When I was in grad school (the 80s), fungal viruses were thought to be exceedingly rare. This was a problem for aspiring yeast molecular biologists, because viral transduction [1] was still the predominant form of genetic engineering of bacteria, and the yeast folks wanted to get in the game.
But it turns out that fungal viruses – mycoviruses – are ubiquitous [2] . Or almost ubiquitous. The one place they are not found is the one place that everyone was looking – the environment. Although animal and bacterial viruses are found everywhere you look – water, soil, air – fungal viruses are rarely found outside fungal cells themselves. Once we figured out how to find them in cells, they began turning up in all classes of fungi.
From Mycoviruses: Meaning, Types and Replication
Mycoviruses seem to lack any means of autonomous environmental transmission (which is why we don’t find them in the environment). Instead, they spread in fungal spores, and by cellular fusion between fungi. Apparently this is good enough. Their genomes are predominantly double-stranded RNA, rather than dsDNA like bacteriophage.
From Hypovirulence: mycoviruses at the fungal-plant interface.
Mycoviruses definitely affect fungi – a problem if you are a mushroom farmer, a feature if you are trying to control fungal blights and plant diseases [3] . Given their inability to spread outside of cells and spores, there is not much prospect of using them to control fungal infection of humans – which have never been very treatable.
Footnotes
[1] Transduction – An Introduction to Genetic Analysis – NCBI Bookshelf
[2] 50-plus years of fungal viruses.
[3] Biological control of chestnut blight: an example of virus-mediated attenuation of fungal pathogenesis.