In the Netherlands and Ireland the symptoms associated with virus X are cream to light-brown coloured mushrooms growing in between white mushrooms. There's nothing wrong the quality - it's excellent firm and robust - but the discolouration of the paper thin layer on the mushroom cap makes them unacceptable for the fresh trade. In The UK the symptoms linked to virus X have far more disastrous consequences - delayed harvest bald patches drop in production and deterioration in quality.
Certain farms seem more vulnerable to virus X than others, but the virus also appears more prevalent in certain batches of compost than others. This means that the same compost can show virus X symptoms at one farm, but not at another. It's even feasible that in one and the same growing room one batch of compost will have virus X symptoms while another batch doesn't. It seems that virus X shows no symptoms in ‘low’ concentrations, but manifests itself under stressful conditions during compost production or cultivation.
No solution has yet been found for this problem. To keep it under control, provide nutritious compost stimulate evaporation prevent unbalanced stress situations and make sure everyone adheres to strict hygiene!
Con Hermans, AdVisie ‘The mushroom cultivation advisors’