By Dr. Daren Mueller
With the 2018 growing season coming to an end, it’s time to bust out the record books and begin logging the most prevalent diseases we’ve seen this year in Iowa soybeans. It’s important to review these diseases so that farmers, agronomists and all of our partners can make informed decisions ahead of next year’s season. These decisions include variety selection, choosing seed treatments, developing pre-germination checklists, evaluating the cost-effectiveness of a fungicide application, and cultural practices such as crop rotation. These strategies are all part of integrated pest management. While risk of many soybean diseases was low this year, future growing seasons may be different. .
1. Frogeye Leaf Spot
Rounding out the top of our list is frogeye leaf spot, caused by the fungus Cercospora sojina. Back in late July/August, Iowa State researchers and agronomists began reporting the tell-tale symptoms of round-to-angular lesions, with a light tan center surrounded by a reddish-purple margin on soybean leaves.
Frogeye leaf spot’s impact on yields can vary greatly, depending on the timing of disease appearance, varietal susceptibility to disease and weather conditions during soybean reproductive stages. We won’t know the full impact of the disease on yield until harvest concludes. If the disease begins late in the reproductive stages (after growth stage R5.5), or disease severity is low, the yield impact will likely be minimal. Conditions favorable to disease development include warm, humid weather, with frequent rains that persist over an extended period of time. Several days of overcast weather can also increase the spread of the fungus. If conditions are favorable to the disease, and there are severe disease outbreaks early or just after flowering, yield losses can be up to 35 percent.
With frogeye at number one on this list, it’s important to remember that while the disease might not be impactful this year, it can become a problem in future years, due to the fact that the spores can overwinter in Iowa crop fields. The resistance gene, known as Rcs3, has been effective against all races of this fungus known to occur in North America. Crop rotation and tillage can also be effective in reducing the amount of fungus available to infect the next soybean crop. Long rotations may be necessary if the disease has been severe in a particular field. Well-timed foliar fungicide applications can effectively control frogeye leaf spot. Research shows that applying a fungicide during pod development (R3-R4), is the most effective for managing the disease. Fungicide applications are generally more profitable when disease risk is high, however.
2. Soybean Sudden Death Syndrome (SDS)
Coming in second place this year was SDS — probably the most recognizable soybean disease in North America — caused by the fungus Fusarium virguliforme. Reports of this disease were found at Iowa State research farms beginning in mid-July. Symptoms of SDS include yellow spots between leaf veins that turn brown as the spots expand, eventually leading to defoliation. Underground, roots will be discolored and show decay (sometimes with no foliar symptoms). When soils are wet, the SDS fungus can reproduce on root surfaces, producing a mass of spores that are purple-blue in color.
As with all diseases, yield loss can vary greatly with SDS, and is dependent upon multiple factors. Cool, wet field conditions shortly after planting favor early root infections and disease establishment. Frequent or heavy rains midseason can favor early symptom expression. Yield losses can be substantial in years when susceptible soybean varieties are planted in cool, wet fields with a history of SDS and soybean cyst nematode (SCN; caused by Heterodera glycines). If symptoms develop later in the season, or weather is not conducive for disease development, yield losses can be minimal. At the research farms, soybeans were around R3-R5 when symptoms were observed. In some areas of Iowa, high volumes of rain were the typical forecast. This disease often occurs in patches. If foliar symptoms appear at or after growth stage R6, yield loss may be minimal.
While yields might not have been severely impacted, SDS can still become a problem for soybean farmers, especially in the presence of SCN, and in cases where there is no crop rotation. Because SCN causes wounds to the root system, the plants are more susceptible to the Fusarium fungus. There are no soybean varieties completely resistant to SDS, but partially resistant varieties are available. There are seed treatments that are effective, such as the fungicide fluopyram (ILeVO®, Bayer CropScience), which has efficacy against SDS. As a seed treatment, fluopyram has reduced SDS severity and protected yield on susceptible varieties, compared to a base seed treatment in several research trials conducted by extension plant pathologists in the North Central United States.
Planting partially resistant varieties does not ensure complete control of the disease, but it will minimize yield loss. It’s important to remember that weather and field conditions drastically influence disease severity. Testing your fields for an SCN number, as well as purchasing a SCN-resistant variety could delay SDS. Research shows that tillage can have an effect on reducing SDS, but research also shows that no-till can also reduce the disease.
3. Septoria Brown Spot
Last on our list is Septoria brown spot. Brown spot is usually the first leaf disease to occur on soybean in Iowa, and may be observed on soybeans as early as the V2 stage. Reports of brown spot began arriving in late July, when soybeans were between R3 and R4, and mostly with full pods. Unlike many foliar diseases of soybean, brown spot starts in the lower canopy and continues upwards.
Currently in Iowa, brown spot hasn’t been making much of an impact on yields. It’s a disease that’s common in many fields each year, but fungicide application will not likely be profitable unless the disease spreads into the upper canopy. The symptoms you’d observe would be small purple or brown lesions on lower canopy leaves, progressing to irregularly shaped dark brown lesions on higher canopy leaves, as well as leaves yellowing around spots that leads to foliage death. Favorable conditions for brown spot development are warm temperatures (in the 80s) and rainy weather. Considering the wet start and end of the growing season, conditions favored the disease’s development.
To stop brown spot from occurring, it’s important to know that no cultivars are resistant to the fungus. Fungicides are not recommended due to the fact that the disease’s impact on yields is minimal and fungicide application is not likely to be profitable solely for brown spot management. Rotating to a non-host crop such as alfalfa, corn or a small grain is beneficial for reducing disease.
White mold takes the spot of honorable mention, due to the fact that we saw some instances in far northern Iowa, as well as some field plots in Minnesota. While conditions were not conducive to white mold this season, it’s important to be mindful of the fact that the disease is a significant problem in the north central region. The Crop Protection Network has a great disease guide for white mold that I highly recommend those interested to read. In addition, our colleague Damon Smith at the University of Wisconsin-Madison has helped develop a model predictor for white mold, that can help you make a calculated decision on whether or not to spray a fungicide for white mold.
Dr. Daren Mueller is an associate professor and extension plant pathologist at Iowa State University.