Martin Chilvers, Associate Professor and Field Crop Pathology Specialist, Jill Check, PhD candidate, Austin McCoy, postdoctoral associate, Mike Staton, MSUE soybean educator
The answer to this question will depend upon three factors that must come together, the prevailing weather, presence of the white mold fungus and susceptibility of the soybean variety.
White mold of soybean is an annual problem for some producers and can be particularly severe when conditions are cool and wet during flowering. This disease is often most problematic in soybean fields having productive soils, planted at a high population with narrow row spacing (<30 inches). Canopy closure provides a humid, shaded environment that is conducive to white mold apothecia (mushroom) formation and plant infection, especially in cool, wet years. No soybean variety is completely resistant to this disease, however, as shown in the figure below, even within a company there are varieties which are far more tolerant and perform better under white mold disease pressure. Selecting a variety that is tolerant to white mold, as well as planting lower soybean populations and utilizing wider row spacing can help manage the severity of this disease. Company catalogues do provide white mold ratings, but it is always wise to check with your seed dealer, especially if you have had issues managing white mold in the past. Knowing variety susceptibility can also help inform in-season management decisions such as fungicide applications.
The weather that we receive shortly before and into July will heavily dictate the degree of white mold disease that develops later in the season. A cool (<85o F), wet July means favorable conditions for the white mold fungus to produce apothecia (mushrooms) that release spores and infect the plant primarily through the flowers or fallen flower petals. Disease won’t be readily apparent until August when cottony mycelia of the white mold fungus start to be produced from infected plants and plants begin to wilt. In addition, producers that irrigate will be at increased risk of white mold and should consider less frequent but heavier waterings. Irrigators should also hold off watering as much as possible until the R3 growth stage (one pod 3/16” long on the upper four nodes of the main stem having a fully developed leaf node).
Finally, the third in the trifecta of white mold disease development is the presence of the white mold fungus. Unfortunately, the white mold fungus can survive for many years in the soil as sclerotia. Seed lots can also be infected or infested with the white mold fungus and can act as another source of inoculum. White mold inoculum typically does not move very far from the source. However, in the white mold epidemic of 2014 it did appear that the excessively foggy, wet, and cool July may have seen dispersal of white mold spores into fields with no previous inoculum source. Tillage can also play an important role in inoculum movement and survival. Tilling fields will bury the sclerotia resting bodies where they are more likely to survive and then germinate once brought to within the top inch or two of the soil profile. Sclerotia buried deeper are unlikely to be able to produce an apothecia (mushroom) on the soil surface. Under no-till systems there is evidence that sclerotia do not survive as well on the soil surface.