Research Highlights

Under Pressure: Factors Allowing Phytophthora to Overcome Genetic Resistance in Soybeans

In this article, you’ll find details on:

• Soybean breeders use two types of genetic resistance to protect soybeans from Phytophthora stem and root rot, but the pathogen that causes the disease is changing so that resistance is becoming less effective.
• Understanding the factors that exert selection pressure on the disease-causing pathogen, Phytophthora sojae, will help develop management options.
• While the Iowa State University research team is still looking for answers, they have confirmed the value of characterizing pathogen populations to inform soybean variety selection.

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By Laura Temple

Pressure forces change — usually to overcome the pressure source. For example, in response to pressure from glyphosate, weed populations have changed. Fields now have more weeds that glyphosate never controlled well, as well as weeds that have developed glyphosate resistance.

That principle applies to soybean diseases as well. Alison Robertson, professor and extension field pathologist at Iowa State University, is exploring the factors creating pressure that has allowed Phytophthora stem and root rot to overcome common types of genetic resistance in soybeans. The soilborne pathogen Phytophthora sojae causes this disease.

“In 2023, we saw more Phytophthora in Midwest soybeans than we had in a long time,” she says. “We know that the single-gene resistance that should provide complete protection has been losing effectiveness, and we want to understand why.”

By then, Robertson was already leading research to investigate the pressures increasing the virulence of the pathogen against soybean varieties carrying genetic resistance. The Iowa Soybean Association began funding this research with a soy checkoff investment in 2022. She notes that Phytophthora consistently ranks among the top five pathogens causing economic losses in U.S. soybeans each year, according to agronomic surveys. 

Single-Gene vs. Partial Resistance

Robertson explains that soybean breeders use two different types of resistance to provide in-plant protection from P. sojae infections. 

• Single-gene resistance provides total protection. These genes, called “resistant to Phytophthora sojae” or Rps, recognize proteins secreted by the pathogen when attacking the plant. In response, the genes activate the soybean’s natural defenses to prevent infection. 

• Partial resistance refers to combinations of many minor genes that provide defenses against Phytophthora when a soybean plant gets infected. While the plant does become diseased, it is still able to produce a yield. This is often referred to as field tolerance.

While almost 30 Rps genes have been identified in soybeans and more are discovered each year, only five are currently available in commercial soybean varieties: Rps1a, Rps1c, Rps1k, Rps3a and Rps6. Each gene recognizes a specific protein released by P. sojae. Some Rps genes were not deployed commercially because populations of the pathogen could already overcome them.

“We don’t know all the genes involved in partial resistance, but it is only triggered when soybeans reach the first true trifoliate growth stage,” she says. “We have also seen decreasing effectiveness of partial resistance against Phytophthora. This may be a function of measuring partial resistance, which can be difficult.”

This research looks at the selection pressure both types of resistance put on P. sojae to understand how the pathogen populations are changing in the field to overcome those soybean genetics. She believes this knowledge will help develop better management options, even as breeders combine resistances in some soybean varieties.

Applying Selection Pressure

“P. sojae is changing, becoming more complex,” Robertson says. “For example, field screening shows that 100% of the population in Iowa can cause disease on soybeans with Rps1a. The same isolates may cause diseases on soybeans with different types of Rps genes.”

Her team wants to figure out what causes these changes.

“We hypothesize that P. sojae populations change more under selection pressure from partial resistance, or field tolerance,” she explains. 

The working theory is that because partial resistance allows the disease to take hold, the pathogen produces more sexual spores, called oospores, that can survive for many years. Sexual reproduction increases genetic variation in the population. Under single-gene resistance, Phytophthora produces very few oospores.

However, testing the theory has proved complicated. The team aimed to collect and isolate P. sojae from soybeans inoculated in hydroponic plots with different conditions, including a rotation of Rps resistance genes each cycle. Comparing populations before and after using different types of resistance would show how the population changes over time.

Researchers characterize P. sojae into pathotypes based on which Rps genes an isolate can overcome, infecting the plant and causing disease. This information enables breeders and farmers to develop and select varieties with Rps genes suitable to be deployed in a field.

The method to pathotype Phytophthora isolates requires a couple weeks. Robertson’s team is testing a quicker method that allows pathologists to identify and characterize field populations more efficiently, within several hours rather than weeks. They have yet to confirm the accuracy of this method. 

“We have learned that it’s important to characterize P. sojae populations in a field so farmers can monitor how they are changing and select resistance accordingly,” she says. “And we continue to explore more efficient methods to accurately characterize the pathogens.”

While the team doesn’t have answers yet for the factors exerting the selection pressure behind these changes, the data her team is gathering is directing their ongoing search. And it could support other research, as well.

“In another research project, we are looking at other ways to use genetic information to silence or stop diseases caused by pathogens like P. sojae,” Robertson explains. “In the meantime, we know that improving our understanding of the susceptibility of soybean varieties to Phytophthora and which Rps genes can protect against local populations of the pathogen can help farmers choose varieties most likely to mitigate risks in given fields.”

Additional Resources:

• Phytophthora Root and Stem Rot – SRIN information page
• Phytophthora search results on SRIN 
• An Overview of Phytophthora Root and Stem Rot – Crop Protection Network

Meet the principal investigator on this project: Alison Robertson

Published: Sep 2, 2024