Craig Grau, a plant pathologist at the University of Wisconsin-Madison, conducts research on managing soybean diseases. He said he wasn”t looking for any relationships between soil pH and soybean cyst nematode or brown stem rot.

Nevertheless, after four years of study, Grau has found that soybean cyst nematode is more prevalent in soils with high pH, a situation that can result in lower yields. The opposite was true of brown stem rot – the lower the soil pH, the greater the chance the disease would hurt yields.

Grau”s research, conducted in collaboration with Greg Tylka, a soybean nematologist at Iowa State University, is part of a multistate effort to better understand a variety of stresses that affect soybean yields.

“Looking at soybean cyst nematode, it appears there is minimal yield loss at soil pH levels of 5.9 to 6.0,” Grau said. “Between 6.0 and 6.5, there”s probably low risk. But the risk of yield loss due to soybean cyst nematode rapidly increases as soils reach pH 7.0 and above.”

When the soil pH level reaches 6.5, Grau said nematode damage can reduce yields by as much as 10 to 20 bushels per acre, depending on the year and the variety planted. In his research, yield loss estimates were based on yield differences between soybean cyst nematode-resistant and soybean cyst nematode-susceptible soybean varieties.

The type of soil also may have an influence. The effect of soil pH on nematodes appears to be more pronounced in silt loam soils and less so as sand content increases, Grau said.

When examining brown stem rot”s effect on yields, early results indicate that higher soil pH levels are desirable. That may be due to how different soybean varieties react to different strains of the brown stem rot pathogen, Grau said. Research indicates that populations of the pathogen are composed of two major strains, one highly pathogenic and one weakly pathogenic.

“A question we have yet to answer is: Is the highly pathogenic strain of the brown stem rot pathogen not present or is it less active at the high pH levels?” Grau asked.

Grau emphasized that attempts to modify soil pH would be expensive. Instead he urged soybean producers to use soil tests to determine the status of soybean cyst nematode and inspect the crop at the full-pod stage to determine the status of brown stem rot.

“Growers can manage brown stem rot with more confidence compared to soybean cyst nematode,” he said. “If you have a low pH level and minimal nematode problems, that”s where you want to stay. But be aware of the brown stem rot risk. More commercial varieties are available with resistance to the disease. Also, not planting soybeans in a field for two years greatly reduces the risk of brown stem rot.”

Grau”s research will continue on relationships between soil pH and the two yield-robbing diseases. One study will evaluate data on both new and old soybean varieties that are either susceptible or resistant to brown stem rot. Other studies also will explore the effect of soil pH on viruses in soybean fields.

Grau”s research is part of the Yields Project, a multistate research program funded by Iowa and Illinois checkoff dollars through the Soybean Research and Development Council. The Yields Project is one of the largest soybean research projects ever undertaken to understand factors that limit soybean yields. The project involves scientists at Iowa State University, the University of Illinois, the University of Wisconsin and Southern Illinois University.