Biology, epidemiology

• Storage, sources of inoculum

Thielaviopsis basicola is maintained for a very long time in the soil, thanks to its chlamydospores (Figures 1 and 2). It is able to colonize organic matter and infect many host plants, whether cultivated or not, which will help to multiply and conserve it. Indeed, more than 120 species have been identified belonging to at least 15 different botanical families. Among the vegetable crops, one can quote the bean, the pea, the cucumber, the melon, the watermelon, the carrot, the eggplant, the tomato ... These hosts do not have the same sensitivity to this fungus and therefore multiply it. More or less good. In addition, the strains of Thielaviopsis basicola also show different pathogenic powers.

In nurseries, dust from contaminated soil is an important source of inoculum. It is also preserved on the material used for the production of the plants.

• Penetration and invasion

Chlamydospores, to a lesser extent endoconidia (Figure 3), germinate near the roots and enter them either directly through the epidermis or through wounds. The fungus quickly colonizes the tissues of the cortex and the vessels that it causes to rot.

• Sporulation, dissemination 

In damaged tissues, it produces numerous chlamydospores (Figures 1 and 2). It also forms on the surface of the roots, along with a multitude of endoconidia (Figure 3).

These structures are easily dispersed by water and soil dust. It is likely that the soil present on the tools used to work the soil helps to propagate them. The same is true for melon plants that may be contaminated.

• Conditions favorable to its development

T. basicola is best known to be serious on various plants when growing conditions are difficult. This is the case during cold and wet springs. Under these conditions, the root development of plants is reduced, or even stopped, and this fungus takes the opportunity to severely attack. It therefore appreciates wet soils, but especially cold. Its thermal optimum is normally around 17-23 ° C. The soil's pH influences its behavior; at acidic pH (around 5.6) it is normally less active. Adding calcium to the soil can increase its development.