Project Description:
Frogeye leaf spot (FLS) resistance to strobilurin fungicides has been documented in Southern IL, and many farmers don’t have access to useful information on the distribution of insensitive FLS in Illinois. This work includes a two-year survey for FLS in IL and characterizes the sensitivity profiles of pathogen isolates.
Summary/Project Scope:
Summary of Project
Frogeye leaf spot (FLS) is a potentially yield limiting disease in Illinois, and the most impactful foliar disease encountered in the state. Resistance to QoI (strobilurin) fungicides was documented nearly a decade ago in Southern Illinois. Currently our clientele do not have access to up to date information on the distribution of QoI insensitive FLS in Illinois. In addition, we hypothesize that increased use and exposure to SDHI fungicide active ingredients may be resulting in a buildup of insensitivity within FLS populations, particularly in southern Illinois where the disease is more frequently encountered. Lastly, many producers in the state would like to better understand the relative profitability of managing FLS in the state. In this project we will conduct a two year survey for FLS in Illinois and characterize the sensitivity profiles of isolates recovered to SDHI (group 7) and QoI (group 11) fungicide modes of action. In addition, we will set out multiple small plot trials throughout the state to assess the effectiveness and profitability of variety and fungicide selection for managing FLS in Illinois using established metaanalytical methodology. Data from this project will be used to develop Illinois-specific educational materials on fungicide resistance development and integrated management of FLS, and provide data that could be used to generate profitability tools in the future. It will be the first work we are aware of to assess SDHI sensitivity profiles in C. sojina in the United States. Finally, funds will support the next generation of agricultural researchers, helping to support a student with interest in applied agriculture in pursuing their MSc. degree in crop sciences at UIUC. This is a proposal for year one of a two year project.
Problem Statement
Frogeye leaf spot (FLS), caused by the fungal pathogen Cercospora sojina, is the most yield limiting a foliar disease of soybeans in Illinois. In 2018, an estimated 7,564,944 bushels ($70,955,753 US). The average annual loss due to FLS in Illinois from 2012-2019 is 2,474,909 ($23,953,614 US) [1] . The fungus survives on soybean residue and occasionally can be seed borne. Spores of the fungus are dispersed locally from residue and from within foliar lesions to unaffected soybean foliage and plants. High relative humidity and warm temperatures favor spore production and infection of leaves. Yield losses exceeding 30% can occur following significant reductions in foliar leaf area [2]. In Illinois, FLS historically was only of significance in the southern half of the state. In these areas, repeated use of foliar fungicides in the strobilurin class [quinone outside inhibitor-QoI (Fungicide Resistance Action Committee ‘FRAC’ group 11)] resulted in the development of elevated QoI resistant populations throughout the region. Recently, we have had many conversations with industry, producers, and agronomists about how widespread QoI resistance is within the state, especially in Central and Northern Illinois, where the disease historically has not been yield limiting. The answer is- we don’t know. Currently the only data available is nearly a decade old, and focused on southern regions of the state where the issue started in Illinois
[3-5]. In central and northern Illinois, some producers utilize foliar fungicides containing a QoI mode of action (e.g. Headline, Aproach, Quadris), for “plant health” benefits despite the absence of significant FLS pressure. This practice can inadvertently select more QoI insensitive populations, which results in the loss of efficacy for fungicides with QoI active ingredients over time, and eventually lead to complete failure of the fungicide mode of action to control pathogenic fungal populations.
Once populations are insensitive to a QoI fungicidal active ingredient, they will be cross-resistant to other active ingredients within that chemical group. This means that if a producer applies a QoI to that population, it will not suppress FLS development, and the producer will suffer not only from FLS related losses, but the cost of product and application. A similar issue occurred recently in Iowa, where last season FLS was widespread throughout the state. Where the disease was present at elevated levels, increased numbers of QoI-insensitive individuals occurred. This is a situation we want to avoid in Illinois. A better understanding of the actual distribution of QoI insensitivity issues will help inform soybean producers as to best management practices for effectively managing this disease, including the use of FLS-resistant soybean cultivars and timely applications of fungicides when necessary.
In addition, fungicides belonging to the succinate dehydrogenase inhibitor (SDHI) mode of action are becoming more common in commercial fungicides used in soybean production, and are considered moderate to high risk for development of fungicide insensitivity [6] . Unlike QoI fungicides, industry is only utilizing SDHI active ingredients as part of premix fungicide products containing 2-3 fungicide modes of action. This is due primarily to limited modes of action available for use and in development, as well as a means to extend the lifespan of active ingredients. On first look, this is a way to reduce buildup of fungicide insensitive populations in a typical situation. However, if a fungal population already is insensitive to one of the chemistries in the premix, additional selection pressure will be placed on the remaining active ingredients in the product. This results in the eventual failure of other fungicide active ingredients over time (Figure 1). Such was the case for Didymella bryoniae (gummy stem blight) in cucurbits in Indiana, Georgia, and Florida [7, 8]. In this system, underlying QoI resistance issues were not known, and a premix fungicide containing QoI and SDHI active ingredients was totally ineffective for controlling this disease within three years of its release. Currently the sensitivity profiles of C. sojina to SDHI modes of action in Illinois is unknown. Given that QoI insensitivity has been problematic in southern Illinois for several years now, we should be able to detect insensitivity to SDHI’s if it has developed. SDHI’s are also used in soybean seed treatments (e.g iLeVO), which may impose additional selection pressure on populations of C. sojina in the state. Understanding the potential for SDHI insensitivity to develop in C. sojina will become increasingly important in the future as these active ingredients continue to be released and utilized in soybean production.
Lastly, buildup of insensitive populations of C. sojina can be reduced by utilizing an integrated management plan that includes the use of tillage/rotation, FLS resistant cultivars, and an appropriate fungicide if warranted. A question we often receive is “Do I need to spray a fungicide on my beans” or “What is the likelihood that making a fungicide application for FLS will pay for me?” We can answer these questions through directed, Illinois-centric research conducted across the state, under different environmental conditions and disease pressures. Providing relevant, state-specific measures of the utility and profitability of these practices will help inform and educate the agricultural communities as to how to profitably FLS under Illinois conditions.