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Soybean rust game plan needed Monday, April 4, 2005
Agri News staff writer
NASHUA, Iowa -- Farmers need a game plan if Asian soybean rust is detected in the Midwest this summer, Iowa State University Extension plant pathologist Alison Robertson told farmers at the recent annual meeting of the Northeast Iowa Agricultural Experimental Association at Nashua.
Asian rust is a foliar disease caused by fungi that cannot overwinter in Iowa, Robertson said. The fungus needs green tissue to survive. It is expected to survive on kudzu, an aggressive plant that grows in the South.
"We don't know how kudzu will affect the dispersal of innoculum," said Robertson. "There is no kudzu in the other countries that have soybean rust.''
Rust overwinters in south Florida and southern Texas along the Gulf Coast and is expected to blow north. Robertson cautioned that rust needs the right environment to develop.
Soybean rust enters a field when its spores are blown onto soybean plants. For spores to infect plants, there must be six hours to eight hours of leaf wetness.
"If the spores land on the leaves, and they are dry, they will not infect the soybeans,'' Robertson said.
If soybeans are infected by the spores, pustules form five to eight days later on the underside of leaves.
Robertson said ISU rust expert X. B. Yang has speculated it may arrive later in Iowa than it does in Brazil. Researchers at the USDA, North Carolina State University and ISU are tracking spore development and distribution as it moves northward. They will trace movement of pathogens and the disease at www.ces.ncsu.edu/depts/pp/soybeanrust.
Hundreds of sentinel plots around the United States will track soybean rust. ISU has 30 plots and industry and farmers will also have plots. Sentinel plots are small areas planted to soybeans three to four weeks earlier than the normal.
Early symptoms of rust infection are found on leaves deep in the canopy, and look like tiny black specks scattered within mottled yellowed areas. In older infections, pustules, which produce masses of orange to brick-red spores, may be seen on the underside of leaves.
Robertson said farmers need to use a hand lens to scout.
George Cummins, an ISU Extension field specialist for crops based in Charles City who received training on rust scouting in Brazil, said before pustules form it's difficult to tell if soybeans have rust or something else. He suggests picking suspicious leaves, blowing into a ziplock back and storing the leaves in the bag, checking back every few days.
Cummins said once farmers identify rust in Brazil they manage it by spraying. Robertson said a similar strategy is used in Zimbabwe.
Fungicides have been very effective in controlling Asian rust in Brazil, South Africa and Zimbabwe, Robertson said. Different fungicides attack the disease differently, and farmers need to know how each fungicide works.
Chlorothalonils are preventative. They stop germination of rust spores. They stay on the leaf surface and don't move into the plant. Bravo, WeatherStik, Chloronil and Echo are chlorothalonils.
Strobilurins stop germination and also stop infections. They penetrate the leaves, but movement is nothing like with herbicides.
Quadris and Headline are strobilurins.
Triazoles stop growth of the fungus once it's in the plant tissue. Triazoles are more mobile in the plant. Tilt, Propimax, Bumper, Folicur and Loredo are triazoles. Strego is a strobilurin/triazole mixture.
Timing is essential, Robertson cautioned. Don't spray preventative fungicides too early. Don't spray curative fungicides too late. Coverage and penetration are crucial for fungicides to be effective.
"Once rust is detected in an area, you need to spray every 21 days,'' Robertson said.
A good source is "Using |
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