PESTS AND DISEASES By Helen Grogan, Horticulture Development Department, Teagasc, Dublin, Ireland.
helen.grogan@
teagasc.ie
How mushroom virus X is transmitted
In her role as a senior research officer, Helen Grogan has researched all aspects of Mushroom Virus X (MVX) disease for over 20 years, collaborating with research teams across the UK and Ireland. Results from this work have shaped our under- standing of MVX as well as the knowledge and advice given to the industry today on how to control it. Some key results are outlined below.
MVX is very easily
transmitted and therefore very difficult to
eradicate and control.
Introduction Mushroom Virus X Disease (MVX) seems to have raised its head again in recent months as outlined by Con Hermans in the last issue of Mushroom Business (no. 117). Con offered a good insight into the disease and the challenges around identifying and controlling it. This perplexing virus disease has been with us now for over twenty years and in that time consi- derable knowledge and information has been generated by research teams across the UK and Ireland (Horticulture Research International (HRI), Wellesbourne, UK; NIAB, Kent, UK; and Teagasc, Dublin, Ireland. It was called Mushroom Virus X initially, as it was a new viral complex that we were not familiar with. We now know that up to 18 different viruses can be present in MVX-infected mushrooms, many of which are novel and may be benign (Deakin et al, 2017
https://doi.org/10.1038/s41598-017-01592-9). However, one has been clearly correlated with the brown/beige mushroom symptoms - Agaricus bisporus virus 16 or AbV16, and one is always present with symptoms of crop delay and bare patches - AbV6. Here I outline some key results from those 20 years of research that highlight just how easy it is for MVX to be transmitted and therefore why it can be so difficult to eradicate and control.
Mushroom spawn Commercial spawn is an unlikely source of MVX although it is not outside the realms of possibi- lity. Between 2001 and 2007, almost 50 commer- cial spawns and cultures of 74 wild A. bisporus and other species from the Agaricus Resource Programme (ARP) were tested for AbV6, AbV16 and AbEv1 (another virus in the MVX complex).
20 MUSHROOM BUSINESS
Although the test may not be as sensitive when testing cultures, none of the commercial spawns were positive for MVX while one wild A. bisporus culture (ARP 250) tested positive for AbEv1, cul- tures. Separately, a wild Agaricus arvensis mushroom sample had a feint positive for AbV6. This suggests that MVX may have originated in wild populations of Agaricus species (testing done by HRI).
Virus-infected mushroom strains Most of the work described here was done with MVX 1283 (with AbV6) - a virus-infected strain of A. bisporus isolated in 2000 from a crop with bare patches and crop delay. Another strain was MVX 2735 (with AbV16) - isolated in 2002 from a crop showing brown/beige coloured mushroom symp- toms but no crop delay. Sometimes both viruses are detected in the same crop but generally one set of symptoms dominates. PCR diagnostic tests have been developed for AbV6 RNA-2 and AbV16 RNA-1 but currently there is no commercial diag- nostic service provider. In the past HRI, Fera UK, Teagasc and more recently, Wageningen Univer- sity, offered a service on request but unfortuna- tely the small number of sporadic requests for testing means that the service is generally not available.
Spore transmission. Early experiments using spore suspensions (up to 20 million/ml) showed no transmission of MVX but transmission occurred when freshly-harve- sted infected mushrooms were allowed to drop very heavy spore loads onto spawn running com- post. Thus mushroom spores may not be as important a vector for MVX, as they are in La France virus disease. None the less spores can
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