White Mold in soybeans is probably the number one problem facing soybean growers in the Upper Midwest. What makes it worse is that everything you do to increase yield also increases the likelihood of having white mold as a problem.
White mold winters in the soil. It sends spores up from the ground in late June - early July. If conditions are cool and wet at that time the spores can infect the soybean plant and colonize it, eventually killing it.
White Mold is present in most fields naturally to some degree. Increased occurrence of this disease is probably due to populations building over time as the ground, over the years, is planted to soybeans, snap beans and other host plants .
In the past some of the spreading may have been due to white mold carried in seed. Today any seed company with good equipment should be able to remove the sclerotia (black mouse dropping shaped bodies, which carry the disease back to the soil). Bin run soybeans have a high chance of spreading the disease as they tend to be not as clean.
Control of white mold is nearly impossible because it has so many host plants that it can infect and continue its reproductive cycle. In addition to soybeans and snap beans it can also infect over 90 weed species, trees and shrubs. In addition it is very resistant to fungicides and survives in the soil for years. Unfortunately, once it establishes itself in your field it is there to stay.
There are some control options but I have a hard time recommending them.
You can deep plow but that only works once. Once you deep plow again it will bring the white mold up to the surface to infest once again.
Use of wider rows will slow the canopying of the soybeans reducing the favorable environment white mold needs but the trade off will be in reduced yield and the potential for increased weed pressure.
When you want high yields with your soybeans you want to plant early, plant thick and use narrow rows. What this does is increase the height of your beans and make them canopy earlier. The early flowering widens the window for infection and the taller quicker canopying keeps the environment close to the ground cooler and wetter which will help the white mold.
So do you go for high yield or white mold control? My advice is to still push for yield over white mold. On average you should be able to gain 2-5 bushels of yield by planting early, thick and narrow. With white mold you could see anywhere from 2-15 bushel loss but white mold is not a factor every year which means as long as you don’t have big losses from white mold every other year you are better off going for high yields.
The best advice is to plant a variety with high yield and high white mold tolerance. Currently there is no gene that gives white mold resistance, although; you may see one soon. Current tolerance is simply a measure of historical levels of infestation in a variety. A varieties resistance is probably most dependent on when it flowers and when it canopies.If a variety is flowering or canopied when white mold spores are flying even highly tolerant varieties can be hit hard with white mold. Picking a high yielding and high tolerant line should maximize your return under all conditions.