Soybean Rust Next to Impossible to Predict

February 8, 2005

Editor's note: This is the first of a periodic series on information regarding soybean rust. The goal is to provide media with the latest updates on the disease and Ohio State's role in research and education. These updates are expected to continue throughout 2005.

WOOSTER, Ohio — With soybean rust now confirmed in eight southern states, the question on most minds is will the disease strike Ohio, and if so, when?

Anne Dorrance, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, said that it's next to impossible to predict if, when or even how soybean rust will arrive in the state. But she is hopeful that surveys being conducted in soybean-rust infected states, along with the implementation of a nationwide soybean rust monitoring system, will help provide some clues as to where the disease will show up next.

"Southern states, especially those along the coast, will be assessing whether soybean rust is surviving the winter. If it does survive, the next step is to determine how much innoculum, or active lesions, are present in that particular area," said Dorrance. "We won't have any information on that until mid-March."

In addition to the surveys, the U.S. Department of Agriculture, along with various institutions and universities throughout the country including Ohio State, is finalizing the development of a nationwide soybean rust monitoring system. Dorrance equates the system to OARDC's Web-based head scab forecasting model, in that its purpose is to track soybean rust and aid in determining what areas of the country might be most vulnerable.

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 the United States, is the more aggressive of the two species, causing more damage to soybean plants.

The presence of the disease in other countries, such as Brazil, has caused significant yield losses and high fungicide costs, and there are concerns that the disease could present similar issues for U.S. growers.

Dorrance said that until more is known about the survival of the fungus, growers should continue to educate themselves about the disease. One way is to learn more about the fungicides designed to manage it — currently the only known effective means for controlling the disease.

There are three types of fungicides — chloronitriles, strobilurins, and triazoles — all of which perform differently in relation to the stage of the disease cycle.

Chloronitriles are a group of compounds that are only active on the spore and must be applied prior to any rust spores arriving in a field. Widely used in vegetable production, chloronitriles make a good protectant/preventive treatment, but their main limitation is that they must be applied every seven to 10 days and are subject to weathering. Examples of chloronitriles approved for use against soybean rust are Echo and Bravo.

Strobilurins are also used for preventive treatment, but they are not to be used if there is more than a 3 percent level of disease in a field and they can only be used once in a season for soybean rust resistance management. Headline and Quadris are strobilurins labeled for soybean rust.

Triazoles are curatives and make up the bulk of the fungicides labeled for soybean rust management. A number of products currently have Section 18 status in Ohio, including Bumper, Folicur, Laredo, Propimax and Tilt. Another chemical, called Domark, is still pending EPA approval. Triazoles attack the fungus after it germinates and invades the leaf, so they need to be applied on the plant immediately prior or soon after spore deposition to have the most impact. Triazoles applied too early (two weeks or more) before the arrival of spores are not as effective. Additionally, triazoles cannot be used in successive sprays because of the risk of resistance development.

Some combination products (a strobilurin and triazole, for example) have either been approved (Stratego) or are awaiting approval (Quilt). These products are designed to provide a much broader protection for the plant, both before the disease arrives and after it has infected the plant.

"One of the things to key into with these fungicides are pre-harvest intervals, which are quite long. For chloronitriles, days to harvest is 42, for strobilurins, it's 21 days and for most triazoles, it's 28 days," said Dorrance. "So if rust arrives in Ohio at the end of August, there isn't anything we will be able to spray at that time."

An Ohio State University Extension Crop Profit Game satellite series focusing on soybean rust biology and management will be held Feb. 15 from 7 p.m. to 9:30 p.m. Contact a local OSU Extension county office or log on to http://cropprofit.osu.edu for more details.

Author(s): 
Candace Pollock
Source(s): 
Anne Dorrance