Soybean Rust Update

from Jim Dunphy, Extension Soybean Specialist, and Steve Koenning, Extension Plant Pathologist NCSU

Asiatic Soybean Rust was confirmed yesterday in South Carolina commercial soybean fields in Bamburg and Dorchester Counties, SC. Orangeburg County, and Barnwell County, SC were confirmed positive for soybean rust last week. Although soybean rust has now been found in 4 SC Counties, all are below the lakes in South Carolina. This still does not put rust close enough to warrant a recommendation from us to spray for the disease. The closest confirmed rust on soybeans to our North Carolina soybeans is now approximately 120 miles from Charlotte, 325 miles from Elizabeth City, 155 miles from Fayetteville, 115 miles from Murphy, 205 miles from Raleigh, 260 miles from Washington, 175 miles from Wilmington, and 185 miles from Winston-Salem, NC. (The mileages in red are changes from our previous update.)

Rust has now been confirmed this year on soybeans in 35 counties/parishes in seven states (LA, FL, AL, MS, GA, SC & AR).

We have received sentinel plot samples the last two weeks from Bertie, Carteret, Granville, Johnston, Lenoir, Scotland, and Wayne counties. Soybean rust was not detected in any sample.

Rust has progressed at a faster rate this year than in years past. With a late soybean crop, the odds on needing to apply fungicides is increased. Now is the time to check spray equipment and be sure to have the proper nozzles for applying fungicides.

We do not recommend spraying soybeans that have not started blooming with a fungicide to control Asiatic soybean rust. Such pre-bloom applications have seldom improved yields. Once soybeans start blooming, we would recommend spraying if rust has been confirmed within 100 miles of the field.

Management of Soybean Diseases with Fungicides: to Spray or not to Spray

Why we don’t recommend fungicides on a regular basis

1. We can’t predict the weather conditions for the next two to three weeks.

2. We can’t predict which disease is going to come to NC, or when.

3. A number of fungicides are available for management of soybean foliar diseases; see http://ipm.ncsu.edu/agchem/agchem.html . Typically yield responses of 1 to 2 bushels per acre are found in North Carolina, unless frogeye leaf spot or target spot are found in the field. There are a number of reasons why other states, particularly in the Delta, routinely report larger yield increases: 1) their shift to the early production system, their use of many varieties which are very susceptible to frogeye leafspot, and soybean maturing in August are more vulnerable to fungi because of the hot humid environment at this time of year; 2) some diseases such as web blight and cercospora blight are more common in the Delta than in North Carolina; and 3) crop rotation is practiced less often in the Delta than in North Carolina.

Some sources for more detailed information on Asiatic soybean rust are listed below:

The USDA soybean rust web site http://www.sbrusa.net/  

The North Carolina Agricultural Chemical Manual http://ipm.ncsu.edu/agchem/agchem.html

 


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