White mold of soybeans
Affected plants turn yellow and quickly become distressed. On the stems of these plants, whitish-gray to light brown, stem-encircling discolorations are visible on the lower part. The interior of the stem beneath the discolored tissue is usually hollow. If the stem is cut open lengthwise, black, up to 2 cm large and sometimes even larger fungal survival structures (= sclerotia) and a white, cotton-wool-like network of fungal threads (= mycelium) are found inside the stem. In high humidity or persistently wet weather, the mycelium and the sclerotia appearing on it are also formed on the outside of the stem.
Furthermore, the pods can also be infested. In this case, a white mycelium and sclerotia can also be formed on the outside of the pods or on the inside.
Propagation and transmission
Sclerotinia sclerotiorum forms 5 to 20 mm large and sometimes even larger sclerotia, with the help of which the fungus can survive in the soil, on infested plant remains or on perennial weeds. In the soil, the survival structures of the fungus can remain viable for up to ten years.
The sclerotia, which are located near the soil surface, begin to germinate in the spring. These germinate either directly with mycelium, which can penetrate into the plants via the roots or the root neck, or from the sclerotia grow one to a few centimeters large, stemmed and cup-shaped fruiting bodies (= apothecia) with spores (= ascospores) serving for reproduction. For the germination of the sclerotia, temperatures between 6 and 10 °C must prevail. Shading of the sclerotia and moist soil are optimal for their germination.
Ascospores are shed from the fruiting bodies and spread with the wind. These can infect plants through weakened tissue and/or wounds. Fallen petals caught in leaf forks and side shoot axils favor settlement of spores and their germination. Infections take place in cool and damp weather. The optimal temperature for fungal growth is 20°C, but the fungus can still grow at 0°C.
In addition to independent spread, the fungus can also be transmitted through the seed.
Prevention and control
- Use of healthy seeds
- No cultivation of soybeans in wind-protected and humid locations
- Ensure the earliest possible harvest through timely cultivation
- Crush plant residues after harvest and bury them to a depth of at least 10 cm
- Maintain a crop rotation of at least four years (six years is better) and pay attention to the susceptibility of previous crops. Cereals, corn, beets and potatoes are not susceptible or are less susceptible.
- Control weeds, as they are potential host plants.
- Possibly use plant protection products to control sclerotia disease (see list of plant protection products approved in Austria).
Last updated: 10.12.2021