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Research Highlights

Research Highlights
Boosting Soybean Resistance to Fight Phytophthora Stem and Root Rot

Photo: Crop Protection Network

By Carol Brown

Michigan soybean farmers continue to battle plant diseases, including root and stem rot, but some of their weapons have become ineffective.

To control the disease Phytophthora stem and root rot, soybean varieties have historically been used in Michigan with Phytophthora sojae resistance (Rps) genes. But after a recent survey conducted by Austin McCoy, a Michigan State University (MSU) graduate student, and Professor Martin Chilvers, it became clear that many soybean lines with resistance genes had lost the ability to fight. 

“I have been screening Rps resistance genes, trying to find out which genes were affected in Michigan,” says McCoy. “I’m also examining Michigan’s Phytophthora populations to see if there is resistance to any fungicides. I’m looking at both genetic and chemical management strategies.”

Chilvers, a field crop pathologist in MSU’s Plant, Soil, and Microbial Sciences Department, is McCoy’s advisor. Chilvers says that a survey like this needs to be done occasionally to see what Phytophthora sojae populations are in the fields and whether the soybean Rps resistance genes are still effective. A survey of this kind in Michigan had not been conducted in 20 years.

McCoy obtained more than 100 Phytophthora sojae isolates from soil samples submitted by farmers and crop consultants across the state. He then tested them on a set of 14 soybean lines that each contained a single Rps resistance gene or no resistance gene. Of those 14 lines, only three were effective against the isolates. The other 11 soybean lines succumbed to the Phytophthora sojae isolates.

Figure 1: Graph demonstrating the percentage of Phytophthora sojae isolates in Michigan that are capable of overcoming Rps resistance genes. Note the general increase of Phytophthora sojae isolates that are able to overcome the various resistance genes in 2020 vs 2001. 

“We compared our survey with the survey conducted 20 years ago,” said McCoy. “Effective Rps resistance genes are typically considered those for which 40 percent or fewer Phytophthora sojae isolates cause disease. Alarmingly, the most common resistance genes available commercially, Rps 1c and Rps 1k, appear to have lost efficacy since the last survey.”

Chilvers says that of the three resistance genes that were identified to still be effective, only Rps 3a is available in commercial varieties in Michigan. This means that farmers may not be as armed to fight Phtophthora root and stem rot as they think. 

“This work demonstrates the need for soybeans to have partial resistance, or field tolerance, as well as a seed treatment,” Chilvers says. “That partial resistance doesn’t always initiate when the seed is just germinated. It takes some time for that to get started. Pairing it with a seed treatment helps provide protection against these diseases.”

Photo: Crop Protection Network

Combining a soybean variety with the Rps 3a gene and a fungicide seed treatment will boost the fighting power of the soybean seed. McCoy screened fungicide seed treatments and found that the tested Phytophthora sojae isolates are still sensitive to them. McCoy cautioned that fungicide seed treatments may reduce the disease severity, but not eliminate it.

McCoy has also done preliminary work with Phytophthora sansomeana, another oomycete that causes water mold. It seems to like more than just soybeans as a host. 

Phytophthora sansomeana has been found on corn in the field and, based on my data, it may also use wheat, rye or oats as a host plant. But I’m working to confirm this,” McCoy says. “It seems to cause disease on more plant species beyond soybeans. That could be a problem when farmers are rotating their fields from soybeans to corn. They may just be switching from one host to another, and then you can’t break the disease cycle.”

Work will continue for McCoy and Chilvers in the upcoming crop year on Phytophthora sansomeana to gather more data on how this organism works and how to combat it. 

For more information on soybean Phytophthora root and stem rot, visit the soybean diseases page on this site: https://soybeanresearchinfo.com/soybean-disease/phytophthora-root-stem-rot/

To find research related to this Research Highlight, please visit the National Soybean Checkoff Research Database.