The yield potential of soybean is increased by cultural practices that promote faster closure and greater density of the crop canopy. Greater leaf area and light interception are probable explanations of why higher yields are obtained in plantings with dense canopies. Improved weed control also is an important benefit of fast-closing and dense canopy structure. Unfortunately, anticipated higher soybean yield is not always obtained in high management cropping systems because of white mold.

Faster and more complete canopy closure, associated with high-yield environments, is suspected to result in environmental conditions optimal for development of white mold. The threat of white mold has many soybean growers questioning recommended management practices intended to increase yield. The use of a moderately resistant variety can allow the continued use of management practices intended to increase the yield potential of soybeans.

Early planting date, narrow row width, and high plant populations (>200,000 plants/acre) all accelerate canopy closure and favor activity of the white mold fungus. Bushy soybean varieties and lodging also create a dense, closed canopy which favors white mold growth.

Row width
Row width will determine how quickly and densely the soybean canopy closes. Recent UW research indicates that soybean yield can increase up to 10-20% when planted in drilled (7-10 inch) rows with no white mold pressure (Table 1). In the presence of white mold, however, expected yield increases can sometimes be negated (Table 1). This is because narrow rows lead to a more dense canopy than wide (30 inch) rows, creating an environment that encourages the growth and development of the white mold fungus. A high incidence of white mold usually develops in narrow-row plantings when weather conditions are highly favorable for white mold development.

Although disease incidence is frequently higher in narrow compared to wide row width plantings, yield frequently is similar in wide and narrow row systems. When severe white mold develops in a narrow-row system, the usual perception is that disease incidence would have been less, and yield greater, if the soybeans would have been planted in wide rows. Multiple year trials in Wisconsin suggest this may not be the case. Yield frequently is similar, and in some studies still higher in narrow rows compared to wide row systems in white mold situations. Much of the yield advantage of narrow-row systems can be maintained by planting a moderately resistant variety (Table 1).

Table 1. Summary of soybean performance and incidence of white mold at different white mold potentials in 14 trials planted in narrow (7.5 inch) and wide (30 inch) rows.

Canopy closure occurs earlier in narrower row widths and the microclimate within the canopy favors white mold development. A general observation is that white mold severity is greater in narrow-row width plantings. This is a direct consequence of the earlier observation that canopy acts to buffer and sustain environments favorable to apothecia formation and white mold development.

The relationship between narrower row widths and higher yields is not present at all locations and regions. Varieties react differently to white mold in narrow and wide row widths. Even though higher yields may be obtained in narrow row plantings, increased disease may increase soil inoculum levels. However, we suggest that the sclerotia produced by plants in narrow-row plantings may be smaller and less viable than larger sclerotia produced in plants grown in wider row plantings.

High yields in narrow-row width plantings may result in less pathogen reproduction, even though higher level of disease may result compared to row width plantings. We generally recommend that varieties with greater resistance be chosen for narrow row-width planting if the risk of white mold is present.

Seeding Density
Plant populations above 200,000 plants per acre frequently result in higher yields. When white mold is present, however, the yield benefits one expects from high plant populations can be negated (Table 1). Wisconsin studies suggest that white mold incidence is slightly higher in soybean fields planted with high seed density (>200,000) and final plant populations of 200,000 plants per acre (Table 3).

Table 3. Effect of seeding rate and plant population on white mold incidence and soybean yield in Wisconsin.

Although management for high yield such as high seeding density can lead to greater white mold incidence, variety selection can reclaim some of the lost yield potential. As with row width, white mold negates the higher yield potential expected with plant populations above 225,000 plants per acre, but a moderately resistant variety can reclaim a portion of the lost yield potential.