As soybean farmers set their sights on skyrocketing market prices, it’s important to keep one eye on the budget. Before going all-in on cropping bells and whistles, keep overall crop goals and just as importantly, the budget, in mind.
“When we’re trying to make decisions about whether or not to apply a fungicide, really the main thing we are thinking about is what we are trying to achieve,” said University of Missouri Extension plant pathologist Kaitlyn Bissonnette. “My main goal would be to try and manage the disease, but I’m biased from a plant pathology perspective. The other thing most people would keep in mind is whether the field has a history with a particular disease that develops some sort of risk for the crop.”
During the Stateline Crops Conference, a cooperative effort between Kansas State University and MU Extension, Bissonette released soybean fungicide efficacy trial data from the last three years and pointed out key points for area producers.
Three on Three
For Bissonnette, the three major diseases that affect soybean plants appeared over the course of their trials are also three of the most important diseases to scout for.
Target spot, frog eye leaf spot and septoria brown spot all appeared in Bissonnette’s Missouri trial plots, with septoria appearing consistently in 2018, 2019 and 2020.
Target spot, frogeye leaf spot and septoria brown spot can all be treated with fungicide applications, of which there are three different groups: demethylation inhibitors, succinate dehydrogenase inhibitors and quinone outside inhibitors.
There are a wide variety of different classes in those specific fungicide groups. Bissonnette said most producers will use more than one mode of action in fungicide applications to limit susceptibility to fungicide resistance.
“When we talk about managing fungicide resistance, we also have to talk about managing and rotating these different resistance sources and using multiple modes of action,” Bissonette said.
Regional Extension and Producer Trials
In 2018 and 2019 at Bradford Research Farm and in 2020 at Bradford Research Farm and Greenley Research Farm, Bissonette conducted a series of research trials on the efficacy of all classes of soybean fungicides.
“We looked at soybeans that had a moderate level of susceptibility to frogeye leafspot which is the primary disease that you’re looking to control with a foliar fungicide application,” Bissonnette said. “We had four to six replications of small plots that were 27 feet long and we rated them for disease at multiple times in the season over three years.”
Over the course of the years, the plots in the trials were exposed to a wide range of possible weather conditions, from high rainfall events to drought conditions.
“We found that 2018 was a drought year, so we had very low disease levels regardless of whether we did or did not treat the plots with fungicide, which was not unexpected,” Bissonnette said. “There were very low levels of disease and conditions were not right.”
In 2019 and 2020, conditions were a lot better for disease development, Bissonette said. Conditions were especially ripe for fungal disease in 2020 and the plots showed a higher level of disease in the non-treated control plots compared to the fungicide-treated plots in both 2019 and 2020.
“Overall, you can see that there is a lower level of disease in this trial if you sprayed a fungicide as compared to if you had not sprayed a fungicide, which is
what you’d expect,” Bissonnette said. “However, there was no significant difference in yield as compared to the non-treated control plots.”
It’s important to note the data in Bissonnette’s study is for septoria brownspot alone, as it was the only disease consistently present over the multi-year study.
When Bissonnette compared her data with a similar study on soybeans from 2005 through 2018, more positive results for farms that invested in fungicide applications over time appeared.
“When we sort all of the data of all fungicides across time, the data shows a net positive yield response to fungicide over time,” Bissonnette said. “What happens is the use of a fungicide is a positive in most scenarios from 2005 through 2018.”
In combination with the data from research farm trials and the multi-year outside data, Bissonette studied the same fungicide efficacy trials in on-farm scenarios through MU’s strip trial program. Fourteen farms across the region participated to test the way farmers are actually applying fungicides outside of clinical researcher applications.
The on-farm trials also showed notable positive influence of fungicide use for soybeans, especially when applied at the R3 growth stage.
“Most likely fungicides are increasing yield at the R3 timing, though that benefit was slight at just over 1 bushel per acre and less than 2 bushels per acre,” Bissonnette said. “More than half of the 14 farms in the strip trials had a positive response to applying a fungicide.”
Overall, the studies concluded that the difference in applied fungicide, versus no fungicide is relatively low.
When researchers looked at the probability of offsetting fungicide cost based on market price and yield difference, market price was the main determining factor on the feasibility of applying a fungicide and capturing the added value they provide.
“The lower the cost of the soybeans, the less likely the program will pay as the cost of fungicide application gets higher,” Bissonnette said.
Three modes of action or two modes of action cost more, Bissonnette said, but they offer less opportunity for resistance. In situations were producers are using multiple modes of action, the costs associated are less about application and more tied to the herbicide itself.
“It becomes more and more difficult to justify those costs because the return needs to be higher,” Bissonnette said.
With soybean prices high, 2021 might be an ideal year to investigate whether or not soybean fungicide applications can be viable on-farm. Bissonnette reminded producers that her research is related to only one disease treated by a foliar fungicide application and that septoria brown spot rarely causes significant yield impacts in Missouri.
“Part of this, if we think about the actual biology of the disease of septoria is that it is a lower canopy disease so using a fungicide at the R3 timing and getting full canopy penetration to be able to mitigate disease effectively is difficult,” Bissonnette said. “Another thing is we often don’t see yield loss as a result of septoria in Missouri, so getting effective control of this disease on the one hand can be a benefit from an aesthetic point of view but we might not be seeing yield responses.”