WOOSTER, Ohio — With soybean rust now officially confirmed in the United States, the more educated Ohio growers are about the fungus, the better prepared they will be to manage it if it is ever diagnosed closer to home.
The U.S. Department of Agriculture recently diagnosed the presence of the disease on leaf samples from two soybean fields in Louisiana. This is the first confirmed report of the fungus in the contiguous United States. Hawaii has been home to the disease since 1994.
The presence of the disease in other countries, such as Brazil, has caused significant yield losses and high fungicide costs. There are concerns that the disease could present similar issues for U.S. growers.
Soybean rust is an aggressive fungus similar to the rust fungi that cause wheat leaf rust and corn leaf rust. It is caused by either of two fungal species, Phakopsora pachyrhizi, also known as the Asian species, and Phakopsora meibomiae, the New World species. The Asian species, the one found in Louisiana, is the more aggressive of the two species, causing more damage to soybean plants.
Anne Dorrance, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center, said that this is not the time for growers to rush out and stock up on fungicides.
"The goal here is to emphasize to growers that the need to stock pile fungicides is not warranted. The thing that we've learned from other countries is that this disease is manageable with fungicides, but there are still many questions that need to be answered," said Dorrance. "Questions such as will the disease survive and overwinter and which fungicides are the most cost-effective. Growers should go and enjoy the holidays and by January we should have a lot of these questions answered."
Dorrance said that education remains the key to properly managing the disease. Ohio State University Extension specialists, in conjunction with the American Soybean Association, have aided growers in this respect by offering statewide and national workshops on soybean rust. Such workshops will continue this winter.
Materials such as a pocket-sized soybean rust I.D. card that growers can carry into the field are being developed, along with a fungicide manual. The development of both materials is part of a multi-state project. OSU Extension, with the help from the Ohio Soybean Council and the check-off program, provided a diagnosis of soybean samples through Ohio State's C. Wayne Ellett Plant and Pest Diagnostic Clinic during the 2004 growing season. Efforts are under way to continue that program in 2005.
OARDC research in preparation for the arrival of soybean rust includes studying the efficacy and phytotoxicity of fungicides used in conjunction with herbicides and insecticides.
"We already have two years of data of applying fungicides with Round-Up. Some of the initial reports from some countries have found that the vegetative stage would be the time to apply chemicals. We don't know if that is the case for Ohio, but we thought we'd get the data ready," said Dorrance. "We have found that spring Round-Up at the proper rate along with fungicides produced no phytotoxicity and had better control of the weeds."
Researchers are also studying the possibility of using fungicides along with insecticides used to control the soybean aphid.
"We don't know if we'll have rust in the fields at the same time as soybean aphids, so we need to find out if both chemicals will burn the plant," said Dorrance.
Researchers with the Ohio State Soybean Breeding Program are also moving forward with new variety developments, in the event any of them show resistance to soybean rust.
The first signs of the disease are the development of small tiny brown or red brick spots on the top of the leaves. The spots are less than half the size of a leaf hair and a hand lense is needed to spot the lesions. On the underside of the leaf, the lesions break open to reveal cream-colored spores. The process will continue as long as conditions are favorable for fungal development, causing premature defoliation leading to yield losses, fewer seeds per pod, decreased number of filled pods per plant and early maturity.
Currently, there are registered fungicides that work well against soybean rust and U.S. Environmental Protection Agency Section 18 Quarantine Exemption requests have been submitted by several states to expand the number of fungicides available. Ohio is one that has submitted a request.
The products, however, are expensive; Ohio State researchers are currently studying ways to help producers save on application costs.
Ohio State plant pathologists, the Ohio Department of Agriculture, and the U.S. Department of Agriculture Plant Protection and Quarantine have drafted a management plan to combat the spread of soybean rust.
"I think we are well prepared if the disease makes it to Ohio," said Dorrance. "We will have in place nationwide a series of soybean plots that are monitored throughout the season to determine if and when the disease reaches certain states. We already have this in place for corn rust and wheat rust to let growers know when they need to spray."
Researchers speculate the mode of transmission of soybean rust to be via four specific routes: the Central American land bridge; hurricane winds via the Caribbean; spores on debris in shipments of seed or meal; or an act of bioterrorism with the final two being the least likely to occur.