Foliar soybean diseases reduce yield by an estimated 70 million bushels in the US. This project mainly targets the pathogens Cercospora sojina (frogeye leaf spot; FLS), Cercospora flagellaris (Cercospora leaf blight; CLB), Septoria glycines (Septoria brown spot; SBS), and soybean viruses (e.g., Soybean Vein Ncrosis Virus; SVNV). C. sojina and C. flagellaris have many types. Defining the genetic basis of variability in the populations of these pathogens is critical for breeders to identify cultivar resistance. Our findings reveal that C. flagellaris dominates as the primary cause of CLB in the US, and that C. flagellaris has four dominant genomic lineages that span the US soybean production regions.
FLS remains one of the main targets for foliar fungicides. As fungicide use increases, so does the potential for pathogen resistance. Resistance to strobilurin fungicides has been identified in over 20 states now. Additional focus on sensitivity to other fungicide classes, such as triazoles and SDHIs, is essential since these are the primary chemicals currently providing efficacy. A multi-state field study was conducted, and the efficacy of multiple fungicide combinations against FLS was tested.
Furthermore, there is a need to monitor soybean cultivars and fungicides to watch for resistance development and determine the best management options. Fungal isolates (C. sojina, C. flagellaris, S. glycines, and Corynespora cassiicola – Target Spot) were collected from soybean production fields from different states and their sensitivity to different classes of fungicides was tested. Results from this research were shared through publications, talks, blog articles, and soybean field days.
One of our objectives was to identify sources of resistance and develop resistant varieties and elite germplasm for C. sojina. One hundred twenty-seven lines from two elite populations were screened in greenhouse and field experiments for resistance to C. sojina. We identified two resistant lines and four moderately resistant lines with protein content > 43% and oil content > 17% that will be used for breeding purposes.
Soybean viruses have been reported in all soybean-producing areas of the US. Effective strategies are needed to manage these viruses. The primary activities related to soybean viruses undertaken in this project included monitoring the occurrence of flower and soybean thrips, both vectors of SVNV, in samples collected from suction traps from the suction trap network (https://suctiontrapnetwork.org/data/). We noted that those thrips are widely distributed and that their populations peaked in May, June, and July for flower thrips and August and September for soybean thrips.
Also, in this project, 356 soybean plant introductions were screened; soybean lines with resistance to SVNV-infected thrips and SVNV-infection were found, although the vast majority of soybean plant introductions and cultivars screened were resistant to both thrips and virus SVNV.