On the surface, Asian soybean rust doesn't appear to be the sort of thing that could cost a soybean farmer his farm, says Monte Miles, a plant pathologist with USDA's Agricultural Research Service at the University of Illinois.
“It looks like a little bit of brown sugar on the underside of the leaf,” says Miles, considered USDA's foremost authority on the disease. “It doesn't look like it could cause much damage.”
But looks can be deceiving, says Miles, and farmers in Brazil and Africa who did not take the disease seriously at first have found out just how damaging it can be. Yield losses of 40 to 60 percent have not been uncommon, and crop losses of 100 percent have been reported.
Speaking at a recent seminar, Miles said Asian soybean rust has caused disruptions for farmers in most of the places where its spores have landed.
“Since soybean rust was first discovered in Brazil in 2002, the recommendations from the government of Brazil and the Extension agencies have changed three times,” he said. “The first year they said spray at R-5 just like you would for any other late season disease. The second year, it was spray at R-3. The current recommendation: Spray at first detection anytime after R-1.”
Since soybean rust has become endemic to Brazil, 30 percent of the acreage has changed hands, according to sources there.
Why should U.S. farmers worry about soybean rust?
“If you have a good environment for growing soybeans you have the perfect environment for this disease,” he said. “The only thing that seems to slow it down is drought.”
Farmers generally have three weapons against a disease like soybean rust — genetics, cultural practices, and fungicides — but don't count on much help from the first two in the short-term.
“We've evaluated about 16,000 germplasm accessions from the USDA collection,” says Miles. “Out of that, we have 700 to 800 that might have some usable resistance. But we're talking 10 years to move any of that resistance into commercial varieties.”
Cultural practices — planting dates, row widths — also don't hold much promise. “The thing with row spacing is that if you have a tight row the disease gives you a tight focal point,” he notes. “If you have wide rows it blows through your field that much faster.
“The best cultural recommendation we've got for soybean rust is to choose the agronomic practices that are going to return the most on your farm. It's a lot easier to start with a top yield and try to protect it than it to start giving up yield by modifying cultural practices.”
That leaves fungicides as the primary defense against soybean rust for the near future.
“Our cooperator in Brazil asks his producers to leave a skip in the field to remind themselves why they're paying all those big bucks for fungicides,” says Miles, who coordinates the USDA fungicide testing program in Brazil, Paraguay and South Africa.
“He's never had a producer come back and say, ‘I wasted my money with these fungicides.’”
Miles said U.S. growers should realize they may have different formulations of fungicides than those available in Brazil and in Africa.
“Not all the fungicides in those countries will be available for U.S. producers,” he notes. “The risk cup may be too high for EPA. Some of the products may just be too toxic.”
USDA cooperators in Africa have been providing information on fungicide applications that could benefit U.S. growers. Clive Levy, a scientist stationed in Zimbabwe, has looked at application timing and the number of fungicide applications needed to control the disease.
“If you wait until R-4 or R-5 to begin spraying, you have already lost half your yield,” says Miles. “Based on his information, early vegetative applications may not give you an economic return. That's in his environment. We're all going to have to learn what works in the United States.”
Levy also tried to determine if growers could achieve control with a single fungicide application. “You can get away with a single application that's almost as good as two applications, but if you're off one way or the other by 10 days, you're going to take a significant yield hit.”
EPA has labeled four products in the United States for soybeans and soybean rust: two chlorothalonil products, Bravo and Echo, and two strobilurin products, Quadris and Headline.
Other fungicides have received Section 18 emergency exemptions: Myclobutanil or Laredo; propiconazole or Tilt and Propimax; pyraclostrobin or Headline; tebuconazole or Folicur; propiconazole and trifloxystrobin or Stratego; and tetraconazole or Domark. Domark has just received a Section 18 exemption for a single application each year.
“You have to know what products are going to be available in your state. Every state won't have the same products, so you will have to check to see what your state regulatory agency put in their Section 18 list.”
The fungicides are not the same. “This is not Roundup,” he said. “Think back to the pre-Roundup days and all the choices you had. Each of these products has different strengths and different weaknesses.”
They can be divided into two groups — curative and protectant. The curative products may be used before or after infection, but the protectant product may only be used before an infection has occurred.
“The triazoles tend to be curative, but not all triazoles are curative,” says Miles. Some have more curative properties than others. Protectant products — the strobilurins and chlorothalonils — have to be used in a protective manner. Once the disease is in the field they don't provide protection.”
The products also differ in the rates they are absorbed into the tissue and how they translocate in the plant. None of the fungicides are systemic, he notes. “They will move up and out, but they will not move down to the roots like a herbicide.”
Over the last three seasons, USDA has been conducting tests in Paraguay, Zimbabwe and Brazil comparing fungicides that would be available to the U.S. producer when Asian soybean rust arrived in the United States.
The studies have used a defined protocol with internal spreader rows and borders to maximize the development of the disease. The tests have also been arranged in a split-plot design to allow researchers to compare two and three applications and to learn the strengths and weaknesses of the products.
“So we're at 20 days between applications in these tests,” he notes. “If you read the recommended interval on the product label, some are as little as seven to 14 days and others 20 to 21 days between applications.”
Miles showed the results from a trial in Zimbabwe that was selected because researchers knew the exact time the disease started, which was between the second and third applications of the fungicides. “So we have a protective type program in place,” he said. “We also know that the disease was severe and that it developed very quickly.”
The study was in a high yield area with the two locations in the study averaging 90 bushels per acre. The two locations also had different varieties, one determinant and one indeterminant.
“All products gave us yield protection, so all the products can be used to control soybean rust,” he said. “In most cases, three applications gave us a little bit of a yield bump. Sometimes it was very significant; other times there was not much difference.
“But when you look at the disease ratings before the plants began defoliating, you can see that all of the products are different. On this end, you can see we have products that have no disease in the field. These products have a very high curative activity and a very long residual. Others did not do as well.”
The bottom line on the tests? Growers should make the first application at or near flowering and plan on two applications. “Last year in Brazil, farmers made an average of 1.5 applications; this year, it's 1.9,” says Miles. “The Brazilians are learning.”
Strobilurin fungicides must be used as a protectant. “If disease is not in your field or in your county, but is expected, you can use a preventive or protectant fungicide,” Miles notes. “If the disease is not in your field, but in your county and downwind from you, you should use a triazole or triazole mixture. It's too late for the strobilurins.”
Miles cautioned growers not to make two applications of strobilurins in their fields. “If the disease is present or nearby, you are limited to a triazole or triazole-strobilurin mix,” he notes. “You can have an infection in your field and not see it for seven to 10 days. If you use a strobilurin, you will not control the disease in that field.”
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