Late August finds in NW MN corn and soybean fields

Until recent days, the 2025 growing season had been largely uneventful for the Minnesota Soybean Research & Promotion Council-sponsored scouts that traveled the countryside scouting soybean fields this year in northwest Minnesota. Today, I decided to take a drive about 20 minutes away from the Northwest Research & Outreach Center (NWROC), stopping at corn and soybean fields along the way to see whether there was anything that folks may have an interest in learning about. I was not disappointed.

Corn disease and insect injury

Insect injury

There was evidence of considerable insect injury on ear tips in a corn research trial at the NWROC in Crookston (Figure 1), with many kernels sustaining injury. In addition to the physical injury caused by northern corn rootworm beetles (Figure 2), picnic beetles and other pests, there was evidence of opportunistic pathogens that took advantage to colonize remaining kernel tissue.

Figure 1. Feeding injury on kernels near the tip of an ear of corn. Picture: Angie Peltier

Figure 2. Northern corn rootworm beetle feeding on kernels near the tip of an ear of corn. Picture: Angie Peltier

Upon further investigation, the hybrid planted in this field was an 85 day hybrid with Bt traits. However, there are above-ground and below-ground Bt traits. It turned out that this hybrid expressed the VT Double Pro series of Bt proteins, which provides protection against European corn borer, corn earworm, southwestern corn borer, and fall armyworm. However, this hybrid did not express traits geared toward the most yield-limiting below-ground pests of corn, western and northern corn rootworm. If one was able to observe this level of feeding injury above-ground, it stands to reason that considerable feeding on root tissue occurred before adult beetles emerged from their pupae. 

As corn acreage in northwest Minnesota increases, expect pest and disease problems experienced further south to increase in prevalence. For more information about which traits confer protection against each of the pests of corn in Minnesota, see the Handy Bt Trait Table compiled by Chris DiFonzo at Michigan State University.

Corn disease

In a corn field near the Crystal Sugar factory in Crookston, symptoms of a corn disease that I hadn’t yet seen this far north -bacterial leaf streak- were evident (Figures 3&4). While farmers in northwest Minnesota are all-too-familiar with bacterial leaf streak caused by the bacterial pathogen Xanthomonas translucens pv. undulosa in their hard red spring wheat, bacterial leaf streak in corn is caused by a different species of the Xanthomonas genus: Xanthomonas vasicola. While the boxy lesions delineated by leaf veins characteristic of grey leaf spot can be easily confused with symptoms of bacterial leaf streak, not only does grey leaf spot not occur this far north, one need only hold a symptomatic leaf up to the sky to tell the difference between these two diseases. Lesions of bacterial leaf streak can grow across leaf veins, tend to have wavy margins and are ringed by a yellow ‘halo’.

Figure 3. Symptoms of bacterial leaf streak observed on corn plants in Crookston, MN. Note that the lesions are able to grow across leaf veins and when held up to the light are outlined by a yellow “halo”.  Pictures: Angie Peltier.

Figure 4. Symptoms of bacterial leaf streak observed on corn plants in Crookston, MN. Photo: Angie Peltier

Soybean

Soybean diseases

While it had been likely a quite boring field season for IPM scouts, some disease symptoms are becoming a bit more prevalent, including bacterial leaf blight and Septoria brown spot.

Digging up some struggling soybeans in one grower field revealed that cysts of soybean cyst nematode (SCN) are still visible to the naked eye (Figure 5). Should you not yet know whether your fields are infested, please ‘move heaven and earth’ to get soil samples pulled and samples analyzed for SCN egg counts this fall. Knowing your field egg counts is essential information for those interested in maintaining soybean and dry bean yield potential into the future.

 

Figure 5. Cysts are the dead females of the parasitic nematode soybean cyst nematode. Cysts are filled with ~200 eggs and harden to provide a protective shell in which they can survive between soybean and dry bean crops. Picture: Angie Peltier.

Symptoms of bacterial blight and Septoria brown spot were also evident on soybean leaves. Bacterial blight lesions occur on the upper leaves of soybean plants. Individual lesions can sometimes coalesce and/or drop from leaves and result in significant losses in photosynthetic leaf area (Figure 6). Septoria brown spot lesions, as referenced in the common name of the disease, are brown in color, begin on lower leaves and similar to bacterial blight lesions, are surrounded by yellow halos (Figure 7). While a ubiquitous disease in Minnesota soybean fields, with particular prevalence in wet growing seasons, Septoria brown spot seldom reaches the mid- or upper soybean canopy to cause significant yield potential losses.

Figure 6. Bacterial blight lesions on a soybean leaf. With time, lesion tissue becomes brittle and can drop from the leaf. Photo: Angie Peltier

Figure 7. Septoria brown spot lesions can now be observed on lower leaves of many soybean plants. Picture: Angie Peltier