- Survival and sources of inoculum

Sclerotinia sclerotiorum can survive in the soil for several years thanks to the many large sclerotia it produces on infected tissues (especially in the stem, figure 1) and / or the mycelium present in plant debris. Moreover, it is a very polyphagous fungus that can infect over 360 different plant species, all of which then serve as sources of inoculum. Among the cultivated host plants, sometimes grown in rotation with tobacco, are sunflower, alfalfa, soybean, rapeseed, and some vegetable crops


- Penetration into the plant and invasion

Infection can occur in two ways. One is from the mycelium from germinating sclerotia. This infection occurs at the soil level where plant stems are in close contact sclerotia. The other is through ascospores released from apothecia formed on sclerotia near the soil surface (figure 2), which land on tobacco tissue. These spores germinate and penetrate the plants either by various injuries or directly through the cuticle and epidermis of senescent leaf tissues. Contaminated flower corollas falling on leaves can facilitate leaf infections "dead-blossom leaf spot"). Infection requires a moisture period of 16 to 72 hours. Once infection has occurred the fungus quickly invades the tissues causing the onset of rot in 48 hours. The mycelium and sclerotia are formed on and inside the diseased tissues.


- Dissemination of the fungus

Apothecia can be present in or around the field throughout the season. These organs ensure the sexual reproduction of the fungus. They are formed from the sclerotia, especially when temperatures are low, producing wind borne ascospores that spread the disease over several hundred meters.


- Favourable conditions for the fungus development

This fungus can grow at temperatures between 5 and 30°C. Its optimum growth temperature is around 20°C. It is favoured by periods of wet and rainy weather.

Light soils, rich in humus are more conducive to its development. It is very sensitive to carbon dioxide. This fact is the reason why the active population of the fungus is located in the very top centimeters of soil.