“When is the best time to spray fungicides?” is a question that is central to managing many plant diseases. A good answer, however, is rarely simple and several factors, from the economics of fungicide applications to the biology of the pathogen, need to be considered. For the past four years, a group of researchers has been tracking the spores of the pathogens causing Cercospora leaf blight (CLB) of soybean to get a better answer to this question and provide stakeholders with the information to efficiently manage the disease with fungicide applications.

Cercospora leaf blight is the focus of several research projects within the LSU AgCenter because it has been the main foliar disease of soybean in Louisiana and across the Midsouth for a few decades. Leaf symptoms of CLB are frequently observed in the field when the crop reaches pod-fill stages, and symptoms can be diverse ranging from bronzing/purpling to blight on leaves and consequent defoliation. Purple lesions can also be observed on seeds and petioles.

The economic impact of CLB on soybean growers is significant. Over the past three years, losses due to CLB in Louisiana alone were estimated to have been $34 million. For this reason, our research that aims to translate the improved understanding of CLB epidemiology into more effective disease management tools is supported by the Louisiana Soybean and Grain Research and Promotion Board and the U.S. Department of Agriculture.

Researchers’ fundamental understanding of the epidemiology of CLB has changed significantly in recent years. For a long time, CLB was thought to be caused by the fungus Cercospora kikuchii, with seeds thought to be the main source of the pathogen inoculum. It is now known that two other species of this fungus, Cercospora cf. flagellaris and Cercospora cf. sigesbeckiae, are the predominant pathogens causing this disease in Louisiana, and C. kikuchii is rarely found.

This finding changed the approach of disease management because unlike C. kikuchii, which was known to infect only soybean, the newly identified species could infect several plants present around soybean fields. In experiments where pathogen-free soybean seeds were planted in the field, the soybean plants continued to develop leaf blight symptoms during the season. Taken together, these findings suggested a new source of the pathogen, the air, and a new opportunity to learn about aspects of pathogen biology that would provide avenues to control.

To determine whether airborne spores may indeed play a role in CLB onset, we have been collecting spores of the pathogens from the air to determine when it is most prevalent (spore peak). Several types of spore traps (Figure 1) have been placed around research fields at Winnsboro, Baton Rouge and Alexandria since 2019. The spores on the traps were quantified using newly developed analyses to detect the different pathogen species causing CLB.

CLB symptoms are usually observed on soybeans in late August and early September after the beginning seed (R5) growth stage. However, by quantifying the spores of the CLB pathogens present in spore traps throughout the growing season, we have been seeing one to two spore peaks in June through August in most locations/years, prior to symptoms appearing on soybean.

In 2021, we began experiments to target these spore peaks with fungicide applications. Only one fungicide, Revytek from BASF, was tested, and the main question was whether spore peaks could serve as “susceptibility windows” for CLB management. In other words, we explored if targeting spore peaks with fungicide applications could be more effective than the current recommendation of applying fungicides when the crop reaches beginning pod (R3) and/or beginning seed (R5) growth stages.

Although there were no statistically significant differences in yield, two spore peak-based applications were numerically more effective than two growth stage-based applications in Baton Rouge and Winnsboro (Figure 2). Analysis of spore traps has since shown there is considerable variability in spore peaks by location and year and more research to optimize fungicide applications is necessary.

The yield data was also subjected to economic analysis, which indicated that applying fungicides was certainly better than not applying any fungicide (untreated check). Within the different timing of fungicide applications, spore-peak-based applications, specifically treating the second spore peak, provided the greatest protection against the risk of reduced profits due to CLB.

Fungicide trials and spore trapping were conducted again in 2022 (data analysis is ongoing) and will be continued in 2023. The question remains, however, whether spore peaks identified at one location or year can be extrapolated to other locations and years to improve fungicide targeting. We are now working to identify weather patterns that correlate with spore release to allow for such extrapolation. No matter how complicated the pathogen biology, we hope to provide a simple answer to producers wanting to know when the best time is to spray.

For now, the best time to spray for CLB continues to be beginning pod (R3) and/or beginning seed (R5) growth stages. However, through spore trapping efforts in Louisiana and across the Midsouth, we continue to search for better windows to target fungicide applications to get a greater bang for the buck.

Sara Thomas-Sharma is an assistant professor, and Vinson P. Doyle is an associate professor, both in the LSU AgCenter Department of Plant Pathology and Crop Physiology. Nelomie Galagedara is a doctoral student in the department. Trey Price is an associate professor at the AgCenter Macon Ridge Research Station. Boyd Padgett is a professor based at the AgCenter Dean Lee Research and Extension Center. Lawson Connor is an assistant professor in the Department of Agricultural Economics and Agribusiness, University of Arkansas. Rajan Dhakal is a doctoral candidate in the LSU Department of Agricultural Economics and Agribusiness. Tri Setiyono is an assistant professor in the LSU AgCenter School of Plant, Environmental, and Soil Sciences.

This article appears in the summer 2023 edition of Louisiana Agriculture magazine.

Figure 1: Spore traps placed around soybean fields in Louisiana include the Burkard spore trap, wind vane spore trap and garden motor spore trap. These are used to monitor peaks of spores of the pathogens causing Cercospora leaf blight of soybean. Photos by Sara Thomas-Sharma

Figure 2: Mean yield (solid triangle within each box) in 2021 trials with Revytek fungicide was numerically higher in most treatments compared to untreated checks (pink box). Two applications to target spore peaks (purple box) gave higher mean yields than two applications at R3 and R5 growth stages (green box), at Baton Rouge and Winnsboro.