Biology and Epidemiology

- Survival and sources of inoculum

Thanatephorus cucumeris has a high saprophytic potential, allowing it to survive in soil in the absence of susceptible hosts. It can be observed in the form of mycelium and sclerotia (figure 1), often in organic matter which it easily colonises. It is a very polyphagous parasite fungus that can attack and survive on diverse hosts and on their debris. This is not exactly the case for the epidemiology of its perfect form which involves strains with a smaller parasitic spectrum. It can also be found in some soil-less growth media.


- Penetration into the plant and invasion

Infections can occur in two ways, either through the soil by mycelium or from germinating sclerotia. As it grows, the mycelium comes in contact with the tissues of tobacco plants and penetrates them. It often invades them directly and quickly (figure 2) thus rotting them, which can be accentuated by other micro-organisms. This parasitic process causes damping-off and collar cankers. It is also involved in the primary contamination of leaves close to soil line.

Infections can also be initiated by basidiospores. These generally come from a hymenium (figure 3) which could have been formed on the soil surface, on already infected tobacco or other infected hosts. Once on the leaves, the basidiospores germinate and penetrate the tissues directly through the cuticle or more rarely through the stomata. They quickly invade the tissues and produce occasionally a hymenium at the margins of lesions.

- Sporulation and dissemination of the fungus

In diseased tissues, the fungus forms mycelium which will spread on the tissues and soil, and thus to other healthy plants. It also produces basidiospores (figure 4) on the hymenium formed on the soil or on the leaves. They are spread by wind over long distances. Its dissemination is also facilitated by infected plants and tools contaminated by soil.


- Favourable conditions for the fungus development

Thanatephorus cucumeris will survive in humid and heavy soils as well as lighter and drier ones, with acidic or basic pH values and at temperatures between 15 and 26 ° C. Too dry or too wet soils seem to inhibit the fungus. Quite often it takes advantage of cold temperatures which are unfavourable for tobacco. The development of the perfect stage (Thanatephorus cucumeris), which is airborne, is favoured by weather conditions involving long wet periods combined with moderate temperatures between 20 and 30 ° C.