The fact that this disease occurs only rarely is little consolation when it has messed up your crop. The culprit is a virus which is transmitted by a soil borne fungus Olpidium brassicae. Something to do, you might think, with Brassica crops but there is little evidence that the disease is worse after a Brassica crop. In fact our knowledge about this disease and what are the predisposing conditions are quite limited. It has been recorded in the Netherlands since 1924, where the name ABC disease was adopted after the way in which the symptoms were first descibed viz. (A) dark, slightly raised, dark brown patches, (B) dark, almost sunken patches often resembling horseshoes and (C) lighter groups of brown patches with often parallel cracks. Apparently the virus is not transmitted from seed to daughter tubers. Eersteling (Duke of York), Doré and Bintje appear to have been most frequently affected. A couple of cases have been brought to my attention in the UK in recent years, one in Markies and another in Melody.Clearly anyone who has had the experience of a spoiled crop as a result of this virus will wish to prevent a repeat. So how much do we really understand about the causal agent? Tobacco Necrosis Virus (TNV) can cause bean stipple streak, tulip necrosis and cucumber mosaic and is also found infecting roots of a wide range of species without causing symptoms. Transmission is through the zoospores of Olpidium brassicae, a soil born fungus. It is capable of forming resting spores , which survive for many years and these may account for the unexpected way in which the disease occasionally crops up. In order for disease to occur there has to be a resident population of the fungus which carries the virus.
There are a lot of things we do not know about the disease. For example can symptomless tubers transmit disease? We have no indication of the proportion of fields which have either the fungus with or without the virus. What part do the resting spores play? So any strategy to limit the spread is highly speculative, but after an outbreak it makes sense to do whatever is reasonable to prevent fresh fields from becoming contaminated. Transfer of potentially contaminated soil from fields where crops have been affected should be avoided, applying practical hygiene.
