Causal agent(s) and transmission
Tobacco necrosis virus (TNV) affects numerous cultivated plants (tobacco, tulip, cucumber, parsley, spinach, beans, strawberry, fruit trees, vines, etc.) and weeds (chenopods, sowthistle, etc.) all over the world.
The virus is transmitted in a non-persistent mode by the zoospores of the root infecting fungus, Olpidium brassicae, the causal agent of hernia in Cruciferae.
Transmission depends on having the suitable combination of virus strain, fungus race and host species. The virus does not survive in the resting spores of the fungus.
TNV is not often found on the potato, on which it causes a disease often known as ABC. It has been reported in the Netherlands (on cv. Eersteling), in Southern Europe, especially in Italy (on cv. Sieglinde), in Tunisia and in the United States.
Symptoms on tubers
Tubers show brown-black surface lesions with radial or reticulated cracks which may be confused with those of common scab, caused by bacteria of the genus Streptomyces.
Circular lesions and light brown plaques may also be observed, as well as blisters which form on tubers during storage (photos 1 to 4).
The symptoms caused by TNV are also called ABC-disease because of the initial description of the three types of symptoms induced by the virus:
- A symptom: black-brown blisters;
- B symptom: rots or deep sunken lesions developed in storage from the type A blisters;
- C symptom: scab-like pale brown lesions with star-shaped cracks to brown spots without cracks, visible at harvest.
The vector, the fungus, O. brassicae, is favoured by moist, even excessively moist, soils and relatively low temperatures (10-20°C).
The virus does not persist in resting spores of the fungus, formed in unfavourable conditions. The mode of transmission to the potato is unclear, but with the other host species the root system is clearly the primary site of penetration.
Systemic infections of the potato are rare and infected tubers produce healthy plants (stems, leaves and daughter tubers).
It has never been possible to reproduce the symptoms in the tuber by mechanical transmission.
Outbreaks of TNV on potatoes have occurred in North America, Europe and Mediterranean areas but the reasons for these sporadic epidemics are unknown.
Because of its extreme variability, it is not easy to identify this virus in the laboratory.
There are several strains and variants found in one or other of the crops infected. The potato may be infected by isolates of the two serotypes (A and D). The Elisa test should use antibodies that recognise both serotypes.
Biological diagnosis is also possible in the greenhouse by inoculating tobacco plants or chenopods.
No method of control has been recommended for this virus, apart from crop rotation and thorough weed control.