Update:
Sclerotinia disease pressure was moderate in field trials conducted in 2016. Average Sclerotinia incidence (across all row spacing and seeding rate treatments) ranged from 2 to 21% across soybean varieties.
• In Oakes, where hot weather during early bloom and cool weather during mid to late pod-fill resulted in late disease onset, wide row spacing conferred a decrease in end-of-season Sclerotinia incidence in upright soybean varieties exhibiting delayed canopy closure but not in bushy soybean varieties, and seeding rate had no impact on Sclerotinia disease levels. The adoption of narrow (7-inch) or intermediate (14- or 21-inch) rows conferred sharp (7 to 11 bu/ac) increases in soybean yield. Increasing seeding rate from 132,000 to 198,000 pls/ac was associated with a loss of 1 bu/ac in one variety and gain of 3 or 4 bu/ac in three varieties.
• In Langdon, wide row spacing was associated with increased Sclerotinia incidence in one variety and had no impact on Sclerotinia disease levels in two varieties; seeding rate had no impact on Sclerotinia disease levels. The adoption of narrow (7-inch) or intermediate (14- or 21-inch) rows conferred yield gains of 1 to 8 bu/ac. Increasing seeding rate from 132,00 to 198,000 pls/ac resulted in no change in yield for one variety and a gain of 3 bu/ac in two varieties.
• In Williston, wide row spacing (30 inches) conferred a statistically significant decrease in end-of-season Sclerotinia disease levels (P<0.05) in a soybean variety with upright architecture but not in a bushier variety. Increasing seeding rate from 132,000 to 198,000 pls/ac had no impact on disease levels. The adoption of narrow (7.5-inch) or intermediate (15- or 22.5-inch) rows was associated with a -2 to +1 bu/ac change in soybean yield, depending on variety. Increasing seeding rate from 132,000 to 198,000 pls/ac resulted in a loss of 1 bu/ac in one variety and no change in yield in one variety.

Combining results from multi-location field trials conducted in 2013, 2014, 2015, and 2016 facilitated a preliminary identification of thresholds at which intermediate row spacing (14 to 22.5 inches) optimizes soybean yield under Sclerotinia disease pressure without reductions in market grade due to contamination of grain with sclerotia (resting structures of the Sclerotinia fungus).
• Impact of row spacing on Sclerotinia: Except when Sclerotinia incidence was low (< 13%), changing from wide (28 to 30-inch) to intermediate (14 to 15-inch or 21 to 22.5-inch) row spacing consistently increased Sclerotinia incidence.
• Impact of row spacing on soybean yield in soybeans grown under Sclerotinia disease pressure: With very few exceptions, when Sclerotinia incidence was less than 50% in soybeans seeded to intermediate row spacing (14 to 15-inch or 21 to 22.5-inch), yields were maximized when soybeans were seeded to intermediate row spacing. When Sclerotinia incidence exceeded 50% in soybeans seeded to intermediate row spacing, yields were often (but not always) maximized by seeding soybeans to wide (28 to 30 inch) rows.
• Impact of row spacing on contamination of soybean grain with sclerotia (resting structures of the Sclerotinia fungus) in soybeans grown under Sclerotinia disease pressure: When Sclerotinia incidence in soybeans seeded to intermediate row spacing (14 to 15-inch or 21 to 22.5-inch) was below 40%, contamination of the grain with sclerotia (resting structures) of the Sclerotinia fungus rarely increased by more than 0.1 percentage points relative to soybeans seeded to wide (28 to 30-inch) rows. It never increased by more than 0.5 percentage points, and the increased contamination was never sufficient to result in a change in soybean market grade. When Sclerotinia incidence in soybeans seeded to intermediate (14 to 22.5 inch) row spacing exceeded 40%, the adoption of intermediate (rather than wide) row spacing was often associated with a reduction of soybean market grade due to increased contamination of the grain with sclerotia.
• Impact of seeding rate on soybean performance under Sclerotinia pressure: Increasing seeding rate from 132,000 to 198,000 modestly increased the risk of white mold but was also often associated with slightly higher yields. The increased Sclerotinia disease pressure associated with the higher seeding rate reduced soybean market grade due to increased sclerotia in the grain once out of 14 varieties evaluated across multi-location trials in 2015 and 9 varieties evaluated across multi-location trials in 2016.

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

Executive Report:

In fields where Sclerotinia (white mold) is a concern, soybeans are often seeded to wide rows (generally 30 inches) to minimize disease development. However, previous research suggests that even under significant white mold disease pressure, soybean yields may often be maximized by seeding soybeans to narrower rows. The goal of this study is to identify how row spacing impacts soybean yield and market grade under different levels of white mold disease pressure. Testing was conducted at three seeding rates representing the range of seeding rates commonly employed by North Dakota producers.

Field studies were established at the NDSU Carrington, Langdon, and Williston Research Extension Centers and at the NDSU Robert Titus Research Farm in Oakes. Fourteen soybean varieties representing a mix of upright and bushy types were evaluated in four row spacings (7, 14, 21, and 28 inches or 7.5, 15, 22.5, and 30 inches) at each of three seeding rates (132,000; 165,000; and 198,000 pure live seeds/ac). To create conditions favorable for white mold, supplemental overhead irrigation was applied. A severe hail storm on July 9 in Carrington defoliated the soybeans at bloom initiation, precluding disease development, but studies at the other locations were successful. Results from 2016 were combined with data from 2015 and from preliminary studies conducted in Carrington in 2013 and 2014.

Wide row spacing minimized Sclerotinia incidence but often did not optimize soybean yield. When Sclerotinia incidence was less than 50% in soybeans seeded to intermediate (21 or 22.5-inch) row spacing, yields were maximized with the intermediate row spacing. When Sclerotinia incidence in soybeans seeded to 21- or 22.5-inch row spacing was below 40%, the increased contamination of the grain with sclerotia (resting structures of the Sclerotinia fungus) associated with the narrower row spacing was never sufficient to result in a reduction in soybean market grade. Parallel results were observed for soybeans seeded to 14- and 15-inch rows. Increasing the seeding rate from 132,000 to 198,000 pure live seeds/ac contributed to a modest increase in Sclerotinia disease pressure but also a modest increase in soybean yield.

The results suggest that if Sclerotinia incidence is not expected to exceed 40 to 50 percent, seeding soybeans to an intermediate row spacing (14 to 22.5 inches) is likely to maximize soybean yield under white mold pressure without reductions in soybean market grade.