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Research Highlights
Genetic Research Identifies Major Source for Phytophthora Resistance

A discolored lower stem is a common symptom of Phytophthora sojae. Photo: Ohio State University

By Laura Temple

Soybeans rely on genetics for protection from Phytophthora sojae, a common soil-borne disease and the second-most yield-limiting soybean disease in Ohio, according to Leah McHale, Ohio State University professor. Soybean fields contain diverse, complex populations of Phytophthora, with numerous races, and strong protection from this disease requires complex genetics.

Phytophthora can cause damping-off as well as stem and root rot. The Ohio Soybean Council and United Soybean Board are funding research to identify and develop germplasm with improved resistance to Phytophthora and address challenges inherent in that genetic resistance. 

“Once a race of Phytophthora sojae can overcome a cultivar’s to genetic protection, it can quickly move through a field,” says McHale, who is leading the research. “Providing genetic material with varied types of resistance to this disease is key to protecting soybean yields.”

Identifying Significant Resistance Genetics

McHale describes two types of genetic disease resistance. Qualitative resistance provides 100 percent resistance to a specific race of Phytophthora sojae. Quantitative resistance provides partial resistance to all races of the disease. 

“Because fields contain multiple races and the disease can adapt to overcome qualitative genetic resistance in soybeans, we additionally focus on quantitative resistance,” she says. “However, it’s a challenge to breed for many genes that each offer a small amount of Phytophthora resistance.”

Phytophthora sojae reduces yield by more than 30 million bushels annually nationwide. Photo: Ohio State University

In 2019, this research published the characterization of a genetic region, or locus, in soybeans proven to significantly impact Phytophthora resistance. Since that discovery, McHale and her team have been working to better understand these genes and incorporate them into germplasm for farmer use.

“This locus of quantitative Phytophthora resistance provides 35 to 40 percent of the increase in disease resistance in the lab and greenhouse, which is a much higher percentage than most quantitative genes,” she explains. “We’ve seen it provide a 13 to 29 percent increase in yield in fields with disease conditions.”

Genes that confer genetic resistance work in different ways. McHale provides examples that some may affect root architecture to avoid the pathogen, while others may release a chemical defense. Understanding how the resistance from this locus works will help soybean breeders make informed choices to better join resistance mechanisms that work together well. Combining complimentary resistance mechanisms rather than redundant genes that work the same way strengthen soybeans against races of Phytophthora developing resistance to new genetics.

The team is using marker-assisted selection to breed this resistance into soybean lines. This tool helps efficiently find and cross soybeans with the desired genetics.

Combining Resistance Genetics

As this work has progressed, McHale and her team have identified another challenge as they incorporate this resistance into more germplasm.

“The locus for this Phytophthora sojae resistance seems to be located near the genetic region for soybean cyst nematode (SCN) resistance,” she says. “From what we can tell, these regions don’t overlap, but they are close enough to each other that we’ve seen problems in breeding soybeans to have both types of resistance.”

When genes are located near each other, the next generation often receive the same combination of genes as one of the parents. That makes it difficult to cultivate a plant with both resistance characteristics from parents that each carry one type of resistance.

McHale sees this as a challenge and opportunity. While they work to develop soybean strains that contain resistance to Phytophthora and SCN, they are learning more about how these genes work. And once they develop a soybean strain with both resistances, it will be much easier to select for increasing levels of combined resistance. 

“We release germplasm material as soon as we are able, so it is available to other breeders working in both commercial and public breeding programs,” McHale says. “Within five years, the resistance in that germplasm can be available in soybean varieties for farmers to plant.”

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.