Plant Health Initiative

White Mold - Scouting

Figure 1. These mushroom-like structures on the soil surface are the beginning of the white mold disease cycle

Figure 2. These bird's nest mushrooms on the soil surface can be mistaken for apothecia of the white mold fungus.

Figure 3. White mycelium covering a soybean stem. Photo credit: Craig Grau, University of Wisconsin

Sclerotia are hard, dark, structures that form on the inside and outside of the stem. Photo credit: Craig Grau, University of Wisconsin

The white mold disease cycle begins when sclerotia of Sclerotinia sclerotiorum germinate to form mushroom-like structures about 1/3 inch in height called apothecia (Figure 1).

Under the mushroom cap of the apothecia, microscopic spores (ascospores) are produced which are ejected onto the dying flower petals adhering to young pods. Moderate air temperatures and frequent rain events during flowering favor germination and infection at this stage.

Scouting for mushrooms (apothecia)

The best time to scout for apothecia is at canopy closure. In the North Central region, this is generally in late June and early July, and will vary with row spacing. Check high soil moisture areas, especially during flowering. Check areas where moisture collects due to fogs, extended dew periods, and in pockets of poor air drainage.

Sometimes the apothecia of the white mold pathogen are confused with reproductive structure of the bird's nest fungus, a harmless fungus that decays organic matter (Figure 2). Scouts should be aware of the difference between these two fungi.

Scouting for white mold-infested plants

During pod set in August and September, scout for signs of infected plants.

Inspect stems for lesions, which may be covered with a thick white mold, the vegetative body of the fungus (Figure 3).

As the fungus grows from the young pods to the nodes and stem of the plant, it eventually girdles the stem and disrupts the transport of water and nutrients. Once the fungus girdles a stem, all the plant tissue up from the infection dies. Leaves and stems turn brown and stand erect above the soybean canopy.

Hard, dark structures called sclerotia (Figure 4) form on the inside and outside of the stem and pods. The sclerotia fall to the soil during harvesting or remain in crop debris where they survive for many years.

In the spring, sclerotia may germinate to form the apothecia, starting the disease cycle again.