Drought and extreme heat in the lower Southeast has slowed movement of soybean rust into the Carolinas and Virginia. However, as growers in the Delta and lower Midwest will attest, rust spores can move quickly on tropical systems and the disease is still a distant, but realistic threat to late planted beans in the upper Southeast.
Citing recent research at The Ohio State University, Virginia Tech Ag Engineer Bobby Grisso says growers can significantly improve fungicide application and by doing so — soybean rust control — by using proper spray equipment and application techniques.
Grisso says Ohio research indicates medium spray volume, rather than fine or coarse provides better droplet penetration into the soybean canopy. Though labels on most fungicides used for soybean rust suggest a 200-350 micron, or fine to medium spray volume, Grisso says the optimum for rust control is probably closer to the bottom, or about 250 microns, than to the top end of the recommendation.
Grisso contends that using air assisted sprayers, regardless of cone type, generally provides better canopy penetration and plant coverage than conventional sprayers.
When using sprayers without air assisted application, a mechanical canopy opener or rigid bar that bends the top of the canopy ahead of the spray boom improves fungicide penetration to the middle and lower leaves of the soybean plant.
However, when using conventional sprayers, a flat cone spray pattern performs better than hollow cone spray patterns. When air assisted sprayers were used, there was little difference in performance of the two spray patterns, according to the researcher.
Timing, or more precisely the growth pattern and stage of the soybean plant, also effects fungicide application efficiency, regardless of spray delivery system used.
When spraying a fungicide for soybean rust control, good coverage on the lower and middle leaves is critical, because these are the most likely spots on the plant for the disease to form. Applying fungicides to shorter, low density plants will provide better results, again regardless of the spray system used, according to Grisso.
Most beans susceptible to rust in 2007 will be late-planted, double-crop soybeans.
While a single flat fan provides better spray coverage and disposition than cone or dual fan nozzles in tall, dense plant canopies, most of the late planted beans won't have this type growth habit, so the more popular nozzles and spray patterns should provide adequate coverage.
If plants get taller and vegetative growth is denser, research consistently shows a twin flow pattern nozzle, such as the conventional Twin Jet nozzle, did not perform as well as a single flat fan nozzle.
When using a conventional boom sprayer, a flat fan nozzle provides a single spray flow pattern that will likely work better than cone nozzle or a twin pattern flat-fan nozzle, Grisso explains.
Research from various locations clearly indicates an air-assisted sprayer is the best equipment option available to achieve the best coverage of soybeans with a fungicide. Unfortunately, a commercial-scale sprayer with the air assistance may add from $10,000 to $15,000 to the price tag of the equipment. However, this one-time cost may well outweigh the income lost due to soybean rust in one growing season.
Proper operation of an air-assisted sprayer is important for taking full advantage of the air assistance, plus improper air flow may even be counter-productive.
The amount of air flow should be adjusted depending on the canopy density. Also, air directed 30 to 45 degrees forward or backward seems to provide better penetration of droplets into the canopy, and especially better leaf underside coverage, than when the air flow is straight down, according to Erdal Ozkan, the Ohio State ag engineer who conducted the tests cited by Grisso.
When asked which is more important spray volume, pressure or droplet size Grisso says spray volume is most important. At least 15 gallons, and preferably 20 gallons, per acre of spray volume are needed for ground applications.
At least 5 gallons per acre are needed for aerial applications.
This volume will be needed to penetrate and thoroughly cover all leaf surfaces in the soybean canopy.
Next in order of importance is the droplet size. A medium droplet size (200 to 300 microns) is required to penetrate the canopy and adequately cover the leaf surfaces. Fine to very fine droplets, like those produced from hollow cone nozzles, will not penetrate the canopy and are prone to drift.
Large droplets will not give adequate coverage, may bounce off leaves, and because the total number of droplets is less, are more likely to miss the target and land on the ground.
Pressure can be an issue, especially if coarse droplet nozzles, such as those used for herbicide application are used. If these low drift, coarse droplet nozzles are used, higher pressure may reduce droplet size into the medium size range, though this is not guaranteed, Grisso stresses.
Two families of fungicides are currently used for soybean rust management in the U.S. — triazoles and strobilurins. Use rate for triazole products on the market range from 3-8 ounces per acre. Use rate for strobilurins is 6-15 ounces per acre. Combination of the two families in such products as Quilt and Stratego is still in the 5-10 ounce per acre range.
The triazoles tend to move better within the soybean plant and generally provide better curative results, while the strobilurins have longer residual activity within the plant and are generally better as a preventative.
Used together, both families of chemicals provide optimum protection, but all share one common factor — ultra-low rates of active ingredient per acre.
The cost of these highly efficient fungicides, plus their ultra-low volume make it essential for soybean growers, even with prices in the $8 per bushel range, to apply them correctly and at the right stage of soybean growth for optimum profitability.