The genetic make-up and transmission
of Virus X

The ‘Pathogens & Pests II’ section of the ISMS Congress on Monday afternoon showed a vivid discussion on MVX or Virus X disease. Their were four presentations on this subject.

By Anton Sonnenberg, Applied Plant Research

Viruses are common in most fungi. In the past six years, however, apparently new viruses have shown up in mushroom crops. They are associated with a variety of symptoms in crops in the UK, such as bare patches, delayed crops, premature opening of mushrooms and brown discolouration of mushroom caps.

The presentations were focussed on the molecular analyses of the genetic make-up of the viruses, i.e. dsRNAs and the transmission of viruses.

HRI

Adriana Soares and co-workers of the HRI at Wellesbourne, UK, presented new data on the molecular analyses of dsRNAs found in crops with the symptoms described above. Up to 26 different dsRNA elements were found, that are present in different combinations and frequencies in crops with symptoms. Only the smaller dsRNAs, however, showed a clear associated with one symptom, the browning of mushroom caps. The first data on sequence information of a number of dsRNAs indicate that we are dealing with a complex of viruses. In vitro studies showed that dsRNAs can be transmitted horizontally (by hyphal anastomosis) as well as vertically (via spores).

Helen Grogan and co-workers, also from HRI, presented data on the transmission of dsRNAs and symptoms using tissue cultures made from mushrooms of affected crops. Here experiments showed that infected mycelium can transmit dsRNA and symptoms to a crop at any stage of the cultivation, i.e. at spawning, after spawn run (bulk handling) and at casing. Moreover, very low levels of infection, even at a late stage of a crop, can cause symptoms. This indicates that at the compost yard and at the mushroom farms infection can occur, indicating that hygiene is the issue in controlling this disease.

Applied Plant Research

Anton Sonnenberg, Applied Plant Research, Horst, The Netherlands, described the association of dsRNAs and browning of mushrooms. Up till now, the brown discolouration of mushroom caps seems to be the only symptom seen in the Netherlands and Belgium. He showed that there was a quantitative correlation between the smaller dsRNAs and the severity of symptoms. In addition, the highest concentration of these dsRNAs are found in the peel of the discoloured caps, exactly on the place were the symptom occurs. No correlation was found between the other dsRNAs and abnormalities in crops.

A sensitive test was designed for the symptomatic dsRNAs, which makes it possible to detect low concentrations of one of the symptomatic dsRNAs. The test showed that also in white mushrooms of an effected crop this dsRNA is present, although in low concentrations. Even in some crops without any symptoms this dsRNA could be detected. Further screening should make clear if symptomatic dsRNA are wide spread or only present in crops with either symptoms or a history of symptoms. The test will also be used to screen spawned compost and other sources.

PennStateUniversity

Peter Romain and co-workers of Penn State University, USA, described the molecular analyses of the so called hybrid virus of A. bisporus. This virus has three dsRNAs is seemingly benign since no symptoms have been reported associated with these dsRNAs. He showed that the largest dsRNA represent the complete genome of this virus and that the other two dsRNAs are part of the former. The concentration of the virus has decreased over the last 15-20 years.

Researchers of the different grouped together to discuss a better cooperation in order to tackle the MVX or Virus X disease. More sequence information is needed to make quick and sensitive tests that will be helpful to unravel the source of these viruses, and help to understand how transmission occurs. That will help to prevent further outbreaks of this disease that has already caused a considerable economical damage to crops in the UK, Ireland, The Netherlands and Belgium.