Research Highlights

Improving Tools for Soybean Breeders to Fight White Mold

By Carol Brown

Minnesota farmers face several soybean foes including white mold, which has been an issue in the state since the 1990s. It has since become widespread across the north central region. Megan McCaghey, a plant pathologist at the University of Minnesota, is working to improve methods that can reduce the severity of this yield-robbing disease.

Through funding from the Minnesota Soybean Research and Promotion Council, McCaghey leads a two-fold project: one aspect aims at improving tools that breeders use to screen soybeans for resistance to Sclerotinia sclerotiorum, the fungus that leads to sclerotinia stem rot, another term for white mold. Another objective looks at how soybean plant architecture affects the fungal growth in the soil and white mold development. 

“In Minnesota, Sclerotinia sclerotiorum isolates have differing abilities to infect the soybean plant,” explains McCaghey. “Some are really good at infecting and killing the plant. Others are not as good; they may cause lesions on the soybean, but it doesn’t usually result in plant death. Depending on which Sclerotinia sclerotiorum isolates that breeders use to screen lines, soybeans will be affected with either stronger or weaker resistance.” 

She and her research team are developing an isolate panel for breeders to use when screening soybean varieties. When tested, the different isolates will cause a representative response so breeders have a better idea of how soybean varieties will react in the field toward the pathogen. 

To study the different Sclerotinia sclerotiorum isolates and their levels of aggressiveness, McCaghey needed to gather many samples. She collected white mold samples at the Crookston research farm and St. Paul research plots, and from other faculty in the plant pathology department. McCaghey also asked growers across the state for samples and received several. To date, she has gathered nearly 30 isolates and is happy to add more to the collection. Farmers are welcome to submit white mold samples to her.

In a separate project funded by the USDA National Sclerotinia Initiative, McCaghey is looking at the genetic determinants of aggressiveness to understand what influences the level of damage Sclerotinia sclerotiorum inflicts on the soybean and other susceptible plant species. This could lead to biotechnological approaches for disease control, she says.

Soybean Architecture Connections

The soybean plant’s shape can impact the formation of apothecia, the structure that holds the Sclerotinia spores. The apothecia release spores that settle on the plant to cause the disease, says McCaghey, and they are sensitive to specific light and moisture conditions. The shape of the soybean plant impacts how much and what type of light reaches the soil surface, which affects whether the apothecia form. 

McCaghey and UMN colleague Aaron Lorenz, associate professor in soybean genetics, are looking at soybean characteristics including branch angles, leaf internode slope, leaf size and shape. All of these physical, or phenotypic, traits can affect how soon the plants reach canopy closure. They tested more than 150 soybean lines at two locations to learn when canopies begin to close with each variety. McCaghey says white mold can develop starting at around 50% canopy closure. 

“We also looked at light penetration in this study, as apothecia need a specific intensity of UVB light to form,” she says. “We used a UVB light meter to measure light penetration in the field at each location. We collected the data and scouted for apothecia over the season. But due to drought conditions at one test site, we had less growth than we had hoped.”

This coming crop season, she and her team plan to reduce the study to 20 soybean lines with different architecture traits, which will create a variance of light penetration levels on the ground. They will evaluate these lines for disease response and continue to look for apothecia development. 

“It would be good if we could adjust the environment so less white mold would develop without reducing soybean plant population,” McCaghey says. “If we could combine plant architecture traits with genetic resistance, we could improve disease avoidance.”

Published: Jul 17, 2023