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Department of Agronomy

Kansas State University

1712 Claflin Rd.

2004 Throckmorton PSC

Manhatan, KS 66506

785-532-6101

agronomy@ksu.edu

Extension Agronomy

Management of soybean diseases in Kansas

(Editor’s Note: The following is from the K-State Research and Extension publication Kansas Soybean Management 2015, MF-3154, available online at: http://www.ksre.ksu.edu/bookstore/pubs/MF3154.pdf)

 

Numerous soybean diseases attack soybeans throughout the growing season. Long-term estimates predict a 12.5 percent increase in soybean yields in Kansas if diseases could be eliminated. Approximately 25 diseases might occur in any given year, a much smaller number are responsible for the bulk of disease losses.

Early in the season, seed rots and seedling blights reduce yields an average of 2.5 bushels per acre. The responsible pathogens primarily include Pythium, Rhizoctonia, and Fusarium, although occasionally others can be involved. Seed treatment is an effective means of dealing with seedling blights. Numerous products are available that provide good to excellent control of these early-season problems. Use products containing two or more active ingredients to broaden the spectrum of control.

General recommendations for seed treatment are that all fields planted before May 15 should be treated with a fungicide. Use seed treatment in no-till fields at least through May 31. With the expense of high-tech seeds, many growers are now using seed treatment as insurance even on double-cropped soybeans.

Two other important diseases, soybean cyst nematode and soybean sudden death syndrome are best managed at planting. That is because resistant varieties are the best way to manage both diseases. Research has shown that soybean cyst nematode is a predisposition agent to sudden death syndrome, that is, you rarely see a field infected with sudden death syndrome that does not already have soybean cyst nematode in it.

A recent two-year survey indicated that approximately 20 percent of Kansas soybean fields are infested with soybean cyst nematode. In two counties, Cherokee and Doniphan, that number is near 100 percent. Unfortunately, fewer than 10 percent of growers indicate they soil test for soybean cyst nematode. While nearly all soybean varieties have some level of soybean cyst nematode resistance, their effectiveness in the field can be highly variable. Growers should continuously monitor nematode levels in known infested fields to make sure appropriate varieties are being grown. It is recommended that all fields be tested after every third soybean crop to confirm the nematode has not become established, or that it is being properly managed.

Recently, seed treatments have become available for soybean cyst nematode control and a new, soon to-be-registered product shows promising control of sudden death syndrome. The cost effectiveness of this product is currently being evaluated.

The most significant soybean disease in Kansas is charcoal rot. While this pathogen infects soybean roots early in the season, it does not make itself known until the reproductive stages of growth when hot, dry weather occurs. Under heat and drought stress, the fungus becomes active and slowly kills the plant. Plants that die prematurely typically have smaller seeds and reduced yields. While all soybean varieties are susceptible, some are more susceptible than others.

Careful observation of varietal differences can be useful in management. Also, shorter MG varieties tend to express disease symptoms more than late MG varieties. Irrigation and any type of moisture-saving cultural practices can reduce disease losses. The most effective management strategy is to reduce seeding rates to approximately 100,000 seeds per acre. At this rate, there are fewer plants competing for moisture in a dry year. In wet years, plants still have the ability to branch and compensate for fewer plants per acre.

In 2014, Kansas suffered its most severe outbreak of Phytophthora root rot in many years due to soaking rains for much of early June. Resistant and field tolerant varieties are the best means of management.

There are several foliar and late-season stem and pod diseases that reduce soybean yields. These include frogeye leafspot, brown spot, pod and stem blight, anthracnose, and Cercospora leaf blight/purple seed stain. Fungicides can be profitable in certain instances, most notably for frogeye leafspot control. Pod and stem diseases are tricky to manage because at the time fungicides need to be applied, it is not apparent as to whether or not the diseases are likely to appear. Pod and stem diseases are favored by late-season rains.

When a fungicide is necessary, it should be applied at the R3 to R5 growth stage for maximum effectiveness. Growers should be cautious about overuse of strobilurin fungicides. Strobilurin-resistant frogeye leafspot has already been reported in 11 states; fortunately, Kansas is not yet one of them.

Soybean rust has only occurred once in Kansas, in 2007, since its introduction into the United States in 2004. Each year, its spread is tracked through a national reporting network. Should it threaten Kansas in the future, numerous outlets will update growers as to the need for fungicide usage.

All other diseases, including bacterial blight, downy mildew, aerial blight, Sclerotinia white mold, stem canker, bean pod mottle virus, bud blight, and soybean vein necrosis virus occur either too infrequently to warrant control, or there are no effective control measures.  

 

Doug Jardine, Extension Plant Pathology
jardine@ksu.edu

From Kansas Soybean Management 2015, MF-3154, available online at: http://www.ksre.ksu.edu/bookstore/pubs/MF3154.pdf