Biology and Epidemiology

- Survival and sources of inoculum
 
Verticillium dahliae is not very "performant" at the soil level. However, its overseasoning is ensured by microsclerotia which can survive in the soil for fifteen years (figure 1). Its wide host range allows to attack many cultivated plants (pepper, eggplant, tomato, gourd, cauliflower, cotton, potatoes) or weeds (black nightshade, pigweed, and others) that promote its multiplication and survival. Repeated tobacco crops could select for particularly aggressive strains.
 
 
- Penetration into the plant and invasion

Infections generally occur either directly or through wounds. They are also favoured by injury from root-knot nematodes and Pratylenchus spp. (see the different types of root nematodes). Once infection has taken place, the fungus invades the vascular system of plants and colonises it gradually. The plants react to the vascular invasion by forming gum or tyloses thus preventing its progression, which blocks xylem tissues and causes wilting. Subsequently, the fungus produces microsclerotia and fragile verticillate conidiophores forming ovoid conidia (figure 2).
 
 
- Dissemination of the fungus 
 
Dissemination of the pathogen is possible through movement of soil, contaminated farm equipment and plant debris. Soil dust harbors microsclerotia and / or conidia which are easily dispersed by wind, as well as water splash and soilborne insects.
 
 
- Favourable conditions for fungus development 
 
The optimum temperature for growth of V. dahliae is between 20 and 32 °C. Short photoperiods and low light make plants susceptible to the disease, and it is more severe in neutral to alkaline soils. Monoculture of susceptible plants and too short or poorly chosen rotations contribute to increase the incidence of this disease in some fields.