Update:
In 2019, we conducted replicated field experiments at seven ISU Research and Demonstration Farms across Iowa near Crawfordsville, Nashua, Kanawha, Sutherland, Armstrong, McNay, and Ames. Planting was delayed by about three weeks due to excessive rains in May in many locations. We evaluated 20 fungicide treatments representing different chemistries and fungicide resistance action committee (FRAC) code for their performance for foliar disease management and yield. Fungicide treatments included newly registered fungicides and/or new ways of applying existing fungicides. They were applied at R3 and the rate was based on company recommendations. In addition to statewide fungicide trials, we also established a fungicide resistance management trial at Ames location, where we sprayed fungicide from all fungicide classes to monitor resistance in frogeye and brown leaf spot pathogens. We assessed foliar fungal diseases between R5-R6 soybean growth stages. We collected foliar disease severity ratings on 10 leaves from upper canopy across all locations. Septoria leaf spot was rated for its height in plant canopy in 1-3 scale in addition to the disease severity on top leaves.

An article summarizing the disease and yield data from all seven locations was published in ICM news. In 2018, Septoria brown spot and frogeye leaf spot were two main diseases observed in all locations. Frogeye leaf spot was observed at greater levels in all the locations compared to previous years. This might be due to a combination of Quinone outside inhibitors (QoI) fungicide resistant strains of C. sojina, which were confirmed throughout the state in 2017, coupled with the high rainfall in August. Septoria brown spot was also observed at all locations. Sudden death syndrome was observed at high level in Ames location. Heavy rainfall in September and October challenged timely harvest in many locations. Average yield in untreated plots was 54.9 bu/A. Yield response to fungicide was statistically significant, and the yield response ranged from 0.8 to 7.4 bu/A. Management of foliar diseases and use of foliar fungicides was one of the main topics of discussion at winter meetings. We presented our data from this trial at over 25 meetings with more than 3,000 attendees. Highlights of these talks included managing frogeye leaf spot, selecting the best foliar fungicide, and managing fungicide resistance.

We are summarizing data from 2005 to 2018 using meta-analysis approach to determine the yield response of fungicides on soybean yield in low or no disease conditions. The overall mean yield response for a generic fungicide was about 1.6 bu/A (a 2.6% increase in yield); however, a premix containing a combination of two or more fungicide classes produced greater yield than solo fungicides. Year effect was significant meaning yield benefit was greater in some years than the others. The highest yield responses occurred when soybeans were planted at the optimal time and when total precipitation between planting and the R3 application date was higher than historic averages. These results suggest that while foliar fungicides tend to increase yields, meeting certain site and weather conditions may further boost yield responses. The result was presented in in NCERA and Southern Soybean Disease Workers meeting in Pensacola, FL in March 6-7, 2019. A manuscript writing is in progress and will be submitted soon to Plant disease.

A manuscript entitled “Widespread Occurrence of Quinone Outside Inhibitor Fungicide-Resistant Isolates of C. sojina, Causal Agent of Frogeye Leaf Spot of Soybean, in the United States" has been published in Plant Health progress (Plant Health Prog. 19: 295-302. https://doi.org/10.1094/PHP-04-18-0016-RS). Cercospora sojina strains resistant to QoI fungicides were found for the first time in Iowa in 2017. Extension articles and press release have been published. Result has been presented in several meetings and conferences as well.

In 2018, we also evaluated a few isolates of Septoria glycines that causes brown spot in soybean collected around central research farm Ames for resistance to QoI fungicides. They were found resistant to the QoI fungicides as well. We are working in collaboration with Dr. Carl Bradley's lab at University of Kentucky to test the isolates for fungicide resistance. Result was presented by Dr. Carl Bradley’s postdoc at the Southern Soybean Disease Workers meeting in Pensacola, FL in March 6-7. We will include the result in future talks as well. More isolates of C. sojina, S. glycines will be tested in this fall.

We are working on establishing fungicide resistance management plan for soybean pathogens in Iowa. In addition to the frogeye leaf spot, we are also working on Septoria brown leaf spot (Septoria glycines) isolates for resistant to QoI fungicides. In 2019, we are collecting frogeye infected leaf samples from farmers field in several counties an addition to ISU research farms to develop a county level maps with QOI fungicide resistance. We are also working in collaboration with Dr. Heather Kelly’s lab at University of Tennessee to monitor resistance in other fungicide classes besides QoI.

A graduate student, Joshua Viggers, working on fungicide coverage with Dr. Daren Mueller for the evaluation of fungicide coverage study recently graduated with his master’s degree. He compared comparing two different spraying methods traditional ground sprayer vs ground sprayer with undercover nozzles. Manuscript writing is on progress. Based on preliminary data, no significant differences in disease control and yield between these two spray methods were found but Undercover nozzles had better coverage of middle and lower canopy than the traditional ground sprayer.

We published a manuscript in Plant Disease (Plant Disease 102:140-145) comparing differences in ISU small plots research plots and On-Farm Trials, conducted through Iowa Soybean Association, for yield response to foliar fungicide in Soybean. We utilized data from field experiments evaluating fungicides in previous years (2008-2015).

We evaluate foliar fungicide products in seven locations across the state. This provides farmers annual updates on how effective these products are. The 2018 data were probably the most valuable to date, as this was the first year where we evaluated products with widespread resistance issues (see next paragraph). The effectiveness of fungicide products and the subsequent yield response varied greatly, which is really the first time this has happened since we started evaluating products. The 2018 results were published on the ICM News.

Fungicide resistance is now a huge issue. The two main fungi that foliar fungicides target (pathogens that cause frogeye leaf spot and brown spot) are now resistant to one of the main groups of fungicides (QoI or strobilurin). This matters because there are solo strobilurin fungicides still being used and they do not work at all. More importantly, the strobilurin fungicides are still being used as premixes in almost every product, and their inclusion in these premix products is also not helping farmers at all (in fact it is only hurting them because they have to buy a product that does not work at all and is misleading farmers into thinking they are spraying a premix (doing the right thing)). Our group is documenting what pathogens are becoming resistant to fungicides. We are working closely with other pathology labs across the nation to monitor for resistance in other fungicide classes as well.

We continue to evaluation other ways fungicides may be better used - for example, we have evaluated the importance of fungicide coverage and started looking at using predictive models to time fungicide application.