Biology and Epidemiology

- Survival and sources of inoculum
 
Thielaviopsis basicola survives very long time in soil thanks to formation of chlamydospores (figure 1). It is able to colonise the organic matter and infect many host plants (over 120 species have been identified) that contribute to the pathogen multiplication and long term survival. In nurseries contaminated soil is an important source of inoculum. It also survives in the equipment used for seedling production, and in the crevices of the polystyrene trays used for floating seedbeds. In some countries peat-based substrates can also be contaminated.
 
 
- Penetration into the plant and invasion
 
Chlamydospores, and to a lesser degree, endoconidia (figure 2) germinate near the roots and penetrate the epidermis either directly or through wounds. The fungus rapidly colonises the cortex tissue and vessels causing them to decay. It produces numerous chlamydospores in damaged tissues. Chlamydospores are also formed on the root surface along with a multitude of endoconidia (figure 3).
 
 
- Dissemination of the fungus 
 
Chlamydospores and endoconidia are easily spread by water and soil. It is likely that contaminated soil adhered to tools used for soil cultivation contributes to spread of chlamydospores and endoconidia. Similar can be the contribution of infected tobacco plants.
 
 
- Favourable conditions for the fungus development 
 
This disease is particularly severe when the tobacco plant encounters adverse growing conditions. That is during cold, wet springs. Under these conditions root development of plants is reduced or stopped and Thielaviopsis basicola may develop and cause severe damage. It likes wet soils but especially cold ones. When temperatures rise, tobacco plants are less sensitive. Soil pH influences the behavior of this fungus; at acid pH (around 5.6) it is usually less active. The addition of calcium in the soil increases disease incidence.