Frogeye leaf spot (FLS) of soybean is caused by Cercospora sojina and typically affects leaves in the upper canopy. Under favorable conditions, FLS can cause up to 35% yield loss [1]. FLS is severe when soybean is grown continuously in the same field, particularly in fields where tillage is reduced, since this is a residue- and seed-borne disease. FLS was first identified in 1924 in the southeastern United States [2] and has now spread past the northern border of Nebraska and was common across the eastern third of the state in 2017[3]. Over the past few years, generic fungicides have become available to producers at a very low cost. Even though there are newer products with multiple modes of action for management, producers are selecting single mode of action products which will push resistance to become widespread
[4]. In 2019, resistance to QoI fungicides was confirmed in 10 isolates of C. sojina from 10 counties in Nebraska (Fig. 1). Fungicide resistance is a serious concern because it threatens the ability to use QoI fungicides, which are some of the most effective and widely used fungicides for managing FLS. The proposed study is aimed at: 1) developing a DNA-based method to provide rapid diagnostics to identify FLS-resistance at no direct cost to growers in the next three years, 2) developing new information on the distribution and spread of fungicide resistant FLS in Nebraska, and 3) develop educational materials and outreach programs on resistance management practices.

[Figure 1: Maps depicting counties known to have QoI-resistant Cercospora sojina, the causal agent of Frogeye Leaf Spot (FLS). (Left): In 2019, Nebraska counties in yellow had one C. sojina isolate test positive for QoI fungicide resistance (Right): From 2010-2017, 14 states have QoIresistant C. sojina isolates, with color-coding by year of discovery [5], with first report of fungicide resistance in Tennessee in 2012.]

1. Develop a DNA-based protocol for rapid identification of QoI resistant C. sojina
2. Solicit FLS samples coming from Nebraska soybean grower fields by contacting crop consultants, commercial agronomists, and Extension educators, and use the DNA-based tool (from Obj. 1) for rapid diagnosis of FLS fungicide resistance at no cost to the grower
3. Perform targeted follow-up sampling of selected soybean fields to be able to characterize the within-field distribution of fungicide resistance
4. Characterize the population structure and genetic diversity of C. sojina populations (from Obj. 2 and 3) from soybean to better understand the epidemiology of resistance in Nebraska
5. Develop/deliver educational materials and outreach programs on resistance management

Timeline and Milestones (Year 3) – The survey will continue in year 3 (summer 2023). Existing genetic markers will be used to determine the genetic relationships between isolates obtained from throughout Nebraska. Significant results obtained will be presented at scientific meetings and shared with local growers during the Extension program. The graduate student will prepare results for publication and included in the dissertation.

Results will be broadly disseminated and reported directly to growers, Extension educators, and crop consultants. The results will be shared during the Extension program, growers' meetings, Nebraska soybean management field days, Crop Production clinics, online and in UNL CropWatch Newsletters. Growers (their crop consultant, agronomist, or local Extension educator) that submit the diseased samples will receive FLS fungicide-resistance diagnosis reports and educational materials on managing fungicide resistance free of cost. The Principal Investigators and the graduate student will deliver presentations at regional and national meetings. Support of a graduate student in this project will accomplish broad training for the student in the area of applied soybean pathology. The student will be expected to publish results from this survey in various media output including crop watch and peer-reviewed publication. The graduate student will work directly with Dr. Tamra Jackson-Ziems to communicate the results/create educational material through educational flyers, magazines, and social media outlets.

In 2019, resistance to QoI fungicides was confirmed in 10 isolates of C. sojina from 10 counties in Nebraska. Fungicide resistance is a serious concern because it threatens the ability to use QoI fungicides, which are some of the most effective and widely used fungicides for managing FLS. A major limitation is that we do not presently have capacity within Nebraska for QoI resistance screening of C. sojina. The proposed study is aimed at: 1) developing a DNA-based method to provide rapid diagnostics to identify FLS-resistance at no direct cost to growers in the next three years, 2) developing new information on the distribution and spread of fungicide resistant FLS in Nebraska, and 3) develop educational materials and outreach programs on resistance management practices.