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Minnesota Crop Conditions - Sept. 10, 2017

The USDA Minnesota crop condition report for the week ending September 10, 2017  reports that ninety-six percent of the corn crop was at or beyond the dough stage while seventy-one percent of the corn crop reached the dent stage, nine days behind last year and five days behind the five-year average. There were scattered reports of corn reaching maturity in southern Minnesota. Corn for silage harvest has begun and is ten percent complete, eight days behind last year.
Corn condition improved slightly to eighty-two percent good to excellent. Forty-seven percent of the soybean crop was turning color with thirteen percent dropping leaves compared to twenty-five percent for the five-year average. Soybean condition declined slightly to seventy-two percent good to excellent.

White Mold Disease in Soybeans
Recently in Minnesota, many soybean growers have noticed soybean plants in their fields exhibiting white mold (sclerotinia stem rot) disease symptoms. White, cottony mold can be seen on the lower stem and black, hard sclerotia may be present. These sclerotia can also be embedded inside the stem. Leaves wilt and turn grayish green between veins but remain attached to the plant.
Yield loss is more severe when plants die prematurely or stems are girdled. In addition to causing yield loss, white mold can impact seed quality and reduce grain price because of foreign material at the elevator if sclerotia are present. After harvest, check seed lots for sclerotia and infected seeds. Infected seeds are usually smaller, lighter, white, and cottony.

Life Cycle of White Mold
Sclerotinia sclerotiorum survives in the soil as sclerotia, which are hard, black structures that resemble mouse droppings. When soils are shaded, moist, and cool (40-60°F), sclerotia within the top two inches of the soil profile can germinate to produce apothecia. Apothecia are small (approximately 1/8 to 1/4 inch in diameter), tan, cup-shaped mushrooms. Apothecia produce millions of spores called ascospores that typically infect soybean plants through senescing flowers. Infection is favored by cool maximum daily temperatures (lower than 85°F) and moisture from rain, fog, dew, or high relative humidity. A dense canopy during flowering (growth stages R1 through R3) may provide an ideal microenvironment for white mold development.
Taking accurate notes about where and how much white mold occurs in each soybean field is important for future disease management planning. Tracking disease levels across years will also help determine the potential sclerotia (inoculum) load and the disease risk that may be present in a particular field.

Management of White Mold
Seasonal and long-term factors favoring white mold risk in soybean include a high yield potential crop with a dense canopy, planting a susceptible variety in a field with a history of white mold, and a history of susceptible crops in the rotation. Factors favoring a dense canopy and white mold risk include early planting, narrow row width, high plant populations, and high soil fertility.
Choosing resistant soybean varieties is the best method of reducing white mold, especially in conjunction with other best disease management practices. Fields at high risk for white mold at flowering stage (R1) may require a fungicide application and possibly a follow-up application at beginning pod stage (R3) in severe epidemic conditions.
Source: Dave Nicolai, Extension Educator, Crops, UMN Extension
References: White Mold, Crop Protection Network CPN – 1005, Soybean Disease Management 2015

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