Minnesota Corn, Soybean: Late Season Disease Notes

Tar Spot on corn with Black raised “bumps” and small necrotic fisheye symptoms. Photo: University of Illinois

Although it is getting late in this growing season, it is worthwhile to take note of corn and soybean diseases that have developed in fields in Minnesota and the region. Several different diseases have been increasing in corn and soybean, and we continue to watch development of tar spot of corn in nearby states.

Corn diseases

Several different diseases have developed in corn fields in Minnesota this year. Physoderma leaf spot was a concern in southwestern Minnesota, especially in mid-summer. Bacterial leaf streak and Goss’s wilt also developed in multiple fields in southern and/or northwestern Minnesota, with bacterial leaf streak being most common again in fields with sweet corn. More recently, northern corn leaf blight, rust, and gray leaf spot have been easy to find at low levels to moderate in many fields across Minnesota.

Although most of these diseases developed too late to have significant effects on corn yields, their presence shows that inoculum is present and there is a risk of these diseases developing to greater levels in the future when conditions favor them.

Tar spot

While still not found or confirmed in Minnesota, tar spot of corn continues to be a potential problem for Minnesota. Tar spot developed in 2018 in Illinois, Iowa, Indiana, Wisconsin, and Michigan to high and yield-damaging levels in many fields. In 2019, this fungal disease has developed again in many of the same areas. The current distribution map is shown here.

Tar spot has generally been slower to develop and not as severe in most of those areas as it was last year, although it has once again reached significant levels in some fields. Frequent rain or irrigation appears to create favorable conditions for development of tar spot.

Thus, based on the reports of tar spot in counties in Iowa and Wisconsin near the southeastern border of Minnesota and the frequent rains this summer, southeastern counties appear to be the area of highest risk for tar spot in Minnesota. With the corn crop behind normal development, there is still time to scout for tar spot with the goal of helping to manage this disease in the future.

Initial symptoms are small black lesions, often on leaves below the ear. Symptoms become easier to see as the disease develops as you can see Figure 1. They can look similar to rust pustules until we look closer. The tar spot pustules are attached firmly and do not release spores when a finger is rubbed across them as happens with rust pustules. More photos of symptoms and information on tar spot can be found here.

If suspect samples are found, please contact Dean Malvick (dmalvick@umn.edu) or send samples to the Plant Disease Clinic at the University of Minnesota.

Figure 1. Early tar spot symptoms on lower corn leaves. Click Image to Enlarge

Soybean diseases

Soybean also has not escaped from disease this year in Minnesota. The late planting and the slow crop development in many areas have also seemed to slow disease development. The common diseases include sudden death syndrome (SDS) and white mold that have been favored by the wet summer, and are significant in scattered fields.

Bacterial blight was very common in many areas at low levels this summer, along with Septoria brown spot. Pod and stem blight and brown stem rot (BSR) have been developing, with BSR most common where resistance levels are low in planted varieties.

Figure 2. Frogeye leaf spot symptoms on soybean from southern MN in 2019.

Once again, as was the case in 2018, frogeye leaf spot has been common in some areas in southern and central Minnesota (Figure 2). This is a relatively new problem in Minnesota that seems to be spreading. It is a significant disease is many parts of the U.S, but was previously uncommon in Minnesota.

Frogeye leaf spot has been favored by the unusually wet and humid summer weather. Because of its potential to cause damage, this disease should be monitored. Fungicides can be used to manage frogeye leaf spot and we are working to determine if the pathogen in Minnesota is resistance to the QoI (strobilurin) fungicides as reported in many other states. More information on this disease can be found here.


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