The unusual weather this season in Minnesota has created favorable conditions for various diseases in corn and soybean crops. Some diseases have been appearing since June due in part to delayed planting and crop growth and abundant rainfall in many areas. This article focuses on diseases that have been recently been confirmed and have raised concern. In corn fields, above average levels of common rust are being reported and Goss's wilt was confirmed in two fields. In soybean, Phytophthora root and stem rot and the less important but often noticed leaf diseases bacterial blight and Septoria brown spot are widespread. This is a good time of the year to scout fields for crop diseases.
Corn diseases reported and of interest include common rust and Goss's wilt of corn, which are both developing in Minnesota. Lower levels of other diseases including eyespot, northern corn leaf blight, and others are also likely to be developing in many fields.
Common rust of corn is being reported at levels higher than normally seen, especially in southern Minnesota. The greater amount of common rust is due in part to cooler than normal temperatures and delayed crop development. There are no absolute guidelines for thresholds or timing for management of common rust. Fortunately most field corn hybrids have enough resistance to this disease to prevent measurable yield loss.
Goss's wilt was confirmed in south central Minnesota in two fields in the past week. This disease can cause significant yield loss and is of much greater potential importance than common rust. Based on the size of the lesions in the samples, initial infection probably started in mid to late July. Recent wet weather and storms may also have increased the spread of this disease in fields. I expect that Goss's wilt is also appearing in other fields, and thus it is a good idea to check fields for this disease. It's important to know where Goss's wilt is developing to help with future crop management plans and possibly hybrid selection.
Soybean diseases of recent interest are also developing. A destructive soybean disease in many fields across Minnesota is Phytophthora root rot. This disease was favored by the wet and warm soil conditions that occurred in June. This disease has been noticed over the past few weeks killing plants and causing the typical brown lesions that develop on the lower stems above the soil line.
Two other diseases of soybean that are widespread and often noticed in fields are Septoria brown spot (a fungal disease) and bacterial blight. Septoria brown spot typically develops on the lower leaves of the plant, whereas bacterial blight typically occurs on the upper leaves. Both of these diseases are typically superficial and rarely result in yield loss in soybean.
Weather and soil conditions this season have also been favorable for two other diseases of soybean that can cause considerable damage. These are white mold and sudden death syndrome (SDS). Although I'm not aware of any confirmed reports of these diseases developing yet, I expect them to be appearing in many fields soon due.
In summary, diseases are developing in corn and soybean fields in many areas in Minnesota, including Phytophthora rot of soybean, and common rust and Goss's wilt of corn. The weather condition and heavy rains and storms that went through the state this week will be likely increase some of the problems. Keep your eyes open and scout fields over the next 2-3 weeks so that we can understand where disease is reducing yields this season and where disease management efforts should be focused in the future. Please see the Minnesota Crop Diseases web site (http://www1.extension.umn.edu/agriculture/crop-diseases/) for more information and photographs of these diseases.