A Look Back at Soybean and Corn Diseases in Minnesota in 2018

Dean Malvick, Extension plant pathologist

White mold in soybean. 

This article is a recap of some of the most common and problematic diseases of soybean and corn in Minnesota in 2018. Looking back at diseases that affected crop growth and yield can help explain why yields were not as expected in some areas and help prepare for next year. This reflects a broad overview, although much variation occurred across Minnesota fields based on weather, variety, cropping practices, soil type, and geography.

Soybean diseases

Pod and stem blight

Pod and stem blight was a problem in many fields, especially in central and southwestern Minnesota. This disease killed all or parts of plants and caused top dieback. The fungal pathogen that causes this disease is widespread in Minnesota. Frequent rainy weather was favorable for this disease, although other diseases and plant stress may have contributed. Soybean varieties vary in susceptibility to pod and stem blight.

Sudden death syndrome

Sudden death syndrome (SDS) was common in some areas, especially in western and southern counties. Heavy and frequent rains were favorable for this disease, and it was found in areas where it had not been previously reported. Our field research studies in Minnesota, as well as studies in other Midwestern states, continue to show the efficacy of using resistant soybean varieties and selected seed treatments for managing SDS.

Rhizoctonia root and stem rot

Rhizoctonia root and stem rot caused significant stand loss and led to replanting in some soybean fields, especially in southern Minnesota. This disease occurs most often when soils are warm and moist for the first 2 - 3 weeks after planting. We saw this pattern in some areas where planting was delayed due to frequent rain and soil was warm and wet when plants were emerging in late May and early June. Successful diagnosis of this and other seedling diseases requires that plants be sampled soon after they start to show symptoms, as the plants can die and decompose rapidly to make diagnosis impossible. Various seed treatments can reduce Rhizoctonia damage to soybeans, and  recent research suggests that soybean varieties can vary in susceptibility.

White mold

White mold developed at low levels and in patches in many fields across southern and central Minnesota. Thus, the situation was very different than it was in 2017 when white mold was a major problem in many areas. This disease is develops primarily when weather is cool and wet during flowering periods. This year many areas of the state had sufficient rain, but the warm temperatures in July and August likely suppressed white mold. While we don’t know if white mold will be a problem in 2019, consider planting the most resistant variety available and using low seeding rates in fields with a history of white mold. Timely fungicide applications and soil-applied biological treatments also can be considered for managing this disease in high risk fields.

Frogeye leaf spot

Frogeye leaf spot deserves a special note because it is a relatively new problem in Minnesota. It has been uncommon in previous years, but this year was reported at levels of concern in southwest Minnesota. Frogeye leaf spot has long been a significant yield-reducing disease the southern half of the U.S, but it may be becoming more common in northern states, perhaps in association with unusually wet and warm summer weather. While we do not know how often frogeye leaf spot will be a problem in Minnesota in future years, it should be watched for. Fungicides can be used to manage frogeye leaf spot, but it is important to keep in mind that resistance to the QoI (strobilurin) group of fungicides has been widely reported for the fungus that causes this disease.

Other diseases

Several other diseases of soybean that were reported in fields scattered across Minnesota include Phytophthora root and stem rot, stem canker, green stem syndrome, Septoria brown spot, bacterial blight, anthracnose, and brown stem rot (BSR).

Links to more information

Photographs and basic information for these and other soybean diseases can be found in the new Soybean Disease Diagnostic Card Set (PDF) and the U of Minnesota soybean diseases web site. The Soybean Research & Information Initiative web site is an excellent source for information on soybean diseases and pests (http://www.soybeanresearchinfo.com/).

Corn diseases

Bacterial leaf streak in corn

Bacterial leaf streak

Bacterial leaf streak of corn has been recently increasing and spreading in Minnesota. Symptoms are narrow lesions between the veins of leaves that may be yellow, tan, brown, or orange with wavy-edges. The symptoms are similar to those caused by gray leaf spot. Bacterial leaf streak was confirmed for the first time in Minnesota two years ago in two southern counties. In 2017 it was found in multiple counties in southern Minnesota. In 2018 it occurred at more widely and with higher severity in Minnesota than in previous years, especially in central and southern MN from near Sauk Center to Waseca. Sweet corn appeared more susceptible than field corn, and disease was severe in some fields. The potential for this disease to cause significant yield loss is unknown, but is thought to be low for field corn. More information on bacterial leaf streak can be found at: cropprotectionnetwork.org/library/.

Physoderma brown spot

Physoderma brown spot was reported to be common across southwestern Minnesota. This fungal disease develops most commonly in wet and warm weather, just as it was this year. Symptoms are small (about 1/8”) round-to oval, yellow-brown spots on leaves and sometimes the stalk, and can occur in bands across infected leaves. The symptoms can be confused with eyespot. Node and possibly stalk rot can also develop. This disease typically does not have a significant effect on yield.

Goss' leaf blight and wilt

Goss’s leaf blight and wilt was reported from multiple fields in southern and central Minnesota. Most reports suggested a patchy distribution that rarely was widespread and severe enough to result in measurable yield loss. The common use of hybrids with resistance to Goss’s wilt has greatly reduced problems with this disease, but it continues to something to watch for given its potential to reduce yield, especially when infection occurs by mid July. When disease doesn’t develop or spread by early August, yield loss is typically low or nil. A new summary of information on Goss’s wilt can be found at: cropprotectionnetwork.org/library/.

Other diseases in Minnesota

Other corn diseases reported in Minnesota in 2018, mostly at low levels, this year include common rust, eyespot, northern corn leaf blight (this seemed to develop primarily in September), northern corn leaf spot, gray leaf spot, and stalk rots.

Diseases in neighboring states

Tar spot

Of interest also is the tar spot of corn, which was reported in Illinois, Iowa, Wisconsin, and Michigan at relatively high levels of distribution and severity in some fields. Tar spot (a fungal disease) has not yet been reported or confirmed in Minnesota, even though scouting was done to look for it. Tar spot was found for the first time in the U.S. in 2015 in Indiana and Illinois. Until this year (2018) there appeared to be low potential for this disease as it is known in the U.S. to cause yield loss. But this year there were reports of potential yield loss in some fields. Many foliar fungicides are likely to be effective to manage this disease if reaches levels of concern. As is also true for bacterial leaf streak, we have much to learn about this disease, the conditions that favor it, why it is spreading and becoming more severe, and if and where it may become a problem in Minnesota.

Links to more information

For more information, the Field Guide to Corn Disease is an excellent, inexpensive resource with color photographs and basic information on most of these and other corn diseases (bookstores.umn.edu/product/book/field-guide-corn-diseases). Another excellent resource is A Farmer’s Guide to Corn Diseases (available at my.apsnet.org/ItemDetail?iProductCode=44549).

This summary is based on comments from many experts that spend much time in crop production fields, samples sent to the U of MN, and my own observations. If you saw other diseases or have a different view of those I have mentioned, please let me know.