Brown mushroom symptom expression following infection of an Agaricus bisporus crop with MVX associated dsRNAs

• Cropping experiments demonstrate symptoms expression following MVX infection.
• Expression of brown symptoms in Agaricus bisporus following MVX infection is variable.
• Severe symptom expression occurred following infection at end of spawn run.
• A chromameter is a reliable means of quantifying colour symptoms associated with MVX.

Mushroom Virus X (MVX) is associated with a range of symptoms observed in mushroom crops. The most prominent symptom in Ireland is the occurrence of ‘brown’ or ‘off-white’ mushrooms in white strain crops. The browning symptoms are associated with the presence of four low molecular weight dsRNAs: MVX0.6, MVX0.8, MVX1.8 and MVX2.0, however viral dsRNAs also occur in non-symptomatic mushrooms. Three virus-infected mushroom cultures containing MVX1.8 and MVX2.0 were used to infect experimental crops at different rates and at different times in the crop cycle to test the effect on symptom expression. Mushroom colour was measured by chromometer, and the ΔE value calculated. RT-PCR was used to test for the presence of MVX1.8 dsRNA in harvested mushrooms. Results indicate that following infection, browning symptom expression is variable both within and between crops. Control mushrooms from 1st and 2nd flush had ΔE values of 7–12, with most being <10. In contrast, 1st flush mushrooms from virus infected treatments had ΔE values of 6–25, with most being >10 while 2nd flush mushrooms had ΔE values similar to controls. Only mushrooms with ΔE > 15 appeared visibly brown or off colour. The transient and inconsistent nature of MVX-associated browning symptoms is discussed.