Now that soybean and corn harvest is nearing completion in Minnesota, it is a good time to review some of the diseases issues in these crops across the state in 2015. As is usual, the pattern of diseases across the state is as inconsistent as the weather that drives disease development. Some diseases were common in many fields and areas, but many were much more scattered. This is a summary of some of the most problematic or talked-about corn and soybean diseases in Minnesota.
Northern corn leaf blight (NCLB)
Northern corn leaf blight (NCLB) first received attention in Minnesota in July, primarily due to problems reported in southern Iowa. In Minnesota at that time when this disease had the greatest potential to reduce yields, it was not a significant problem and was reported only at low levels from scattered fields. NCLB never became a significant problem in most areas in Minnesota. In late August and early September, however, when NCLB no longer had much potential to reduce yield, it developed rapidly in many fields in southern Minnesota, especially in the southeast. NCLB caused minimal yield loss this year, but given its recurrence for several years in a row it is a disease to watch for in the future.
Goss’s wilt was seen fairly widely across southern and central Minnesota at low levels, and a few areas suffered significant problems. Overall the weather pattern with few strong storms in July was not favorable for widespread development of Goss’s wilt. However, yield losses over 50% were reported from a few fields, and it is clearly another disease to lookout for given its potential to dramatically reduce yields. If there is strong reason to believe a field may be at risk, e.g. due to previous confirmation of Goss’s wilt, it may be warranted to plant highly resistant hybrids.
Root and crown rots
Root and crown rots developed to levels that killed plants in August in multiple fields in southern Minnesota. Preliminary diagnosis suggested it was due to fungal disease, but the reasons for the pattern of occurrence were not clear. In some fields ears were hanging on stalks and had stopped developing well before plant maturity. In most fields only a low percentage of plants were affected.
Other corn problems
Other reported problems include stalk rots and Physoderma leaf spot. Stalk rots were reported from many fields and raised concerns about harvestability of the crop. Fortunately harvest proceeded ahead of normal in many areas and few fields were exposed to strong winds and rain late in the season that may have caused lodging. Physoderma brown spot also developed in fields scattered across southern Minnesota.
New corn diseases found in the Midwest (not in Minnesota)
Tar spot was found for the first time in corn fields in Indiana and Illinois this year (http://bulletin.ipm.illinois.edu/?p=3423, New corn disease confirmed in Indiana). It was also the first time this disease had ever been found in the United States. Previously tar spot on corn was primarily known to occur at high elevations in Latin America. There are two types of tar spot caused by different fungi, and fortunately the one type found in Illinois and Indiana is not known to cause significant yield loss. Bacterial stripe disease was also reported for the first time in Illinois (http://bulletin.ipm.illinois.edu/?p=3442). Symptoms of this disease can appear similar to Goss’s wilt. The potential impact of bacterial stripe on corn yields appears likely to be minor, but is uncertain. These will be two more diseases to look for in Minnesota corn fields next year.
White mold was probably the most widespread and talked-about soybean disease in Minnesota this year. Wet weather during flowering periods in July along with relatively cool temperatures in many areas were just what this disease prefers. Thus it was more prevalent in wet areas of southern Minnesota than in the drier areas in the northwest. Although white mold was reported widely, its impact on yield was more uncertain. Published data indicates that soybean yield is reduced about 2 – 5 bushels/acre for each 10 % increase in incidence of the disease (% of plants infected). The range in yield loss is largely due to the varying times when plants are damaged to the point that seed growth stops.
Sudden death syndrome (SDS)
The frequent rains in southern Minnesota were also favorable for sudden death syndrome (SDS), and the disease developed in many fields as would be expected under these conditions. Late or delayed development of disease reduced the impact on yield loss. I saw widespread but late development of SDS in many fields in southern Minnesota, including my research plots where disease started to develop as expected in early August but did not fully develop until late August when it had less potential to cause yield loss. Our SDS management trials demonstrated the importance of using soybean varieties with resistance to SDS, as well as the potential value of selected seed treatments, for managing this disease.
Brown stem rot
Weather conditions were also favorable for development of brown stem rot (BSR) in parts of Minnesota. We had reports of this disease from multiple fields, and it developed to moderate levels in our research plots at Rosemount and Waseca. Once again as we have often seen, leaf symptoms of BSR may or may not develop depending on the type of the pathogen present and the environmental conditions. For example, in our plots this year the leaf symptoms only developed in Rosemount and the symptoms in Waseca were mostly limited to internal stem symptoms. This is a good reminder that only way to know for sure if BSR is a problem in a field is to split stems in mid to late-August to look for the browning in the pith.
Other soybean diseases
Other soybean diseases also occurred at notable levels in some fields, including pod and stem blight in western Minnesota and downy mildew in Northwestern Minnesota. One of the most common diseases in 2013 and 2014 in central Minnesota was Rhizoctonia root rot, but this was a minor problem in most areas in 2015.
More information and photographs of the symptoms for most the diseases mentioned in this article can be found on the Minnesota Crop Diseases web site (http://www.extension.umn.edu/agriculture/crop-diseases/).