Soybean Rust Update September 21, 2015
from Jim Dunphy, Extension Crop Science Specialist (Soybeans)
Asiatic Rust has recently been confirmed on soybeans closer to our NC soybeans since our last update on September 15, but still not close enough to trigger a recommendation to spray. It has been confirmed on soybeans in Arkansas, Ashley, Chicot, Desha, Drew, Lonoke, Phillips and Prairie counties AR; Madison County, FL; Sumter County, GA; Alcorn, Choctaw, Kemper, Leflore, Newton, Noxubee, Prentiss, Simpson, Smith, Tishomingo, Webster and Yazoo counties, MS; and Barnwell County, SC. This represents the first find this year of rust on soybeans in Arkansas and South Carolina. The Barnwell County, SC, find is about 140 miles from Charlotte, 360 miles from Elizabeth City, 185 miles from Fayetteville, 205 miles from Murphy, 240 miles from Raleigh, 295 miles from Washington, 210 miles from Wilmington, and 210 miles from Winston-Salem. All but Murphy represent the closest finds to date. The closest find to Murphy remains DeKalb County, AL, at 115 miles.
I do not consider these confirmations to signal any imminent threat to NC soybeans. I still consider Steve Koenning’s (recently retired Extension Plant Pathologist) and my recommendation to not spray blooming soybeans that are more than 100 miles from confirmed presence of rust on soybeans to be a valid recommendation for most soybeans in NC. I do not recommend spraying for rust after stage R6 (full-sized beans in the top four nodes), since soybeans this mature will probably drop their leaves naturally before rust will cause many to drop prematurely. Check the label on the fungicide you prefer to use to see how late in the season that chemical may be sprayed.
The current status of rust in the continental US can be found anytime at http://sbr.ipmpipe.org.