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Research Highlights

Research Highlights
Developing a Comprehensive Management Program for Foliar Diseases of Soybean (Phase II)

Symptoms of frogeye leaf spot.

By Ahmad M. Fakhoury, Southern Illinois University Carbondale

Each year, U.S. soybean growers lose as much yield to disease as the entire soybean production of China, the world’s fourth largest producer. Foliar diseases cost U.S. soybean producers $700 million/year, and some, such as Cercospora leaf blight (CLB) and frogeye leaf spot (FLS), are increasing in geographical range and frequency. To make matters worse, some of these pathogens are evolving fungicide and genetic resistance faster than new fungicides, and resistant cultivars can be developed. This $373,041 program will develop cost-effective, sustainable management options for major foliar diseases of soybean by gaining a better understanding of the biology of foliar diseases.

Many of the pathogens causing foliar diseases of soybean are historically understudied. For example, CLB was historically thought to be caused exclusively by Cercospora kikuchii. From a collection of > 700 pathogen isolates from 10 soybean-producing states, it was determined that C. flagellaris is actually the predominant pathogen associated with CLB in the U.S., followed by C. sigesbeckiae. C. kikuchii has not been found in the U.S.

An association mapping approach was used to identify regions of the C. sojina genome that correlated with FLS, leading to the discovery of two regions of the C. sojina genome that are strongly associated with different races of the pathogen. 

Figure 1. Students from SIU preparing to apply fungicides – Carmi, IL.

Most commercial soybean varieties are susceptible to FLS, and three genes that condition resistance are known, but less than 5% of commercial varieties contain them. Five naturally occurring strains of C. sojina that infected soybean cultivar Davis (the original source of Rcs3) at consistent, albeit low, levels were identified. Advanced breeding lines were evaluated for resistance to C. sojina, and five with high crop potential will be introduced to uniform soybean tests in 2020, and advanced as breeding material.

Toward other goals, strobilurin fungicide-resistant strains of C. sojina were confirmed in 250 counties/parishes in 15 states from 2010 to 2018. These findings helped soybean farmers make better-informed decisions about foliar fungicide products to best control FLS in their area, increasing awareness and showing the benefit of integrated disease management.

First reported in 2008, soybean vein necrosis virus (SVNV) has since been reported in all soybean-producing areas of the U.S. SVNV is transmitted by soybean thrips, which are difficult to control. Little is known about the yield losses or effective management. Both field and growth chamber studies have been used to evaluate the effects of the virus and thrips on plant productivity and to evaluate management approaches using host plant resistance. Data indicates that while thrips carrying SVNV can obliterate soybean plants, soybean genotypes differ and exhibit various levels of resistance. 

Figure 2. Mirian F. Pimentel and Leonardo F. Rocha, PhD students at SIU,
rating soybean – Carmi, IL.

This work will provide both immediate-term benefits and long-term solutions to minimize the risks of soybean foliar pathogens, including CLB, FLS and Septoria brown spot pathogen. A combination of new disease-resistant cultivars and management practices will benefit growers, and ultimately the entire soybean community. 

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.