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Abstract: Five viruses have been isolated from cultivated mushrooms in Britain, and most of these have now been found in other countries where mushrooms are extensively grown. Four of the viruses have polyhedral particles 25, 29, 35 and 50 nm diameter, one has bacilliform particles 19 x 50 nm. They can occur singly or, more commonly, in mixed virus infections. The relative prevalence of the different viruses has changed considerably in recent years: 6 years ago, virus 2 was widespread but now is less often found, whereas virus 4, only recognized in 1967, is now much the most common (Fig. 1). This may be because virus 4 is so efficiently transmitted through spores. Although it is well known that the mushroom viruses cause degeneration of the mycelium in the compost, the formation of abnormal mushrooms and loss of crop, we have found no consistent or reliable association between the kind of symptoms shown and the type of virus or viruses present. The same viruses can cause waterlogged stipes; or dry, brown and leathery mushrooms; or grey, stunted clumps of dwarfed mushrooms that decay without further development; or thick, short-stalked mushrooms with poorly developed caps; or long, thinstalked mushrooms, according to circumstances. In controlled infectivity tests, we found that symptoms depend far more on the growing conditions, type of spawn, and age of crop when infected, than on the kind of viruses concerned. The importance of early infection has also been well established: infection at spawning can cause 80-90% loss of crop (Fig. 2), whereas infection at casing may cause only 5% loss, for example, with viruses 1 plus 4. Generally, virus 2 is associated with severe crop losses and barren areas, which become progressively worse, whereas virus 4 often causes a steady but less dramatic loss of crop, and sometimes cropping may be resumed in later flushes. Viruses 1 and 3 are usually associated with slight to moderate crop losses, and virus 5 seems the least important.
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