The battle against Asian soybean rust will be won with a 20X hand lens, judicious sampling of suspicious leaves and well-timed fungicide applications, according to the director of fungicide product development for a U.S. chemical company.
“The most important factor for managing Asian soybean rust is initial scouting,” said Jim Bloomberg, Bayer CropScience, Research Triangle Park, N.C. Bloomberg spoke at the National Alliance of Independent Crop Consultants annual meeting in Universal City, Calif.
In March 2004 he visited the Mato Grosso do Sul area of Brazil, where the disease is now firmly entrenched.
“The disease can go from no symptoms to complete defoliation in 10 days under ideal environmental conditions. It is not your typical disease which will nickel and dime you to death. There can be 60 to 70 percent yield loss with this disease.”
The disease is very difficult to detect in the early stages of infection, which typically occurs in the lower one-third to the lower middle part of the canopy due to higher humidity in that part of the plant, according to Bloomberg.
He noted that cultural practices cannot be changed to manage soybean rust. “Rotation and tillage can control certain diseases and weeds, but rust is a windborne disease.”
Here are some important keys to scouting:
- Check high-risk areas first. That would include low areas where there is more moisture. It may be an area where there is a tree line which provides partial shading.
- Scout the lower canopy once or twice a week, even more if it's in a high-risk area.
- Increase scouting after storms or after winds which have come from areas where there is a known rust outbreak.
- Pustules are 1 millimeter or less in size, so at least a 20X hand glass will be needed.
- Look at the underside of the leaf with a hand glass for tan or red lesions. The disease can be confused with brown spot, noted Bloomberg. “Both start in the lower canopy and work up the plant. Bacterial blight also looks very similar to Asian soybean rust, but it tends to occur in the upper part of the canopy. Another disease, bacterial pustule, tends to have a yellow halo around the necrotic spot on the leaf.
- One way to pick up the disease is to hold a suspect leaf up to the sun. You may be able to see tiny brown spots starting to form or some chlorosis.
- A picture pocket guide could also be helpful. Asian soybean rust looks like other diseases that are common in soybean fields. Free guides from the University of Nebraska and Ohio State University clearly pinpoint the differences. They are available through university Web sites.
- Acquire early-detection bags for collecting suspicious samples. Know exactly where the samples were collected. Collect up to 20 leaves from four to five random spots in a field, wrap them in a paper towel and place in early-detection bags. Include information on where the sample was collected, along with your name and telephone number. Send to the person responsible for early detection in your state.
Even though most fungicides do a good job of controlling the disease, all fungicides are not equal for soybean rust control, according to Bloomberg.
“Curative chemistries tend to be absorbed into the plant tissue and have some translocation within the plant. They tend to kill fungal tissues and can be used after an infection has occurred. Preventive chemistries may or may not be absorbed into the plant tissue. They tend to prevent infection, so you need to get them out earlier.”
In addition, some fungicides may offer longer residual control of the disease. In any case, to battle Asian soybean rust, you may need one to three applications, depending on your location and environmental conditions. “In south Louisiana, which has high humidity and is close to overwintering sites, you may need two to three applications.”
The critical period for control of Asian soybean rust is at R-1 to R-6 — flowering through pod development. “Within that range, the most critical is R-3 to R-6, when the plant is forming the bean. Research has shown that if you have defoliation at R-3 to R-5, you may lose 70 percent of your potential yield. If you can get through the R-6 stage, your yield is going to be made and probably will not warrant any fungicide sprays.”
Bloomberg stresses that growers get good coverage of fungicide sprays. “Get the chemical down to the infection. You may have to look at nozzle selection a little differently. You may want to use higher spray gallonages. Likewise, you may want to increase spray pressure to get into the lower canopy. Of course, you don't want the pressure too high or you could get offsite target movement.”
Resistant soybean lines are being evaluated, and some show some field resistance, according to Bloomberg. “But they are years away from commercialization. For the short term, we're going to be treating this disease with fungicides.”