ARTICLE IN PRESSnon-sporulating sectors

We quantiWed the Wtness costs of these and more ‘cryptic ’ virus infections on mycelial growth rate and spore production, and on competitive ability with respect to other strains under diVerent growth conditions. Mycovirus infection proved detri-mental in all these measures. The reduced success in interference competition due to mycovirus infection belies co-evolution of mycovirus and host to a mutually beneWcial symbiosis, like in killer virus systems in yeast and smut and agrees more to recent infections. For a stable virus infection frequency in the black Aspergillus population, Wtness costs and spontaneous loss should be balanced with new infections. Implications of even small viral Wtness eVects combined with the observed transmission limits for host and mycovirus are discussed