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Soybean and Corn Diseases Occurring in Minnesota - July Update

By Dean Malvick, Extension Plant Pathologist

A number of different root and leaf diseases have been appearing in soybean and corn fields across Minnesota. Most are of minor concern at this point, but some have been more problematic. This article focuses on diseases that have been reported or may be favored by weather conditions and have raised questions or concern.

Soybean Diseases Noted

Rhizoctonia root and stem rot has been the most common problem reported this season. Rusty-brown, sunken regions develop on the stem and root and often girdle stems near the soil line. It can be confused with early season Phytophthora root rot. As noted in a previous article this season in Crop News, the warm and wet soil conditions along with delayed soybean development in many fields have been favorable for Rhizoctonia root and stem rot. Significant plant death and stand loss has been reported from multiple fields in southern and central Minnesota. Of the soybean samples received by the University of Minnesota Plant Disease Clinic ( the most common disease that has been confirmed to be killing soybean plants thus far in 2014 has been Rhizoctonia root and stem rot.

Septoria brown spot. Septoria brown spot typically develops first on the lower leaves of the plant and progresses to the mid-to-upper canopy throughout the summer. Brown spots and chlorosis (yellowing) develop on leaves. This disease is typically minor and rarely results in yield loss in soybean.

Bacterial blight. Bacterial blight typically occurs on the upper leaves. Symptoms begin as small, angular, water-soaked spots that turn brown as the tissue dies and are surrounded by yellowish-green halos. Large, dead patches often develop on the leaves that may fall out and give the leaves a ragged look. This disease is also typically superficial and rarely results in yield loss in modern soybean varieties.

Cercospora leaf blight. Symptoms of Cercospora leaf blight include light purple spots and areas on the top surface of leaves that expand and become reddish-purple or bronze. The infected leaves appear leathery and 'sunburned'. This is another disease that is often reported (and usually not confirmed). It is typically superficial and rarely results in yield loss in Minnesota.

Fusarium root rot. The taproot and lateral roots of infected plants are brown to black in color and have root rot, and dark brown lesions may develop on roots. This very common root disease has been the main problem found in a number of stunted seedling samples.

Phytophthora root rot. This disease has been favored by the wet soil conditions that occurred in May and June. This disease causes brown lesions that develop on the lower stems above the soil line and can kill plants throughout the season into August.

Soybean Diseases to Watch For

White Mold. The wet and relatively cool weather (below 76°F) in many areas of Minnesota recently during flowering stages of soybean has favored infection by the white mold pathogen. Although infection may have started in many fields, more cool and wet weather will help to spread the disease. Obvious symptoms may be most easily seen from mid to late August.

Sudden Death Syndrome. Look for SDS leaf symptoms in early to mid-August. Wet and cool soils that were present in many areas during early seedling growth stages have been favorable for infection of soybean roots by the SDS fungal pathogen. The disease may continue to develop and cause significant damage in fields that receive average or above average rainfall in July and early August. SDS may not progress to cause severe damage if the rain shuts off in late July and August as it has the past few years.

Brown Stem Rot. Because BSR leaf symptoms look very similar to SDS leaf symptoms, these two diseases continue to be confused at times. This disease is widespread across Minnesota, and symptoms usually become most apparent in mid to late August. Split stems to look for browning in the pith of plants with BSR.

Corn diseases Reported

Common rust of corn is common in many fields in southern and central Minnesota. Cooler than normal temperatures, moisture, and delayed crop development favor this disease. Fortunately most corn hybrids have enough resistance to this disease and our common summer temperatures above 80°F usually keep this disease in check. There are no absolute guidelines for thresholds for management of common rust with fungicides if it develops to levels of concern.

Anthracnose. Our frequent rains through early July favored development of this and other fungal leaf diseases. Anthracnose causes oval lesions on leaves of young and old plants, and is also a common cause of stalk rot.

Physoderma brown spot. Several reports have come in for Physoderma brown spot on corn. Although we did not confirm the diagnosis, all available information suggests the symptoms were due to this disease. Symptoms appear as small, nearly round, yellow to brown spots on leaves. It can be confused with eyespot. This disease has been favored by the wet conditions in most of Minnesota in June, and is most common where corn has not been rotated. Physoderma brown spot is typically a minor disease. This disease can also cause stalk rot as was reported in Iowa last September.

Eyespot. This leaf disease is much more common in Minnesota that Physoderma brown spot, which causes similar symptoms. Symptoms of eyespot are small spots (1/16' to 1/8" diameter) with yellow halos that develop into a tan spot surrounded by a brown to purple ring and narrow yellow halo. The spots can be scattered on leaves or appear in patches. This disease also is typically minor and rarely causes yield loss, except on susceptible corn inbreds.

Northern leaf blight. This is a common fungal disease that often is seen at low levels, and occasionally reaches severe levels of concern. Typical symptoms are canoe-shaped lesions 1" to 3" long by ½" to ¾" wide that are tan colored and may contain dark areas of fungal growth.

Corn Disease to Watch For

Goss's leaf blight and wilt. I am not aware of any confirmed reports of this disease yet in Minnesota, but the pathogen is widespread in infected corn residue and we have had suitable conditions in many areas to favor infection. Thus, Goss's wilt is likely developing at low levels in some areas, but severe symptoms and damage usually increase and become most visible from early to mid-August. It's important to know where Goss's wilt develops help with future hybrid selection and crop management plans.

Final Notes

Several different diseases are occurring in corn and soybean fields in Minnesota. The weather conditions projected over the next week may increase some of the problems. Keep your eyes open and scout fields so that we can understand where disease is reducing yields this season and where disease management efforts should be focused in the future. Photos and more information for most for these diseases can be found at the Minnesota Crop Diseases web site.
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