Global Plant Protection News

Lauren Quinn University of Illinois

URBANA, Ill. – Septoria brown spot may be considered the “common cold” of soybean diseases, but that doesn’t mean it’s entirely benign. The fungal disease can cause 10 percent to 27 percent yield loss. Many farmers fight it by using fungicide, but a new University of Illinois study shows Septoria can actually increase after fungicide application.

“When we applied the fungicide, most of the fungi on plant surfaces decreased,” said Santiago Mideros, an assistant professor in the department of crop sciences at the University of Illinois and co-author of the study. “But a few of the fungi increased, Septoria among them. It was very surprising.”

Led by Heng-An Lin, a former crop-sciences doctoral student, the study was designed to identify and track the soybean mycobiome – the collection of fungi living on soybean plants – in field conditions.

Lin and Mideros inoculated half the soybean seedlings in their field trials with Septoria. Then using genetic information and bioinformatics analyses, they identified fungal species on leaves throughout the season before and after applying fungicide.

“We chose a mixture of fluxapyroxad and pyraclostrobin fungicides because it’s quite commonly used in the Midwest,” Mideros said. 

The fungicide controlled many fungi, but not Septoria. It removed Septoria’s competitors, allowing the pathogen to flourish, Mideros suggested. The result calls into question the common practice of yield-protective fungicide application.

“We know – based on previous research – that when we spray a lot of fungicide, such as every week, Septoria symptoms are kept in check and yield increases,” he said. “But that application frequency isn’t feasible for farmers. This study is a closer approximation of what producers actually do, with one to three applications during the season.

“I’m not saying fungicide wouldn’t increase yield in some fields. It might. But what I’m learning from the study is that we don’t know exactly what we’re doing when we apply fungicides to protect yield. We need to learn more about the unintended effects of chemical applications. We could be doing things more effectively if we had a better understanding of all the changes to the systems when we do a fungicide application.”

Although there are still questions whether producers should shelve fungicide when battling Septoria, the study provides a look at how the soybean mycobiome interacts. The researchers identified 3,342 distinct fungi on the three soybean lines they studied. Some were pathogenic and others were beneficial. There were still more whose effects on soybeans haven’t been characterized.

Knowing what fungi are on each soybean line and how they interact could pave the way for future disease-fighting tools, such as biocontrol agents.

“One of the things we were trying to address with the analysis was to see which fungi are associated with each other,” Mideros said. “If we found patterns where one fungus seemed to have a suppressive effect on another, it could be used as a biocontrol agent. We did find some negative associations but not many and, unfortunately, none with Septoria. But there are several organisms that have a negative association with other fungi, so it’s something we could study further.

“There’s a lot of interest in finding more sustainable management practices. It could come in the form of biofungicides or manipulations of the mycobiome that could result in less disease and greater yields. There’s a world of hidden microorganisms associated with crops into which we could tap.”

The study was published in Phytobiomes. Visit apsjournals.apsnet.org and search for “Septoria and fungicide” for more information.