Midsummer corn and soybean disease development in Minnesota

Several different crop diseases have appeared in corn and soybean fields across Minnesota. Although most are at minor levels now, diseases are dynamic and it is important to be alert for these and other diseases that may be developing. More information can be found at the Minnesota Crop Diseases web site: www.extension.umn.edu/agriculture/crop-diseases/

Corn Diseases

Goss’s leaf blight and wilt has been confirmed in a few fields in northwest, south central, and southern MN in the past week, including in the areas of Hubbard, Le Sueur, Waseca, and Renville Counties. Mid-July is earlier than we usually have reports of this disease in production fields. The earlier that Goss’s wilt is established, the more it is likely to spread and reduce yield. Recent weather conditions in many areas, especially wind, rain, thunderstorms, and temperatures in the 70’s and 80’s (F), have been favorable In our research plots, Goss’s wilt has been developing rapidly over the past two weeks, providing further evidence of favorable conditions. Due to favorable conditions and the fact that the pathogen is widespread in corn fields, please keep alert for this disease. There are no proven curative treatments for use this season. More information is available at: www.extension.umn.edu/agriculture/crop-diseases/corn/gossbacterialwilt.html.

Northern corn leaf spot has been reported in a few fields in Minnesota at low levels. Typical lesions include light grayish-tan lesions surrounded by a darker border. This disease rarely reaches levels that cause significant crop damage in MN.

Physoderma brown spot has been reported from corn fields in southern MN. Symptoms appear as small, round, brown spots on leaves. It can be confused with eyespot. This disease has been favored by wet conditions and where corn has not been rotated. Physoderma brown spot can also cause stalk rot, but is typically a minor disease.

Eyespot. This leaf disease is usually more common in Minnesota than Physoderma brown spot, which causes similar symptoms. This disease also is typically minor and rarely causes yield loss, except on susceptible corn inbreds. Symptoms of eyespot are small tan spots (1/16′ to 1/8″ diameter) surrounded by a brown to purple ring. Spots can be scattered or appear in patches.

Common rust has been reported low levels in southern Minnesota, as is typical by mid-July. Common rust is favored by cool temperatures (below 75F) and moisture. Given our advanced corn development in many areas and warm weather in the forecast, rust is less likely to reach the levels of concern in dent corn hybrids as it did in the past two years. Keep in mind that most corn hybrids have enough resistance to keep this disease in check.

What about Northern leaf blight? Although NCLB has been reported at low levels in a few fields in southern Minnesota, it doesn’t appear to be a significant problem. Thus, the current situation in Minnesota is quite different than in southern and central Iowa where high levels of NCLB have developed. This is a common fungal disease that often is seen at low levels, and occasionally reaches severe levels of concern as it did last year in some fields. Typical symptoms are canoe-shaped, tan lesions 1″ to 3″ long by ½” to ¾” wide. We should keep a lookout for NCLB over the next few weeks, especially if the weather is wet and cool (<80F).

Soybean Diseases

Bacterial blight is common on the upper leaves of plants in many fields. The symptoms begin as small, water-soaked spots that turn brown and are surrounded by yellowish-green halos. Dead patches often develop on the leaves that may fall out and give the leaves a tattered look. This disease rarely results in yield loss in modern soybean varieties.

Rhizoctonia root and stem rot has been reported this season, but is less widespread this year than last year. At this point of the season most of the damage has been done by this disease, although plants can still succumb and it is not unusual to find superficial lesions on roots and lower stem. Rusty-brown, depressed regions develop on the stem and root.

Phytophthora root rot. The wet and warm soil conditions that occurred in May and June in many areas of the state have been favorable for Phytophthora rot. This disease has been confirmed several fields as expected, but appears to not be a widespread problem. Infected plants develop brown lesions on lower stems and can kill plants throughout the summer.

Septoria brown spot. This leaf disease is also favored by wet weather. It typically develops first on the lower leaves of the plant and can progress to the mid-to-upper canopy throughout the summer. Brown spots and chlorosis (yellowing) develop on leaves. Septoria brown spot is typically minor and rarely results in yield loss.

Sudden Death Syndrome. Environmental conditions have been highly favorable for SDS to begin and continue developing in many areas. In our research plots at Rosemount and Waseca, early SDS symptoms developed in June, indicating good conditions for the early season infection that is critical for full disease development. In production fields, SDS leaf symptoms first appear in late July to mid-August. If regular rains continue into early August, we may see significant levels of SDS in some areas. SDS may not fully develop if the rain stops in late July and early August. More information on SDS is available at: www.extension.umn.edu/agriculture/crop-diseases/soybean/suddendeathsyndrome.html