In recent years, corn and soybean diseases typically observed further south have made their way into Minnesota. “Tar spot is one of these emerging corn diseases in Minnesota,” states Dr. Dean Malvick, University of Minnesota Extension plant pathologist. It was first reported in north central Illinois and Indiana in 2015 and confirmed in southeastern Minnesota in 2019. While it has been detected at low levels in additional Minnesota counties since then, the highest disease levels have remained concentrated in the southeastern corner of the state.
Until recently, researchers thought the disease required a lot of moisture and cool conditions to develop. A recent study in Nature found that cool temperatures were the most important factor and that it may not need as much moisture as originally thought. In fact, extended periods of high humidity, especially with high temperatures, actually seemed to hinder disease progression. Tar spot management includes avoiding the most susceptible hybrids, scouting fields in mid-July to assess risk and applying fungicides if needed.
Southern rust is another emerging corn disease in Minnesota, although it has not been a concern during the recent drought. “Growers should keep an eye out for it late in the season because it has the potential to become a bigger problem,” says Malvick.
Prior to 2017, frogeye leaf spot (FLS) in soybean was uncommon in Minnesota. Since then, it has spread and reached levels of concern in some fields in non-drought areas. In the southern half of the US, frogeye leaf spot has caused yield losses up to 30 percent.
FLS can be managed with crop rotation, tillage and fungicides. However, managing FLS with a fungicide is complicated due to its resistance to QoI (strobilurin) fungicides, so non-QoI fungicides, such as a triazole or SDHI type, need to be selected instead. If a fungicide mixture contains the QoI fungicide, non-QoI fungicides need to be in high enough concentrations in the mixture to be effective.
If we continue to have periods of drought and heat in the summers, charcoal rot of soybean is a disease we may see more of, according to Malvick. It’s common across the U.S. and while we know it’s present in Minnesota, we don’t know how widespread it is.
Charcoal rot is favored by hot (>85F), dry soils. It has a very wide host range and includes soybean, corn, alfalfa, sunflower and dry edible beans. Premature leaf yellowing symptoms usually begin in the driest parts of the field and small black specks can be found in the lower stem or roots. Crop rotation with small grains may be helpful and some soybean varieties have partial resistance.
If you see plants that you suspect of having charcoal rot, Malvick would like to hear from you (dmalvick@umn.edu). He welcomes plants or soil from fields infested with charcoal rot or Phytopthora for analysis and there’s no charge.
Every year is different, and we never quite know what we’re going to find or what’s going to develop. That’s where Digital Crop Doc comes in (https://z.umn.edu/DigitalCrop-Doc). When disease is developing in the field and you’re wondering what it might be or you’re looking for confirmation, submit photos to the online form and one of our experts will contact you with a diagnosis. If we can’t give a definitive diagnosis, we might request more information or suggest that samples be submitted to the plant disease clinic.
This service is free and open for corn, soybean, small grains, sugarbeet and forage submissions.
For more information on crop diseases and fungicide efficacy, visit the Crop Protection Network (https://cropprotectionnetwork. org). For more information from University of Minnesota Extension, visit extension.umn.edu/ crop-production.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
