Update:
Cercospora sojina strains resistant to QoI fungicides were found for the first time in Iowa in 2017. We worked in collaboration with Dr. Carl Bradley's lab at University of Kentucky to test the isolates for fungicide resistance. Extension articles and press release have been published. Result was presented in Farmer’s meetings and conferences. 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" that was submitted with Dr. Bradley and other collaborators in Plant Health Progress has recently been accepted for publication. We will include the result in future talks as well.
In summer 2018, we established replicated field experiments at seven ISU Research and Demonstration Farms across Iowa near Crawfordsville, Nashua, Kanawha, Sutherland, Armstrong, McNay, and Ames. 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. An untreated control was included in all the locations to compare. Plots were 4 - 6 rows wide at 30-inch spacing and 25 to 40 ft long.
We assessed all 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 addition to the disease severity on top leaves. Septoria brown spot and frogeye leaf spot were two main diseases observed in all locations. Frogeye leaf spot was more severe compared to previous years. Sudden death syndrome was observed at high level in Ames location. We are entering and analyzing disease data.
We have collected leaf samples from fungicide treated and untreated plots with frogeye lesion from 5 of the 7 locations to isolate C. sojina. Isolation is on progress. We will test those isolates for resistant to QoI fungicides. We are working on establishing fungicide resistance management plan for soybean pathogens in Iowa. We are also working with the Plant and Insect Diagnostic Clinic to do routine fungicide resistance screening. The clinic re-ran the test on some of the isolates that were found resistance in previous year and produced the same result. In addition to the frogeye leaf spot, we are also testing Septoria brown leaf spot (Septoria glycines) isolates for resistant to QoI fungicides. In 2018, we collected and sent brown leaf spot infected leaf samples to Dr. Carl Bradley's lab at University of Kentucky to test them for fungicide resistance.
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).
Soybean diseases, mainly Septorial brown spot, frogeye leaf spot, and Cercospora leaf blight were observed across the locations in all years. Overall, Septoria brown spot was the most frequent disease observed in all locations and years. In 2018 field season, frogeye leaf spot was observed at relatively high levels in all the locations. Other diseases were at very low levels (trace amounts). We measured height of brown spot disease in plant canopy, it moved to top leaves in most cases in UTC in 2016, perhaps due to the rainfall during the reproductive growth stages favoring the upward movement of Septoria. Even though it was up to the top level, the severity was not very high to cause yield loss. Septoria brown spot (SBS) progression in the canopy (height) was lower than the untreated control for several fungicides across many locations indicating fungicide spray reduced the SBS progress. Other diseases were not affected by fungicide application. In 2017 and 2018, Septoria was observed in the middle canopy in most cases.
Yield response to fungicide treatments was very minimal and inconsistent. In 2015, responses were mixed; there were both negative and positive responses to various treatments at some locations, but nothing consistent was observed over the seven locations. In 2016, the average yield response to fungicide treatments were positive in all the locations ranged from 0.29 bushels/acre to 4.4 bushels/a across the locations, however the difference was not statistically different at P = 0.10. In 2017, treatment effect was significant only in Kanawha were priaxor produced greater yield than untreated control. Although yield increases can occur sometime with foliar fungicide and/or insecticide treatments, current market prices and application costs may limit profitability when disease and/or insect pressure is low. For the most part, fungicides had minimal effect on moisture.
Soybean yield response variability to foliar fungicide applications was compared between ISU small plot research trials (SPT) and ISA on-farm replicated strip trials (OFT) from 2008 through 2015 in Iowa. A total of 49 SPT and 230 OFT were compared for yield response to Headline, Priaxor, or Stratego YLD. On-farm trials were harvested with farmers’ combines equipped with yield monitors and GPS, while SPT were harvested by small research plot combines. While average yield responses were similar, the residual random yield variation was smaller in OFT than SPT. Power analysis showed that SPT need more replications than OFT to detect same overall treatment differences. To detect a yield response of 134 kg/ha, it would require at least 3 treatment replications with 12 locations in OFT and 7 replications with 12 locations in SPT. Researchers need to acknowledge the differences in statistical power of detecting yield responses to foliar fungicide on soybean in different types of field experiments, especially with smaller plot sizes in situations with less foliar diseases.
Priaxor® (4 oz/A) were applied at R1, R3, R5, 400 hours or 700 hours of RH > 85% beginning July 1 in 2015 and 2016 to determine how weather and application time relates to foliar fungicide response. Although the R1 spray showed the greatest reduction of foliar diseases, yield response to the fungicide was better at R3 spray than any other timings. Yield for the R3.5 and R4 treatments were about 7 bushels/A greater than the untreated control in 2015. There was no clear consistent correlation between disease control and wetness hours. In 2016, no difference was observed among the spray timings for yield.
Green stem disorder was evaluated before the harvest during 2014 and 2015. Higher levels of GSD were observed in 2015 across most locations. Results showed that certain quinone outside inhibitors (QoI)-containing fungicides may result in more GSD. There was higher GSD incidence in southern Iowa locations than other parts of the state. Fungicides slightly increased GSD compared to untreated control. At the Sutherland location in 2015, the Quilt Xcel fungicide had higher levels of GSD compared to the UTC. Soybean farmers should be aware that prophylactic applications of foliar fungicides might increase GSD incidence, especially in cases that plants experience a sink-limiting stress.
Establishment of fungicide resistant plan in Iowa is ongoing in coordinating with Plant and Insect Diagnostic Clinic. In all three years, we isolated several Cercospora sojina isolates from leaves with frogeye leaf symptoms from all trials across Iowa treated with a strobilurin fungicide to screen for fungicide resistance. Cercospora sojina strains resistant to QoI fungicides were found for the first time in Iowa in 2017. We worked in collaboration with Dr. Carl Bradley's lab at University of Kentucky to test the isolates for fungicide resistance. In 2018, we sent leaves with brown spot to Dr. Bradley’s lab. These are currently being evaluated for resistance to QoI fungicides. The protocol for screening isolates for resistance more quickly has been worked out for future years.