Biology and Epidemiology

- Survival and sources of inoculum

Fusarium oxysporum  f.sp. nicotianae is able to survive in the soil for at least a decade by forming thick-walled, resistant chlamydospores (figure 1). It can also survive as a saprophyte by colonizing various plant debris and other organic materials. Some strains can survive on susceptible hosts such as sweet potato and cotton.


- Penetration into the plant and invasion

Usually, the chlamydospores germinate only in the presence of tobacco roots due to exudates secreted by the roots. Subsequently, the pathogen enters the plant either through natural wounds such as those present at the point of growth of secondary roots, or through lesions caused, for example, by root-knot or cyst nematodes. Once in the plant, the fungus  spreads to the vessels and invades them via growing mycelium (figure 2). While in the vessels the fungus produces chlamydospores and micro-conidia. As with Verticillium wilt, plants can respond to this vascular invasion by forming gum or tyloses that impede pathogen progress; however the tyloses further restrict water flow and contribute to the wilting of the plant.


- Spread  and dissemination of the fungus

The disease can be spread from field to field by equipment that carries contaminated soil and plant debris. Soil dust containing chlamydospores is easily dispersed by wind and/or splashing water. In some countries the pathogen is present in the nurseries where it causes infections. In this case the infected seedlings spread the disease by carrying the pathogen  to the field.

- Favourable conditions for the disease development

Fusarium oxysporum  f. sp. nicotianae grows better at high than low temperatures. Its optimum temperature for growth is around 28 °C. At this temperature the disease develops quickly. The disease is more severe in sandy-loam soils, particularly in the presence of root-knot nematodes.