraying down.

• For best results, keep the application rate above 15 gpa for ground and 3 gpa for aerial application.

• Air-assisted spraying usually provides the best coverage and droplet penetration into the canopy.

• Read and follow the recommendations on the fungicide label, in the nozzle catalogs, and sprayer operator's manual.

• Calibrate the sprayer to be sure the right amount is sprayed uniformly across the boom width.

• Spray drift is more likely with fine droplets. Take precautions to reduce drift because it wastes expensive fungicides and is bad from health and environmental aspects.

• If possible, slow down when spraying. Spray coverage usually is improved at slower speeds and higher travel speed often leads to drift.

• Be safe. Wear protective clothing, rubber gloves, and respirators when calibrating the sprayer, doing the actual spraying, and cleaning the equipment.

Details are available in our 5-page OSU Extension publication, "Spraying Recommendations for Soybean Rust", AEX-526, which can be accessed free at: http://ohioline.osu.edu/aex-fact/pdf/0f26.pdf

Erdal Ozkan, Professor and Extension Agricultural Engineer, can be reached 614-292-3006, or ozkan.2@osu.edu.