Soybean Rust Moving Slower than Predicted

June 13, 2005

Editor's note: This is part 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 — Soybean rust is moving northward from the southern United States much slower than predicted, but Ohio's soybean crop may not be out of the woods yet.

To date, only four counties in Florida and one county in Georgia have had positive reports of soybean rust, according the U.S. Department of Agriculture. The slow movement of the disease could be attributed to a variety of reasons, said Anne Dorrance, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center, and Ohio's leading researcher on soybean rust.

"One theory is that there was just very little inoculum that survived the winter, which for U.S. farmers is actually very good. Another reason is that the environment in Florida has not been favorable for rust development," she said. "And another fact is that the weather currents have not allowed spore movement. Storms have pushed spores out into the Atlantic and not back into the gulf (of Mexico)."

The reports spell good news for many, but Ohio growers still need to keep a close eye on their soybean fields.

"We've got a crop in Ohio that's gone in two weeks later than usual and is at a growth stage of about V1-V2, which means that we'll be flowering in late July. Growers will also be harvesting about two weeks later than normal," said Dorrance. "So we've got a delayed crop and a delayed pathogen, and we'll see if these two meet."

While the waiting game continues, growers should use the time to educate themselves on identifying soybean rust in the field, especially since several other soybean diseases share similar characteristics.

"As growers begin scouting their fields, there are certain diseases they are going to see," said Dorrance. "The main ones we run into in Ohio are bacterial blight, brown spot, and some Cercospora blight and frogeye leaf spot."

Bacterial blight affects the mid-to-upper leaf canopy, whereas soybean rust affects the lower canopy. Bacterial blight lesions are also much larger than those of soybean rust and take on a water-soaked appearance with a yellow halo.

Brown spot is like soybean rust in that it affects the lower leaf canopy. However, lesions are much larger than those of soybean rust and they take on a "yellow" appearance.

"One thing about soybean rust is that it tends to follow the leaf veins because that is where the moisture is held longer," said Dorrance. "On brown spot, the lesions are more scattered." Brown spot is already showing up in some parts of Ohio.

With frogeye, lesions start as dark, water-soaked spots and are larger and have more defined margins than soybean rust lesions. Cercospora blight affects the upper canopy and the disease starts out as light purple areas, which eventually spread over the leaf surfaces.

"We are going to have eight field days and twilight meetings in Ohio this summer to help growers identify these diseases," said Dorrance. "The reality is if a grower walks into a field and sees a bunch of tiny pin-prick lesions on the plants, then the best thing to do is just send the sample to the diagnostic clinic for testing. We would rather be deluged with common soybean foliar diseases than to let this one go."

There is no cost this year to send samples in for testing. Growers should send samples to the Ohio State University C. Wayne Ellett Plant and Pest Diagnostic Clinic at Soybean Rust-CWEPPDC,110 Kottman Hall, 2021 Coffey Road, OSU, Columbus, OH 43210.

For the latest in soybean rust development, log on to USDA's soybean rust monitoring Web site at http://www.sbrusa.net. For additional information on soybean rust, log on to Ohio State's Agronomic Crops Network Web site at http://agcrops.osu.edu/soybean.

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