Sclerotinia sclerotiorum, also known as cottony soft root, is a plant pathogenic fungus. It has a very wide range of host plants.
Life cycle and appearance of Cottony soft rot
Cottony soft root (Sclerotinia sclerotiorum) overwinters as sclerotia and mycelium in dead and living plant material in or on the ground. Sclerotia germinate at temperatures between 4 and 26 °C with an optimum between 13 and 15 °C. Germination and infection mostly take place at high soil water content over a prolonged period of time.
The sclerotia germinate directly and form mycelium or produce apothecia (fruiting bodies) containing ascospores. The mycelium infects the stem base and the roots, whereas the ascospores infect the above-ground plant parts. In greenhouses, usually only mycelium is produced by the germinating sclerotia. The fungus grows quite rapidly inside the plant and kills the host cells. On the infected plant tissue white, fluffy mycelium occurs with new dark sclerotia. The fungus also infects seeds and tubers, thereby creating a source of infection for the next crop. The dispersal to neighbouring plants is by mycelial growth.
The first symptoms of a cottony soft root (Sclerotinia sclerotiorum) infection are water-soaked lesions at the stem base. The plant above it wilts and rots. White, fluffy fungal mycelium grows on the lesions, giving this disease its name cottony soft rot. In the mycelium, big, dark sclerotia are formed. They can easily be seen with the naked eye and look like rat droppings. The symptoms may occur on stems but also on leaves, fruits and pods.
Root infection often leads to premature wilting and death of plants.