Figure. 1. Apothecia of the white mold fungus on the soil surface. The structures are 1/4 to 1/2 inch in diameter.

When to scout
Scout plants for signs of the apothecia at canopy closure, generally the last week of June and first week of July. Canopy closure will be mainly determined by the row spacing. The most susceptible areas will be those where moisture collects due to fogs and extended dew periods, and pockets of poor air drainage.

Foliar symptoms at the canopy level later in the season are not very diagnostic. Leaves wilt, and the tissues between major veins develop a gray-green cast while vein tissues remain green. Leaves yellow, eventually die and turn completely brown, but often remain attached to the stem past maturity. These symptoms can be seen at pod setting in August and September.

Foliar symptoms of white mold can easily be mistaken for brown stem rot, Phytophthora root rot, sudden death syndrome, or stem canker.

Symptoms
Although symptoms of white mold can be observed at canopy level about 6 weeks after flowering, a closer inspection would have revealed symptomatic plants 3-4 weeks after flowering. Weather, canopy microclimate and soybean variety will modify this timetable of symptom development. Rain, cool temperatures, high humidity, and moist soil in a closed canopy favor the growth of the fungus.

The white mold disease cycle begins with the formation of tan, mushroom-like structures called apothecia. They form on the soil surface when the soil is moist and dim light is filtered through the crop canopy (Figure 1).

Figure 2. The common "bird's nest fungus" on the soil surface can be mistaken for white mold apothecia.

Apothecia of the white mold pathogen are frequently confused with reproductive structure of the bird's nest fungus, a harmless fungus that grows on decaying organic matter (Figure 2). The common name of this fungus is derived from the brown, nest-like structures.

Spores from the apothecia infect senescing soybean flowers and the fungus eventually grows to the soybean stem. Examine the stems for signs of the white mold fungus, which begin as gray to white lesions at the nodes. Lesions are 3-18 inches long, rapidly progress above and below the nodes, and can eventually encircle the whole stem.

White fluffy mycelium often cover the lesions, especially during periods of high relative humidity. This "white mold" is a good sign to look for (Figure 3). The characteristic black sclerotia are, in time, readily visible in lesions or inside the stem (Figure 4).

Figure 3. White fluffy mycelium or "white mold" is a diagnostic sign of infection.

Figure 4. Hard, dark sclerotia are formed in lesions or inside the soybean stem

Soybean field with symptoms of white mold.