Mycoviruses are viruses that infect fungi. They have been identified in all major fungal families. Most identified so far have had double stranded RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 genomes, often with more than one dsRNA present per virus particle, and have been spherical in shape. To be a true mycovirus, they must demonstrate 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.

Preliminary results indicate that most mycoviruses codiverge with their hosts, i.e. their phylogeny is largely congruent with the one of their hosts. However, many virus families containing mycoviruses have only sparsely been sampled.

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

Mushroom

A mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. The standard for the name "mushroom" is the cultivated white button mushroom, Agaricus bisporus; hence the word "mushroom" is most often applied to those fungi that...

, Agaricus bisporus. It is also known as X disease, watery stripe, dieback and brown disease. Symptoms include:

• Reduced yield
  Crop yield
  In agriculture, crop yield is not only a measure of the yield of cereal per unit area of land under cultivation, yield is also the seed generation of the plant itself...
  
  
• 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.