Paul Gans - Plant Health Management

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Tobacco Necrosis Virus -  a rare occurrence but not a pretty sight (24 June 2008)

 

Tobacco Necrosis Virus (TNV) is really a very rare disease, but last winter I was shown a confirmed case and it is worth checking up on the existing knowledge of the condition.  SSince 1924 sporadic incidences have been recorded of a surface blemish in potatoes in the Netherlands which is caused by Tobacco Necrosis Virus.  The name Potato ABC Disease was adopted after the description of the three types of symptoms viz. A, dark, slightly raised dark or brown patches, B, dark, almost black, sunken patches often resembling horseshoes and C, lighter groups of brown patches with often parallel cracks.  Observations from the Dutch phytosanitory service suggest that the virus is not transmitted to the progeny of infected tubers. 

TNV causes several crop diseases for example bean stipple streak, tulip necrosis and cucumber necrosis.  It is often found infecting roots of a wide range of species without causing symptoms.  Transmission is through zoospores of Olpidium brassicae.  Different strains have been associated with other TNV diseases and symptom types within those diseases.  Transmission appears often dependent on the right combination of virus strain and fungal race.  There are no reports of an association of any particular strain with TNV in potatoes but some form of specialisation may well occur.

Much of our current knowledge of this disease is based on limited publications around 1960 and 1970.  Since that time our understanding of several diseases which are the result of virus transmission by soil borne vectors has increased greatly since the application of molecular techniques, for example Potato Mop Top Virus and Lettuce Big Vein Disease.  However for TNV in potatoes there are still many unknown factors.  For example we do not know if symptomless tubers can carry the virus from one field to another.  We do not know the frequency of fields infested with viruliferous or non-viruliferous O. brassicae populations.  What is clear is that when you find a surface blemish of this nature it is worth having it checked out.


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