Research Highlights

Understanding Potential Soybean Disease Threats Influences Management Decisions

By Laura Temple

Correctly identifying soybean diseases is the first step in making sound management decisions to protect yield and quality. Many soil-borne pathogens can be present in a field, but they only attack when environmental conditions are favorable. While infection often takes place early in the season, symptoms don’t usually appear until soybeans enter reproductive growth stages. 

“Soybeans have very few ways to communicate that they are unhappy,” says Dr. Alyssa Koehler, Assistant Professor and Extension Specialist in Plant Pathology at the University of Delaware. “That means many disease symptoms look similar, though they may be caused by different pathogens.”

Koehler led a survey to characterize the most common fungal pathogens in the Mid-Atlantic with funding from the Delaware Soybean Board. Her research began in 2019, and it is continuing into 2021, as she identifies threats and develops management recommendations for common disease pressure in soybeans. This research parallels a Mid-Atlantic nematode survey funded by the Maryland Soybean Board and the Atlantic Soybean Council.

“We want to know what primary diseases are out there, and what diseases might be under the radar, leaving yield in the field,” she explains. “Understanding what is most likely to be in fields under various weather conditions allows soybean farmers to make better decisions to optimize yield and quality.”

Koehler and her team monitored soybean fields across Delaware and Maryland in 2019 and 2020. Different weather conditions caused different diseases to flourish each year. Diaporthe species, which cause a variety of diseases, and Macrophomina phaseolina, which causes charcoal rot, appeared most frequently. 

In addition to nematodes, these fungi have become the focus of Koehler’s ongoing soybean research program. Graduate student Lexi Kessler collected and isolated samples of these pathogens to learn how aggressiveness varies among species and how they respond to common fungicide treatments in the lab and greenhouse. This will provide a foundation of practical recommendations for farmers to protect against them.

Diaporthe Mistaken for Other Diseases

Diaporthe species can cause pod and stem blight, stem canker and seed decay in soybeans. They tend to thrive in wet conditions. For example, Diaporthe longicolla was associated with widespread seed quality issues when harvest was delayed in 2018. In 2020, frequent rains again allowed Diaporthe species to appear soybean fields.

Many fields in 2020 had chlorotic leaves that led to accelerated maturity and dry down. These symptoms were commonly thought to be Sudden Death Syndrome (SDS), caused by Fusarium virguliforme, but sampling and isolating from plant tissue revealed that Diaporthe longicolla was the primary fungus.

“We shared a diagnostic chart with farmers to help them identify and distinguish common soybean disease symptoms,” Koehler says. “Leaf symptoms can look similar, but plants with SDS will tend to have blue coloration at the base, while small black dots called pycnidia may be visible in rows on stems with Diaporthe. Both diseases are difficult to treat once symptoms are observed, but management decisions can be made for future crops, depending on accurate identification.”

In addition to D. logicolla, the survey also found Diaporthe species associated with southern stem canker, but no northern stem canker isolates have been found yet. 

“Since Delaware and Maryland are transition zone states, we expect to have both types of stem canker,” she adds. “Our work is helping to understand which species is most abundant in what areas, all to inform management decisions.”

Managing Potential Disease Pressure

Diaporthe species often co-exist with Macrophomina in this region. In 2019, more than 50 percent of the surveyed fields that had Macrophomina present also had one or more types of Diaporthe. In 2020, Macrophomina was less frequent, but over 80 percent of plants with Macrophomina also contained Diaporthe species. 

Though they are present, weather will drive which diseases show up in a given season. 

“Soil-borne pathogens live in the soil, but disease symptoms come on when weather conditions and soybean susceptibility meet,” Koehler says. “Understanding weather patterns that favor different diseases let us know what to scout for.”

In 2019, mid-season conditions were hot and dry in the Mid-Atlantic region, which allowed Macrophomina to develop into charcoal rot. Symptoms usually appear after flowering, including tiny black structures on the surface of taproots, stems and through the pith. Alternatively, Diaporthe prefer wet conditions. 

“We are still at the beginning stages of developing recommendations,” Koehler explains. “We do know that timely harvest is key, so if farmers know they have fields with potential to develop diseases from Diaporthe, they can manage those fields to support early harvest. We also know crop residue can foster the presence of these pathogens, something for farmers to think about when selecting varieties and scouting throughout the year.”

Cory Atkins, a Seaford, Delaware, farmer who serves as a director for the Delaware Soybean Board and the United Soybean Board, values local recommendations. “Although both my independent crop consultant and I scout my crops regularly, we need to know what potential disease pressure to be looking for,” he says. “Based on what diseases are likely to show up in my fields, I can choose soybean varieties with resistance and intentionally manage them to protect yield and quality.”

Published: Jul 12, 2021