Elongated light-colored spots are visible after bloom. They first appear on one side and later surround the stem. The infection spreads from the leaf and stem axils.
White mould (bean) [Sclerotinia sclerotiorum]
Broad host plant spectrum (over 360 dicotyledonous plant species), for example sunflower, oilseed rape, celery, lettuce, broad beans, shepherd's purse, and field penny-cress.
Pattern of damage
The spots appear pale yellow to whitish, with grayish centers. The cortex and pith of the stem are destroyed by the fungus, causing the upper branches to die (Picture 1). The infected plants with whitish stems are readily distinguished from the green, healthy plants (Picture 2). Light gray, irregularly-shaped dormant fruiting bodies that eventually turn black can be found in the interior of the stem. Except for a small amount of cottony mycelium, the stems are hollow. Infected pods are pale yellow and desiccate. Small sclerotia can be found between the seeds in the pods. In the early stage of stem infection, white mold can be confused with gray mold disease.
Spread/transmission: The fungus, which can survive in the soil for 7 to 10 years in the form of sclerotia (dormant bodies), can infect the plant at the soil surface or in the uppermost soil layers. Adequate soil moisture and temperature of 7-11 °C are necessary for germination. Small, light brown, trumpet-shaped fruiting bodies emerge from the sclerotia and release ascospores. These spores are transported to the host plant by wind. The fungus germinates and sporulates during plant flowering. Some fallen petals collect in the leaf and stem axils and serve as a breeding ground. Wet-foliage periods of 16 to 24 hours, 85% relative humidity, and temperatures between 0 and 25 °C are required for infection to occur. Warm and periodically wet weather during bloom thus constitute favorable infection conditions. Direct infection of the plant by mycelium is also possible but less common. The mycelium develops in the upper soil layer and infects roots and stem tissue. In the interior of the stem, a mycelium starts to develop and eventually forms dense, compact sclerotia. During harvest, the sclerotia fall to the ground along with the harvest residue and are able to infect new plants.