The pathogen causing soybean rust, Phakopsora pachyrhizi, was first described in Japan in 1902. The disease was important in the Eastern Hemisphere for many decades before the fungus was reported in Hawaii in 1994, which was followed by reports from countries in Africa and South America. In 2004, P. pachyrhizi was confirmed in Louisiana, making it the first report in the continental United States. Based on yield losses from countries in Asia, Africa, and South America, it was clear that this pathogen could have a major economic impact on the yield of 30 million ha of soybean in the United States. The response by agencies within the United States Department of Agriculture, industry, soybean check-off boards, and universities was immediate and complex. The impacts of some of these activities are detailed in this review. The net result has been that the once dreaded disease, which caused substantial losses in other parts of the world, is now better understood and effectively managed in the United States. The disease continues to be monitored yearly for changes in spatial and temporal distribution so that soybean growers can continue to benefit by knowing where soybean rust is occurring during the growing season.

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Core Ideas Annual application of foliar fungicide in continuous soybean increased yields. Breakeven soybean price for applying a foliar fungicide ranged from US$0.13 to $0.27kg−1. Results suggest a high likelihood that foliar fungicide application is profitable. With the expansion in soybean (Glycine max L.) land area in the Southeast, producers are wondering how to profitably manage the disease frogeye leaf spot (FLS) (Cercospora sojina); however, little is known about the profitability of using foliar fungicide on soybean infected with FLS. We determined the effect of total water applied, growing degree days, and foliar fungicide treatment on FLS severity and yield of soybean in maturity group (MG) III, IV, and V. A two‐stage severity/treatment outcome model was estimated and results were used to calculate the breakeven price of soybean for applying a foliar fungicide by MG. Data came from an 11‐yr soybean fungicide (quinone) experiment in Tennessee. Applying a foliar fungicide to soybean in a high disease pressure location reduced FLS severity and increased yields for each MG. Annual application of foliar fungicide in continuous soybean was found to increase yields by 475 kg ha−1 for MG III, 321 kg ha−1 for MG IV, and 408 kg ha−1 for MG V. The breakeven price of soybean, for applying a foliar fungicide, ranged from US$0.13 to $0.27 kg−1 for all MGs, depending on the application cost. A profit‐maximizing producer would apply a foliar fungicide as part of the annual production practices to manage FLS since the soybean price has historically been above the breakeven price. Results from this study will inform Tennessee and southeastern producers on the profitability of spraying soybean with a foliar fungicide to manage FLS.

Corynespora cassiicola is the causal pathogen of target spot in soybean and cotton grown in the United States. With target spot increasing in importance, fungicides are becoming an important tool for control of this disease. Unfortunately, there are reports of C. cassiicola isolates in other crops being resistant to some fungicide classes. The objective of this study was to identify if resistance to quinone outside inhibitor (QoI) fungicides is present in Tennessee soybean and cotton isolates of C. cassiicola. Four isolates of C. cassiicola were evaluated at a range of doses for the fungicide pyraclostrobin. Isolates were also sequenced to determine if the G143A mutation was present in the cyt b gene. Two isolates previously reported to be resistant to QoIs were also used as positive checks. Two isolates of C. cassiicola from Tennessee soybean were found to have the G143A mutation. EC50 values for the two isolates ranged from 15.7 to 121 μg/ml. As a result of this study, C. cassiicola isolates have exhibited resistance to QoI fungicides in Tennessee soybean.