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Soybean and corn disease notes for early July

Dean Malvick, Extension plant pathologist

Soybean and corn are growing well across much of Minnesota. As they grow and become more dense, conditions also become more favorable for diseases. White mold is a concern in many soybean fields. Septoria brown spot, bacterial blight, and frogeye leaf spot are also developing. In corn, bacterial leaf streak has been reported. The amount and frequency of rain over the next 6 weeks will greatly influence the amount of disease that develops.


White mold

Photo 1. White mold in soybean.
White mold could develop soon in soybean fields. USDA NASS reported that as of July 5, 43% of soybeans were flowering in MN.

White mold is favored when the plants are flowering, the rows are filling, soil is moist for 10-14 days, stems and leaves have prolonged wet surfaces, and temperatures are cool (below 68°F for prolonged periods).

In much of Minnesota recently, the day and night temperatures have been too warm to favor white mold. However, cooler day and night temperatures and frequent chances for rain over the next week could increase the risk of white mold.

Partially resistant soybean varieties, reduced plant populations, and wide rows can decrease risk of white mold. At this time of the season, fungicides are one of the few options available that can reduce white mold where the disease risk appears to be high.

 Risk can be assessed in part by the field history and the prevailing crop density, stage, and weather conditions. A risk assessment tool, Sporecaster, has been developed at the University of Wisconsin to help assess risk of white mold in soybeans. It may be helpful to try this to determine how well it works in your area.

 If the risk of white mold is considered high in a field that is in the early flowering stages and fungicides are to be used, they are most effective when applied at the R1/R2 flowering stages. Thus, they need to be applied soon.

More information on white mold and it’s management can be found here:

Bacterial blight and Septoria brown spot

Photo 2. Bacterial leaf blight on soybean.
Bacterial blight (Photo 2) and Septoria brown spot have also been developing in soybean fields. Bacterial blight usually develops on leaves near the top of plant, whereas brown spot typically develops on leaves in the lower half of the canopy.

Neither of these diseases typically develop to levels that result in yield loss to soybean in Minnesota, although they have the potential to cause yield loss.

Photos and more information on these two diseases is found on in soybean leaf diseases:

Frogeye leaf spot

Photo 3. Frogeye leaf spot
Frogeye leaf spot is another disease to look for in soybean fields. Warm and humid weather and frequent rains favor this fungal disease, and conditions have been favorable in some areas.

Typical symptoms are round lesions that are tan in the center and surrounded by a dark border (Photo 3). This disease has potential to cause significant yield loss. It has been reported with increasing frequently and distribution in Minnesota over the past two years.

Fungicides can be used to manage frogeye leaf spot; however, we confirmed last year that the fungal pathogen is resistant to the QoI (strobilurin) fungicides in many areas of MN. Thus, fungicide mixtures with a high enough level of a non-strobilurin active ingredient are needed to manage this disease. For more information, see  Fungicide efficacy for control of soybean foliar diseases.

We want to know the distribution and occurrence of frogeye leaf spot and its resistance to fungicides in MN. If you see this disease in a field, please contact and send photos to Dean Malvick (


Bacterial leaf streak

Photo 4. Bacterial leaf streak
The last disease note for today focuses on bacterial leaf streak (BLS) on corn. BLS has been reported in fields in Martin and Waseca Counties over the past 2 weeks, which is earlier than we had previously seen or heard reports of this disease in MN.

Symptoms are narrow, elongated brown to yellow streaks (Photo 4) that can resemble the symptoms caused by gray leaf spot.

BLS is a relatively new bacterial disease of corn in MN and the USA (since 2016). It tends to be most severe on sweet corn and popcorn, but is often seen on field corn when favorable (warm and wet) conditions occur.

I would also like to see reports and see photos of where this disease is developing. Please email me at

More information and photos for this disease can be found here: Bacterial leaf streak.
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