MARSHFIELD, Wis. – Top soybean diseases in 2016 in Wisconsin were, in order, white mold, Sudden Death Syndrome, stem canker, brown stem rot, and Phytophthora root and stem rot, says Damon Smith, University of Wisconsin-Madison field-crops plant pathologist.

Smith spoke at the Marshfield session of recent Wisconsin Pest Management Update Meetings around the state. Producers in more northerly growing areas battled white mold; those in southern Wisconsin faced more Sudden Death Syndrome.

Damon Smith

Damon Smith

“Soybeans tend to keep us pretty busy most years,” Smith said of disease pressure and university crop-disease diagnoses.

Smith said producers needed 3.5 to 4.5 more bushels this fall to recover the cost of applying fungicide against white mold, figuring beans at $9 to $10 a bushel and fungicide-application cost at $35 to $45. He said timing is everything in terms of fungicide effectiveness against white mold.

Management recommendations for white mold – technically Sclerotinia stem rot — are based on the amount of disease in the field and the relative resistance or susceptibility of the variety planted.

The university recommends:

Field history of less than 5 percent disease

  • – If white mold is present in pockets, avoid planting susceptible soybean varieties, maintain current row width and plant population, and rotate out of soybeans for at least a year. If disease is spread throughout the field, plant partially resistant varieties, maintain current row width but reduce plant population if planting less-resistant varieties, and rotate out of soybeans for at least a year.

5 percent to 25 percent disease

  • – If the soybean variety is partially resistant, maintain current row width and plant population, but rotate out of soybeans for at least a year. If the variety is moderately susceptible, widen row width to 15 to 30 inches and lower the seeding rate accordingly, rotate out of soybeans for at least a year, and consider treating seed fields with fungicide at flowering.

25 percent to 50 percent disease

  • – Select partially resistant varieties, maintain current row width but lower plant population, rotate one to two years out of soybeans, and consider treating the field with a fungicide at flowering.

More than 50 percent disease

  • – Plant varieties with as much resistance as possible; narrow row spacing may be acceptable for varieties with good resistance. If planting 30-inch rows, plant 125,000 seeds per acre. Rotate two to three years out of soybeans. A fungicide treatment will likely be necessary for susceptible varieties; apply when flowers are present, especially on the lower half of the stems, which is between the R1 and R3 growth stage.

Optimum seeding rates for high-risk fields for white mold are 180,000 plants per acre for 7.5- and 15-inch rows, and 125,000 plants per acre for 30-inch rows.

In a 2016 University of Wisconsin white-mold fungicide-efficacy trial at the Hancock Agricultural Research Station, 9 fluid ounces of Approach at the R1 and R3 growth stages resulted in 7.6 bushels more than the untreated check-plot yield of 74.9 bushels. Four fluid ounces of Endura at R1 followed by 4 fluid ounces of Priaxor at R3 resulted in 7 bushels more than not applying fungicide. Those top two treatments were in the face of what Smith described as fairly low disease incidence but quite a bit of severity.

He also shared 2016 fungicide-timing results for white mold at Hancock. While 9 fluid ounces of Approach at R1 and R3 resulted in 15 bushels more than not treating, 8 fluid ounces of Endura at R3 resulted in 14.4 additional bushels, and 9 fluid ounces of Approach at R3 resulted in 11.6 more bushels. Eight ounces of Endura at R1 resulted in 7.6 additional bushels, and 5 fluid ounces of Proline at R4 turned in 7.5 more bushels.

“R3 (application) gave us better control,” he said. “We had a lot of bloom when the weather was conducive for the pathogen (in 2016).”

If white mold isn’t a concern, applying fungicide hasn’t resulted in significant increases in yield for the fourth straight year in University of Wisconsin trials. However if white mold is a significant problem, Endura, Approach and Proline continue to be consistently efficacious products. Within the R1-R3 optimal-timing window, the R3 application timing worked best in 2016. R4 or later applications in most instances didn’t work very well. Smith and fellow crop-disease experts are working on a white-mold prediction model they hope to have ready by 2018 to help producers make decisions on the R1-R3 fungicide application timing.

Sudden Death Syndrome, which posed a problem for southern-Wisconsin growers in 2016, is relatively new in Wisconsin — but nationally it’s one of the top-five yield-limiting diseases. Symptoms are yellow-to-brown discoloration of leaves around veins, which can be mistaken for brown stem rot. To differentiate the two, Sudden Death Syndrome typically expresses earlier in reproductive growth than brown stem rot. Also dig intact roots and split the stem. Sudden Death Syndrome doesn’t lead to brown discoloration of vascular and pith tissue like brown stem rot does. When Sudden Death Syndrome is severe, it’s possible to see blue coloration on the taproot, which is growth of fungus.

The Sudden Death Syndrome pathogen overwinters in soybean debris as resistant fungal spores. The disease is favored by wet soil during vegetative growth and wet cool weather around flowering. Low spots in fields are favorite areas for Sudden Death Syndrome.

There’s a known association of increased Sudden Death Syndrome severity when soybean cyst nematode is present, though each can occur without the other. The university recommends sampling for soybean cyst nematode in any field where a grower notes Sudden Death Syndrome. Management includes resistant cultivars. However little varietal-tolerance information exists in soybeans of maturity group 2 and earlier soybeans in Wisconsin.

Tillage might be considered to increase soil temperature and dry the soil more quickly. Crop rotation doesn’t seem to have much impact on Sudden Death Syndrome; outbreaks have occurred after other crops, including corn. Seed treatments like ILeVO and Mertec 340-F appear to suppress early infection and reduce yield loss.

Visit http://ipcm.wisc.edu/blog/2016/08/sudden-death-soybean-video for information and a video on Sudden Death Syndrome. Contact Smith at damon.smith@wisc.edu or 608-262-5716 for more information.

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