A new tool is available to select for soybean rust resistance in breeding populations, said Glen Hartman, University of Illinois professor of crop sciences and USDA-ARS scientist.

Hartman and his team of researchers successfully used quantitative polymerase chain reaction (Q-PCR) assays to assess fungal DNA in soybean leaf tissue to quantify the level of resistance in individual plants with resistance to soybean rust.

“This is not a new technique,” Hartman said. “But it is a new tool for use in soybean rust resistance breeding, which has typically used phenotyping or visual assessment to measure resistance. We discovered that we can perform more precise and quicker assessments using this molecular technique.”

Visual assessment is subject to interpretation and is not an exact science, Hartman said. However, Q-PCR allows for exact enumeration of fungal DNA in the tissue. This is particularly helpful when plants show similar visual symptoms, but colonization levels vary based on fungal DNA levels.

 “The eye can easily tell us if it’s a plus or minus for qualitative resistance, but Q-PCR tells us the quantitative resistance or the gray that lies between the plus and minus,” Hartman added.

Often qualitative resistance doesn’t last as long as quantitative resistance because it involves a single gene. Pathogens can overcome a single gene more easily, putting soybean breeders right back to where they started with a susceptible reaction, he said.