Biology, epidemiology

• Conservation, sources d'inoculum

Scerotinia sclerotiorum has significant saprophytic potential. It can be maintained in the soil for 8 to 10 years thanks to its sclerotia (figure 1) that it produces on the affected organs and / or on its mycelium present in the plant debris left on the plots. In addition, it is a polyphagous fungus that can be found on many host plants.

reported S. sclerotiorum has been on more than 400 different plant species, cultivated or weeds. It infects many vegetable crops that can sometimes rotate with zucchini and squash (or be grown nearby), such as salads, beans, cabbages, peppers, eggplant, many cucurbits, celery, peas, carrot, swede, potato ... A number of weeds harbor it unnoticed.

These numerous hosts are capable of multiplying it and of serving as sources of inoculum when they are incorporated, after harvest, into the soil with the sclerotia of this fungus.

Contamination of S. sclerotiorum can occur through the mycelium from sclerotia found near organs in contact with the soil. But let us emphasize that this fungus also forms apothecia on its sclerotia. These organs ensure its sexual reproduction and generate numerous asci containing ascospores. Thus, millions of ascospores are released from the apothecia into the air over 2 to 3 weeks; they are the source of airborne contamination, sometimes over several hundred meters. Their germination on plant tissues can only be achieved in the presence of water from rain, irrigation by sprinkling or dew.

• Penetration and invasion

Whatever the nature of the inoculum (mycelium, ascospores), this fungus easily penetrates into living, injured, senescent or dead organs in contact or not with the soil, and quickly invades them. Its mycelium progresses into healthy tissue, which it rots using numerous lytic enzymes. For example, Sclerotinia sclerotiorum produces endo- and exopectinases, hemicellulases and proteases. It also synthesizes oxalic acid, which influences both the expression of its pathogenic power and the receptivity of its host.

When the ambient humidity allows it, this Sclerotinia produces white mycelium more or less dense and sclerotia on the damaged tissues. When crop residues are incorporated into the soil, 70% of the sclerotia are found in the first 8 centimeters of depth.
 

• Sporulation and dissemination

The sclerotia sometimes ensure the transmission of these fungi to other plots, such as they are transported through the soil on plowing tools or plants. As previously reported, S. sclerotiorum (homothallic species) readily produces apothecia (Figure 2) , asci and ascospores disseminating , especially when temperatures are low, between 8 and 16 ° C.

• Conditions favorable to its development

If its optimum temperature is slightly below 20 ° C, Sclerotinia is capable of growing at temperatures between 4 and 30 ° C . It is favored by humid and rainy periods and particularly likes tissues that have reached an advanced development.

Light soils rich in humus are more conducive to the development of S. sclerotiorum . The latter is sensitive to carbon dioxide, which explains its location in the very first centimeters of the ground. The temperature and humidity conditions of the soil also influence the survival of the sclerotia of these fungi. Apothecia are also formed as a result of rains, thunderstorms, irrigation increasing soil moisture.