Biology, epidemiology

• Conservation, sources d'inoculum

Sclerotinia sclerotiorum has significant saprophytic potential. It can be maintained in the soil for 8 to 10 years thanks to its sclerotia (figures 1 to 3) that they produce on the affected organs and / or the mycelium present in the plant debris left on the plots. In addition, it is one of those polyphagous fungi 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 rotate with tomato, such as salads, beans, cabbages, peppers, eggplants, many cucurbits, celery, peas, carrots, rutabaga, potatoes. , the sunflower... 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 melon plants by S. sclerotiorum occurs through the mycelium from sclerotia found near organs in contact with the soil . In addition, this fungus 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. They can only germinate on the leaves in the presence of water from rain, sprinkler irrigation 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 in healthy tissues which it rots thanks to numerous lytic enzymes. For example, it 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, it forms 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 this fungus 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 4) , asci and ascospores disseminating , especially when temperatures are low, between 8 and 16 ° C.

• Conditions favorable to its development 

Its thermal optimum is slightly below 20 ° C, this fungus is able to grow 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 its development. it 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 its sclerotia. Apothecia are also formed as a result of rains, thunderstorms, irrigations increasing soil moisture.