Iron deficiency chlorosis (IDC) is a serious problem when soybeans are grown on high-pH soils.

""Chlorosis in soybeans can occur whenever soybeans are grown on soils with calcium carbonate or lime in the topsoil,"" says R. Jay Goos, North Dakota State University professor in the Soil Science Department. ""Lime in the topsoil can occur on eroded soils where calcareous subsoil is exposed."

However, in North Dakota, IDC is more common on level, poorly drained soils, which also can have lime in the topsoil.

""Soils with a high water table in the spring are the soils most associated with IDC,"" Goos says. ""These soils often have elevated salinity as well, which makes chlorosis worse. Most of the crops we grow aren't bothered by the presence of lime in the topsoil, but soybeans can be. The upper leaves will turn yellow, but the leaf veins will turn green. If the deficiency progresses, the growing point will be damaged."

The most practical control measure is choosing a resistant variety.

""No soybean variety is immune to chlorosis, but there are huge differences among the most resistant and most susceptible varieties,"" Goos warns. ""The wrong choice can lead to reduced yield or even crop failure."

Since 2001, Goos, with the financial assistance of the North Dakota Soybean Council, has screened soybean varieties for resistance to chlorosis.

Five trials with multiple replications and measurements were started in 2009. Two sites in Richland County were lost due to ponding. However, reliable data was obtained at the other three sites. All of the soybean varieties entered in an NDSU yield trial received a chlorosis evaluation.

Among conventional, Roundup Ready, Roundup2Yield and Liberty Link varieties, more than 300 varieties were tested in 2009.

""We report the data two ways,"" Goos says. ""We report the data as the actual chlorosis score, where a score of 1 means the variety did not have chlorosis, while a score of 5 is the most severe chlorosis. We also give each variety a grade based on how it did relative to the other varieties and the standards we include in each trial. We did this last year and received favorable comments from producers."

Goos offers the following interpretation of his grading system:

• Avoid the D's. These varieties are too susceptible to plant on chlorosis-prone soil.
• You can do better than a variety that tests at a C. Stronger varieties are available for chlorosis-prone soil.
• A grade of B or B minus is a good grade. However, for fields with the worst chlorosis problems, stay with varieties with a score of A, A minus or B plus. By sticking with these three categories, producers are going with the top 10 percent in the marketplace for resistance, which will be needed for the toughest conditions.

Goos cautions that these trials do not apply to fields without chlorosis problems.

""Our ratings are only for high-pH fields with chlorosis problems,"" he says. ""For well-drained fields without lime in the topsoil and no history of IDC in soybeans, the chlorosis-resistance score of a variety is of little concern."

Goos also warns farmers with chlorosis problems not to be too fast to ""jump on the bandwagon"" regarding new genetic traits.

""Seed companies are coming out with soybeans with new transgenic traits,” Goos says. “A promised yield advantage could be lost quickly if the soybeans turn yellow with chlorosis. On fields giving chlorosis, farmers should not compromise on chlorosis resistance in order to try something new.""

The 2009 trial results can be found at http://www.yellowsoybeans.com .

NDSU Agriculture Communication