Issue No. 24, Article 5/November 5, 2004
Illinois Soybean and Corn Diseases in Review--Reports and Observations from 2004
Average Illinois corn and soybean yields were high in the 2004 season. Generally good weather across the state made for excellent growing and harvesting conditions in most areas. In spite of these positive reports, diseases affected stand establishment and crop yields. The high average yields may be hiding two important things: low yield in localized areas due to significant disease pressure and whether yields in many fields may have been even higher if diseases had not been present. This article will provide a brief overview of several diseases of soybean and corn that were common in Illinois in 2004 based on reports and observations. This information may be useful in seed selection and field management for the 2005 season.
Seedling diseases, root and stem diseases, and foliar diseases all took their toll. Wet spring weather in many areas favored problems with seedling diseases, and in some areas fields were replanted. Several different pathogens cause soybean seedling diseases. Among infected soybean samples that we received from across the state for diagnosis in May and June, we found the most common pathogens (in order of most to least common) to be Fusarium, Phytophthora, Pythium, and Rhizoctonia.
Root and stem diseases also occurred widely in Illinois. These included white mold, Phytophthora rot, stem canker, sudden death syndrome (SDS), and brown stem rot (BSR). Each of these diseases caused considerable damage in some soybean fields. White mold was of particular concern this year in the northern half of Illinois, especially in the northwest. Cool and wet conditions in July promoted development of more severe levels and incidence of white mold than had been seen for a number of years. SDS was widespread in Illinois based on a survey specifically for this disease, and although BSR was seen in the northern third of the state as usual, it was seen at levels above normal in central Illinois. Stem canker is another disease that appears late in the season that seems to be increasing in many areas in Illinois.
In the past growing season, weather conditions were also quite favorable for foliar diseases. Foliar diseases were reported in Illinois more frequently than normal in 2004. These included Septoria brown spot, downy mildew, bacterial blight, Cercospora leaf blight, frogeye leaf spot, and anthracnose. Many questions came up regarding the potential for yield losses from these diseases and how they should be managed. In many cases, we don't have as much information as we would like, but research is under way to find some of the answers. Information on foliar diseases of soybean and their management will be presented in January at the Illinois Crop Protection Technology Conference in Urbana and at the Illinois Corn and Soybean Classic meetings at six locations in Illinois.
In addition to reports of crazy top in flooded areas and stalk rots, four major disease problems were of considerable concern in some parts of the state: gray leaf spot, rust, northern corn leaf blight, and Diplodia ear rot. Several factors came together, including stage of crop development and favorable weather conditions, to create much concern over gray leaf spot. The most significant levels of this disease were reported in central Illinois, with the west-central area receiving the most attention. Concern was raised to the point that fungicides were applied on some grain production fields (vs. the more common applications on seed production fields); however, fungicide timing was a problem in some areas. Rust was also a problem in some areas due in part to the below-normal temperatures in July, with apparently the greatest incidence and severity in the north-central part of Illinois.
Northern corn leaf blight (NCLB) may be an increasing problem. A few fields in Illinois were reported with significant levels of this disease in 2003, but many more fields seemed to be affected in 2004. The reasons for the increased incidence of NCLB are not entirely clear at this time, but the weather, hybrid susceptibility, and presence and virulence of the pathogen all played some part. One thing is clear--there are differences in susceptibility/resistance among hybrids. Because NCLB seems to be increasing and there should be plenty of the residue-borne inoculum next season in many fields, the disease should be considered when hybrids are selected. Hybrids with resistance to NCLB should be selected if possible to help avoid problems with the disease in 2005.
In summary, crop diseases are opportunistic and can develop to levels not seen for years and in areas where they have not occurred before, as we saw this year. Diseases continue to take a toll on corn and soybean yields and quality in Illinois, and vigilance in management can do much to reduce the effects of disease.--Dean Malvick