Abstract
Frogeye leaf spot, caused by Cercospora sojina K. Hara, is a major soybean (Glycine max L. Merr.) disease that has become more prevalent in the upper Midwest and can be managed with foliar fungicides. Incorporating disease severity into a parameter directly related to yield may better relay the impact of disease on yield and yield components than severity alone. Experiments during the 2018 and 2019 growing seasons in fields located in north central and southwestern Iowa were completed to (i) determine how foliar fungicides affected frogeye leaf spot, remotely sensed plant health indicators, and soybean yield, and (ii) compare the relationship and impact of foliar fungicides and frogeye leaf spot on radiation-use efficiency (RUE) estimated using unmanned aerial vehicle reflectance data. Fungicides affected frogeye severity and yield in one of the three locations; in Lewis 2018, the flutriafol + fluoxastrobin treatment reduced frogeye leaf spot severity by over 50% and increased yield by 19% compared to non-treated controls. Applications of foliar fungicides increased canopy coverage compared to non-treated controls (p = 0.012), but NDVI, SPAD values, and RUE values did not differ between fungicide treatments at all three locations. Estimated soybean RUE values (1.05 to 1.66 g Mj−1) were within the range of known values. Overall, this study indicates that RUE can be a valuable resource to estimate the impact of the disease on yield, however, additional research will be needed to use RUE within certain pathosystems.
Highlights
Frogeye leaf spot of soybean, caused by the pathogen Cercospora sojina K
radiation-use efficiency (RUE) is capable of being calculated on a small plot scale with the help of unmanned aerial vehicle (UAV) reflectance data
The foliar fungicides used in this study resulted in increased canopy cover compared to non-treated controls
Summary
Frogeye leaf spot of soybean, caused by the pathogen Cercospora sojina K. Frogeye leaf spot is common in the southern United States due to the favorable environmental conditions and mild winters, but northern states such as Iowa, Ohio, and Wisconsin have seen a rise in prevalence [6,7,8]. Across the United States from 1996 to 2019, frogeye leaf spot reduced soybean yields by approximately 288,000 metric tons, which equated to an annual loss of $98 million [9]. In Iowa, frogeye leaf spot caused losses of over 43,500 metric tons and. In Iowa during 2018, frogeye leaf spot caused an estimated loss of over 360,000 metric tons and over 140,000 metric tons in 2019, which equates to over $119.4 million in 2018 and $46.5 million in 2019 [9]
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