Phytophthora Root Rot Grabs the Spotlight in Soybeans
A deep, rich brown color is just what you want to see when you’re munching on a chocolate candy bar. On the other hand, it’s the last color you want to find on soybean stems.
Chocolate brown is a defining color for Phythophthora root rot (PRR), and it’s showing up this season in fields across Iowa and other parts of the Midwest.
“We have Phytophthora every year, usually a little bit for the summer. This year, we just seem to have a lot more,” Alison Robertson, Iowa State University Extension plant pathologist, told AgriTalk Host Chip Flory earlier this week.
The disease can infect soybeans at any point in the growing season. One of the identifying features are lesions that start below the soil line and extend up the stems and into the lower parts of the plant, she says.
Hot, Dry, Then Wet
The increased incidence of the disease this year has to do with hot, dry weather followed by what Robertson calls heavy precipitation events farmers received in June and early in July.
“If you remember back to mid-June, we had a couple of heavy precipitation events two to three inches. And what that would do is flood the soil and prime what is called Phytophthora sojae -- oospores produced in infected plant tissue as it decomposes,” Robertson explains. “We had mostly a hot, dry July and that’s just exacerbated the situation, and now we’re seeing all these wilted plants.”
P. sojae is the primary species that causes Phytophthora root and stem rot in most areas, but another unidentified Phytophthora species has been found in the Midwest that can also kill soybeans. The geographical range of this other species is not well understood, according to Dean Malvick, Extension plant pathologist for the University of Minnesota.
Little can be done in-season to address PRR. Where it’s a problem this season, Robertson says farmers need to plant resistant cultivars in 2024.
Malvick adds that the most common resistance genes that are widely effective are Rps 1c and 1k. Rps1a can be effective, but is not as effective in many areas as the other genes. Rps 3 and other genes can also be effective. For areas where the Rps genes are not working well due to the presence of pathogen races that overcome the resistance, cultivars with high levels of field tolerance (or partial resistance) should be planted, he adds.
During the AgriTalk discussion, Robertson and Flory also address a variety of other disease issues, including tar spot and Goss’ wilt. Catch the discussion here:
