Mycovirus

Mycoviruses are viruses that infect fungi. They have been identified in all major fungal families. Most identified so far have had double stranded RNA genomes, often with more than one dsRNA present per virus particle, and have been spherical in shape. To be a true mycovirus, they most demontrate an ability to be transmitted - in other words be able to infect other healthy fungi. Many double stranded RNA elements that have been described in fungi do not fit this description, and in these cases they are referred to as virus like particles or VLPs.

La France Disease
An example of a true mycovirus is the causal agent of La France disease. This is a disease that affects the edible mushroom, Agaricus bisporus. It is also known as X disease, watery stripe, dieback and brown disease. Symptoms include: 

Reduced yield Slow and aberrant mycelial growth malformation Premature maturation Increased post-harvest deterioration (reduced shelf life)  

Mushrooms have shown no resistance to the virus, and so control has had to be via hygenic practises to stop the spread of the virus.