Updated July 6, 2018:

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Michael Wunsch, Ph.D. - Plant Pathologist
North Dakota State University
Carrington Research Extension Center

Project Title:
Optimizing row spacing and plant populations for management of Sclerotinia in soybeans

Executive Report:
Seeding soybeans to narrow and intermediate rows facilitates faster canopy closure, increasing conversion of sunlight into biomass and increasing soybean yield potential. However, it also increases the risk of Sclerotinia (white mold) by trapping additional humidity within the canopy, and producers concerned about white mold often seed soybeans to wide rows (generally 30 inches), sacrificing soybean yield potential in the absence of white mold in order to reduce disease-related losses if white mold develops.

Previous research conducted in Michigan, Wisconsin and Ontario confirms that wide rows reduce white mold but indicates that seeding soybeans to wide rows often results in lower soybean yields even under white mold pressure. In two of three multi-year field studies, soybean yields were maximized in narrow to intermediate rows despite increased white mold.

This multi-year, multi-location study sought to determine when the use of narrow and intermediate row spacing maximizes soybean yield under white mold and to quantify the implications for soybean quality. Field studies were established under overhead irrigation at sites near Carrington, Langdon, Oakes, and Williston in 2015, 2016 and 2017. Each year, two to six locally adapted soybean varieties representing a mix of upright and bushy types were evaluated at each location in each of four row spacings (7, 14, 21, and 28 inches or 7.5, 15, 22.5, and 30 inches) and three seeding rates (132,000; 165,000; and 198,000 pure live seeds/ac).

Narrow and intermediate row spacing increased white mold but almost always maximized soybean yield when end-of-season incidence of white mold was less than 50% (Figure 1). The increase in white mold associated with narrow or intermediate row spacing increased contamination of the harvested grain with sclerotia (resting structures of the Sclerotinia fungus) but the increased contamination with sclerotia was small (less than 0.1% by weight) when end-of-season white mold incidence was less than 40% and never resulted in a reduction in soybean market grade until white mold incidence exceeded 45% (Figure 2).

The results indicate that seeding soybeans to wide (28 or 30-inch) rows is unlikely to be profit-maximizing under white mold pressure unless disease incidence is expected to exceed 50 percent at the end of the season. When end-of-season Sclerotinia incidence is less than 50 percent in soybeans seeded to narrow and intermediate rows, the yield gain conferred by faster canopy closure generally exceeds the yield loss conferred by increased white mold in narrower rows.