Biology and Epidemiology

- Survival and sources of inoculum

Phytophthora nicotianae survives in the soil from one year to another in plant debris or by means of free chlamydospores. They can survive for several years in the absence of susceptible hosts even in unfavourable environmental conditions. When conditions are favourable, chlamydospores germinate and form one or more germ tubes which infect tobacco this is followed by production of sporangia (figure1). They can also germinate directly producing zoospores.


-  Penetration and invasion of the plant

Zoospores are usually the primary source of inoculum. As they swim through soil water they are attracted by the roots, encyst, germinate, and penetrate the root tissues. Spread of zoospores also takes place through air by splashing water. Once infection has occurred, the fungus quickly invades the epidermis and cortex. Sporangia and chlamydospores are formed quickly within infected tissues, and on the root surface. They are responsible for the further development of the disease.


- Dissemination of the fungus

Numerous sporangia formed on infected tissue germinate directly or produce zoospores that ensure the spread of the pathogen and lead to secondary infections. Phytophthora nicotianae also spreads in diseased seedlings, irrigation water from ponds or canals, or by contaminated soil on farm equipment or shoes.


-  Favourable conditions for the fungus development

This fungus is especially favoured by hot and humid climates. Its optimum growth temperature is between 24 and 30 ° C. Its activity is practically inhibited below 10-12 ° C and above 36 ° C. Moisture is essential for the different stages of its parasitic activity.  However, disease losses can be very severe after periods of prolonged drought because of chronic water stress in plants with partially rotted root systems. The severity of Phytophthora nicotianae is greater in the presence of root-knot nematodes.