Sclerotinia rot (Sclerotinia sclerotiorum anam. Sclerotium varium) is a fungus that belongs to the Sclerotiniaceae family. The fungus occurs in a wide range of plants, such as green beans, endive, celeriac, lettuce, oilseed rape, peas, potatoes, flax, chicory, dahlias and sunflowers. Warm, damp conditions are particularly conducive to sclerotinia rot, causing local wilting and dying off in infected plants. The sclerotia drop to the ground and can still germinate and affect new plants 5-10 years later. Sclerotinia rot infection is highly weather-dependent. In spring, the sclerotia germinate once temperatures rise above 5 °C (ideally 10-20 °C) and damp conditions prevail for 10 days. Warm, damp conditions during flowering are conducive to the formation of apothecia (carpophores) from the sclerotia. One sclerotium can produce numerous saucer-like apothecia with a diameter of 10 mm, in which water collects. The water later evaporates, releasing the ascospores. The apothecia eject 2-30 million ascospores a day, which can subsequently germinate if they land on leaf axils and lateral shoots of host plants. The ascospores are not capable of infecting healthy tissue. The fallen petals encourage the ascospores to germinate and serve as a culture medium. The fungus subsequently invades the plant and necrosis (dying tissue) will become visible within a matter of days. The dying plant cells in turn provide nutrients for the fungus. A white fluffy mycelium begins to grow on the stems and, in the case of beans, on the pods, containing black sclerotia which resemble rat droppings. These sclerotia drop to the ground where, after a resting period of anywhere between a few weeks and several years, they germinate. Generally speaking, seed treatment has no effect on these fungi.
