Biology, epidemiology

- Conservation, sources d'inoculum

Sclerotinia sclerotiorum and Sclerotinia minor have significant saprophytic potential. They can be maintained in the soil for several years (8 to 10 years) thanks to the sclerotia (figures 1 to 4) that they produce on the affected organs and / or to the mycelium present in the plant debris abandoned on the plots. In addition, they are very polyphagous fungi that can be found on many host plants.

Sclerotinia sclerotiorum is more polyphagous since it is reported on more than 400 different plants, cultivated or weeds. It infects many vegetable crops that rotate with salads, such as beans, tomatoes, peppers, several cucurbits ...

Sclerotinia minor attacks fewer hosts; it is still reported on more than 90 plant species. In his case, the level of attacks is closely correlated with the number of sclerotia present in the soil.

These numerous hosts are capable of multiplying them and of serving as sources of inoculum when they are incorporated, after harvest, into the soil with the sclerotia of these 2 fungi.

Contamination of Sclerotinia minor occurs mainly through the mycelium from sclerotia found near the lower leaves of lettuce. These sclerotia must have dried for some time before they can germinate.

Contamination of salads with Sclerotinia sclerotiorum can occur in the same way. On the other hand, 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 into the air over a period of 2 to 3 weeks; they are the source of airborne contamination. Their germination on the leaves can only be achieved in the presence of free water, resulting from a rain, an irrigation by sprinkling or a dew.


- Penetration and invasion

Whatever the nature of the inoculum (mycelium, ascospores), these 2 fungi easily penetrate senescent or dead tissues of salads and invade them quickly. They progress to healthy tissue, which they decay with many 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 ambient humidity permits, these two Sclerotinia spp. produce on damaged tissues white mycelium more or less dense and sclerotia . It was observed that Sclerotinia minor could produce up to 12,287 sclerotia per plant, while Sclerotinia sclerotiorum only formed 63 under the same conditions. When crop residues are disconnected and therefore incorporated into the soil, 70% are found in the first 8 centimeters.


- 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, unlike Sclerotinia minor (heterothallic species), Sclerotinia sclerotiorum (homothallic species) more easily generates apothecia (Figure 5), especially when temperatures are low between 8 and 16 ° C. The ascospores produced, several thousand by apothecia, are at the origin of the spread of the disease by the wind, sometimes over several hundred meters.


- Conditions favorable to its development

These two Sclerotinia spp. are able to grow at temperatures between 4 and 30 ° C . Their thermal optima are slightly below 20 ° C. They are favored by humid and rainy periods and are particularly fond of salads that have reached advanced development.

Light soils rich in humus are more conducive to the development of Sclerotinia sclerotiorum . The latter is sensitive to carbon dioxide; this property is at the origin of 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.