Author ORCID Identifier
Date Available
11-29-2022
Year of Publication
2022
Degree Name
Doctor of Philosophy (PhD)
Document Type
Doctoral Dissertation
College
Agriculture, Food and Environment
Department/School/Program
Plant Pathology
First Advisor
Dr. Carl Bradley
Abstract
Frogeye leaf spot (FLS), caused by Cercospora sojina, is an economically important disease of soybean in many parts of the world where soybean is grown, including the United States. A meta-analytic approach was used to summarize a data set of 66 uniform field research trials conducted to evaluate fungicide efficacy against FLS on soybean. The dataset spanned 10 years (2012 to 2021) of experiments conducted across eight states in the U.S., including Alabama, Arkansas, Illinois, Iowa, Kentucky, Louisiana, Mississippi and Tennessee. First, the relationship between FLS severity and soybean yield was investigated. A significant negative slope obtained through random effects meta-analytic models confirmed the negative linear relationship between FLS severity and soybean yield. Additionally, the overall relative damage coefficient was calculated to be 0.51%, indicating that a 1% increase in FLS severity would result in a 0.51% yield reduction. In addition, economic damage thresholds were estimated by using the damage coefficient, for a range of soybean prices and control costs, taking into account three different fungicide efficacies representing low (25%), moderate (50%) and high (75%) levels of disease control. As expected, the threshold values increased as the control efficacy also increased and were affected by different crop prices and fungicide costs. Second, after potential yield losses caused by FLS were identified, the best fungicide options to control the disease were investigated. The results demonstrated that fungicide efficacy against FLS differ among active ingredients and is decreasing over time possibly due to fungicide resistant populations (mainly to the quinone outside inhibitors [QoIs]). The best performing fungicide reported in this study was a mixture of difenoconazole + pydiflumetofen, and the poorest performing fungicide was pyraclostrobin, a QoI fungicide. A statistically significant (P < 0.05) decline in performance was detected for two fungicide mixtures (azoxystrobin + difenoconazole and thiophanate-methyl + tebuconazole) and two single active ingredients (pyraclostrobin and tetraconazole). Greater yields in trials with conditions favorable for severe epidemics were found, which could be explained by the more evident effect of the fungicides among the treated plots when compared to the nontreated control. Accordingly, the most effective treatments were more likely to be profitable under higher disease pressure and, as expected, the less effective treatment reported the higher risk of not offsetting the
costs. Third, the profitability of applying fungicides was investigated in the absence or very low levels of FLS in double-crop soybean by using a different data set of 22
fungicide trials conducted between 2008 and 2021 across five states in the U.S. (Illinois, Indiana, Kentucky, Missouri, and Tennessee). The results showed no significant difference in yield response between the fungicide treatments and the nontreated control. Economic analyses indicated that, due to the lower yield responses, probabilities of breaking even were less than 50% for all the single fungicide classes, or up to 51% for mixtures, depending on fungicide cost and soybean price values. Overall, these research findings may provide useful information for regional risk assessment of potential yield
loss caused by FLS, and for planning fungicide programs to control this important foliar disease. Decisions on fungicide planning must take into account, not only technical information such as control efficacy and yield return, but also profitability and strategies to mitigate fungicide resistance issues, seeking to preserve the lifespan of site-specific fungicides.
Digital Object Identifier (DOI)
https://doi.org/10.13023/etd.2022.383
Recommended Citation
Barro, Jhonatan Paulo, "EPIDEMIOLOGY AND MANAGEMENT OF FROGEYE LEAF SPOT OF SOYBEAN: DAMAGE THRESHOLDS, EFFICACY AND PROFITABILITY OF FOLIAR FUNGICIDES" (2022). Theses and Dissertations--Plant Pathology. 39.
https://uknowledge.uky.edu/plantpath_etds/39
