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Soybean Rust Disease Management Page
Application equipment for effective control of Asian Soybean Rust (ASR)
By Bobby Grisso, P.E., Extension Specialists
Biological Systems Engineering, Virginia Tech
There are no soybean varieties currently available that have high levels of resistance to soybean rust. This leaves Virginian’s with only one alternative option: spraying fungicides that are registered to control this disease. However, regardless of which fungicide is used, an effective job of controlling the disease is questionable if the fungicide is not applied properly with the most effective application strategy.
The single most important factor affecting prevention of this disease is complete coverage of the plant with fungicide. Fortunately, we have the technology; however, the producer may incur expenses with adaptation to the current application equipment.
Before listing a number of specific spraying recommendations for Soybean Rust, remember the 5 major principles that result in satisfactory and economic control of the problem: 1) Positive identification of the disease, 2) Correct pesticide, 3) Select the right equipment, particularly the right type and size of nozzle, 4) Timely application, and 5) Check the accuracy of equipment periodically to confirm correct application amounts based on label recommendations.
Calibrate sprayers frequently to confirm the recommended amount of chemical is applied. Too little fungicide results in poor control and reduced yields, while too much wastes dollars and increases the risk of polluting the environment. How uniformly a fungicide is applied to the plant is just as important as applying the right amount -- both of which can be achieved by using the right equipment at the right time and as accurately as possible.
The single most important factor affecting the outcome of the fight against this disease is to get a thorough coverage of the plant with the fungicide. Research has shown that there is a very strong correlation between coverage and efficacy. We have the technology to achieve this, but it may come with higher equipment cost.
Nozzles producing cone pattern are not recommended for Soybean Rust. The flat-fan pattern nozzles are still the best choice as long as the droplets from these nozzles are categorized as fine to medium (200 to 300 micron). Check the nozzle catalog to find out at what pressure the nozzle you picked should be operated for it to produce fine to medium quality spray (note Table 1). A flat-fan nozzle set up with two spray patterns seems to provide a better coverage of plants with fully developed canopies. Research has shown that hitting the target from two different angles, with one forward and one backward spray pattern, provides a more effective coverage than spraying with just one spray pattern shooting down. Several nozzle manufacturers have either the nozzles that provide a twin spray pattern from one tip, or special fittings/caps that allow the producers to place two nozzles in the same cap (Tables 2), one pointed forward, and the other one pointed backward.
Soybean rust first shows its symptoms usually in the lower parts of the plant and works up towards the top of the plant. Detecting the disease early and using the most effective control mechanism are keys to controlling this disease. Complete coverage of the disease could be even more challenging if the symptoms of the disease are found at later stages of plant growth, when the plant is close to having a full canopy. Air-assisted spraying may help in facing those challenges. A commercial-scale sprayer with air assistance may add from $10,000 to $15,000 to the price tag of the equipment, but the added cost may well outweigh the income lost due to soybean rust in just one growing season.
Here are some specific recommendations to help achieve the best coverage and control when spraying for soybean rust.
- Keep spray volume above 20 gallons per acre for best results.
- Choose the appropriate size and type of nozzles and operate them at a pressure that will allow them to produce small to medium-size droplets (200-300 microns).
- Nozzles producing a flat-fan pattern seem to provide better coverage than the nozzles producing cone pattern when spraying for soybean rust when there is full canopy.
- Choose "low-drift" nozzles, which allow a grower to increase the pressure without increasing the number of small, drift-prone droplets (those at 100 microns or smaller). Operate these nozzles at slightly higher pressures (60-70 psi) than usual. Higher spray pressures usually help the droplets penetrate the canopy better.
- Use directed spraying, if applicable, to improve coverage.
- Use twin nozzle/pattern technology. Research has shown that two spray patterns, one angled forward and one angled backward, perform better than single nozzles spraying in one direction.
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If economically feasible, use air-assisted
spraying. Research has shown that air-assisted spraying, which uses air to help
droplets (100 microns or smaller) reach inside of the plant canopy and
underside of leaves, consistently provides the best coverage and droplet
penetration, especially when beans are at or near their full-growth stage. One
should match air flow to canopy density when using air-assisted sprayers.
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