The trifecta is almost complete for soybean diseases to succeed this summer. Every year hosts and pathogens, two of the three points of the disease triangle (pictured below) are readily available in Iowa. This year the environment will make the triangle complete. According to an Iowa Department of Natural Resources press release, June 2014 was the fourth wettest month ever in 141 years in Iowa. The excess rainfall will make soybean fields and cornfields more susceptible to disease this summer.
After a crop is planted there is little to combat diseases except fungicides. The last few years farmers who applied fungicides hadn't seen large yield responses because dry conditions during the season were not conducive for disease development in the crops. Due to wetter-than-normal weather setting up a better year for diseases, fungicides are expected to have larger yield response this year compared to the last couple of years.
On-Network trials show likelihood of positive return
"Data from the On-Farm Network trials have shown that growing seasons with more rainfall are more likely to have a positive return on investment with a fungicide application than when conditions are drier," says Tristan Mueller, Iowa Soybean Association (ISA) On-Farm Network operations manager for agronomic research.
Several soybean diseases are expected to be particularly prominent this year: white mold, frogeye leaf spot, septoria brown spot and cercospora leaf blight.
Fungicides applied at the R1growth stage (beginning flowering) can give some control of white mold, and depending on the chemistry may give some control later in the season for other diseases. All the previously mentioned diseases except white mold can be managed with a full rate fungicide application around the R3 (beginning pod) growth stage.
For more information on what to look for and how to treat white mold specifically, go to www.iasoybeans.com for an article written by ISA and Iowa State University last week.
Several soybean diseases to watch for this summer
While scouting fields, farmers can look for specific things for each disease. With the frogeye leaf spot, round gray spots with dark red-brown borders will appear on the upper leaves. The best time to scout for frogeye leaf spot is between stages R3-R6 (full seed) and after numerous rains.
Septoria brown spot is a fairly common disease that is found in most Iowa fields. It can be identified by small irregular shaped brown spots developing in the lower canopy. The infected leaves will quickly yellow and drop off the plant. Scouting should occur throughout the season.
Cercospora leaf blight symptoms first appear in the upper canopy as purple spots and gives leaves a leathery appearance. Like with frogeye, scouting is most beneficial in R3-R6 stages. "Scouting for diseases is also important for future applications, says Mueller. "Fields that are susceptible to disease this year are likely to repeat that vulnerability in years to come. Knowing what to expect is a way to get ahead of the disease and improve yields."
Use fungicide calculator to make management decisions
Farmers looking to see if fungicide would be beneficial for their operation can use the fungicide calculator developed by ISA On-Farm Network operations manager Peter Kyveryga. Kyveryga notes that while the calculator doesn't provide disease forecasts or recommendations, it is an example of a decision support system that should help farmers make better data-driven management decisions.
The calculator, when dealing with fungicide applications, uses information from approximately 300 on-farm soybean trials across Iowa. The tool is meant to help farmers estimate potential payback from using fungicide based on real-life data. Users input current price of beans, fungicide application costs and choose a rainfall option to calculate a beak-even yield and expected profit-per-acre.
"The unique value of the calculator is that it is based on data collected by farmers using their common equipment across a range of management practices," says Kyveryga. "The estimated probability values allow farmers to quantify the potential economic benefits or losses from these applications by also considering site-specific rainfall, which helps soybean disease develop during the growing season."
Fungicide application is likely to pay off this year
Even with recent soybean prices, fungicide application is likely to be a key to yield success this season. Application costs vary from $15 to $30 an acre depending on application method and the particular fungicide product you apply, but the costs will be well worth it come harvest. "A good way to see if it was worth applying the fungicide is to leave some untreated strips in your field and submit the date to the On-Farm Network for a replicated strip trial," says Mueller. "Fungicide product is still available for R1-R3 application on soybeans and VT-R2 applications on corn."
For more information on fungicide trail results, go to www.isafarmnet.com, visit the On-Farm Network trial database or call 800-383-1423.