Plant pathologists offer soybean rust identification and management tips

With the confirmation that soybean rust has been detected in the U.S., plant pathologists with The American Phytopathological Society (APS) are offering insight into the management and identification of this disease.

According to Doug Jardine, director of the APS Office of Public Affairs and Education and plant pathology professor at Kansas State University, plant pathologists from government agencies, industry, and universities have been working together to prepare for the appearance of soybean rust in the U.S. for the past two years. “Through this collaboration, we have been able to share research information from around the world and updates on control methods, such as fungicides sprays and predictive weather models,” said Jardine. Plant pathologists have also worked with local extension agents and growers to educate them on disease identification, potential yield loss, and costs associated with managing the disease.

Plant pathologists do not expect soybean rust to affect all soybean growing areas next year. “Growers should not assume that every soybean field will be in danger,” said Jardine. “Based on our models, the disease is expected to be more severe in the Southeast, Lower Mississippi-Delta region, and the Appalachians and less severe in the western Great Plains and northern Great Lakes area,” he said. Soybean rust is caused by two fungal species–Phakopsora pachyrhizi and Phakopsora meibomiae. The more aggressive species, Phakopsora pachyrhizi, is the type that has been detected in the U.S.

Soybean rust primarily affects plant leaves and creates two types of lesions–tan and red lesions with tan being the most severe. The appearance of brown spots on soybean leaves is the first sign of infection. When the leaf is turned over, a plant infected with soybean rust will have raised pustules with rust spores inside. As rust severity increases, premature defoliation and early maturation of plants is common. Soybean rust can go from a few pustules in a field to full field defoliation in a matter of about two weeks.

The main method of controlling soybean rust is through early detection and application of fungicides, said Jardine. “If the disease goes undetected for too long, the fungicides will not have much of an effect,” he said. “Growers must put fungicides on as a preventive measure or at the earliest sign of infection,” said Jardine. Information on available fungicides, rates, and time of application can be obtained through local Cooperative Extension Service offices. Fungicide companies and the Environmental Protection Agency (EPA) are working closely with plant pathologists and growers to provide appropriate products in adequate supplies to use next year as needed, said Jardine.

Plant pathologists have screened all available commercial varieties of soybeans for resistance to soybean rust, but none have proven to be resistant. There are some varieties that are more susceptible to the disease and it will be important to identify these early on to reduce the impact of the disease, said Jardine.

Growers that suspect that they may have soybean rust are encouraged to collect samples and send them to their local state land grant university diagnostic labs for evaluation. The diagnostic labs will be receiving additional soybean rust identification training over the winter, said Jardine. A list of state land grant universities is available at www.apsnet.org/directories/extension/top.asp.