Virus disease

former names: «La France» disease, «Dieback», «Mummy»

The virus disease is the most dangerous and the hardest disease to control. Discovering and identifying the infection of mushrooms with virus is hard and sometimes even impossible. Practically all symptoms of the virus disease aren’t determinant and are very similar to the symptoms of other diseases.

The basic sign and result of the virus activity - a sharp decrease in the yield or sometimes; a complete absence of mushrooms – is unfortunately discovered only after very hard work and a loss of time.

During the spawn run period, the presence of a virus can be determined by diagnostic tests with the use of an electron microscope. But most mushroom growers don’t have the possibility of conducting this kind of laboratory researches. The manifestation of other symptoms, which can help distinguish the virus infection from other diseases, depends on the combination of many factors, for example: the time of infection, the amount of introduced diseased spores or infected mycelium, the concentration and combination of the kinds of virus particles present.

The infection of a mushroom culture with a virus can be identified by the following symptoms:

• mycelium hardly grows into the casing layer, and the partly disappears from it again or doesn’t grow on some areas at all;
  
  
• the first break occurs much later than usual, and there are areas with no mushrooms on the bed;
  
  
• Fruit bodies of a very low quality appear late and grow in thick clumps in areas of the casing soil that haven’t grown with mycelium;
  
  
• The mushrooms look like «drum sticks», very small caps grey or dirty cream in color are one with long and sometimes deformed stipes;
  
  
• The fruit bodies are loosely attached to the casing layer;
  
  
• Mushrooms infected with virus develop slowly, but their caps open quickly, releasing millions of spores infected with virus.

All these symptoms can appear separately or in different combinations. But there were some cases, when yield loss in batches infected with virus happened with the occurrence of the above listed external symptoms of the disease. And the use of research to determine the presence of virus in the mushrooms allowed finding out the reason for low yield.

The main danger of the virus infection lies not only in the loss of yield of one batch. With the help of diseased spores and the remains of infected mycelium, the virus easily spreads through the whole farm and enters new cultures, which can lead to the suspension of production. We already mentioned that mushrooms, affected with virus open fast, releasing infected spores. Sometimes, mushrooms that were formed inside the casing layer come out already open. Spores are easily carried by wind, insets, on implement, clothes and hands of personnel. That is why the possibility for the spores infected by virus moving from the old rooms to new batches is very high. The size of damage depends on the stage on which the infection occurred. If the virus enters mycelium during the application of casing soil or later, the yield loss won’t be very great. In this case, some brownish mushrooms with deformation typical for the virus disease are found during the first break. In subsequent breaks the disease might occur as a bigger or smaller lesion focus. Despite the sufficient mushroom yield, these batches present themselves as a source of disease for other growing rooms.

Infection that occurred before the application of casing soil, has grave consequences, especially if spores or mycelium that contains virus fall into different places at the same time. If this batch allows obtaining some amount of mushrooms during the first break, then at a further spreading of the disease it isn’t possible to obtain mushrooms during subsequent breaks.

When mushroom mycelium is infected with virus during spawning, it can lead to a great yield loss. You should keep in mind that an infection caused by diseased mycelium that enters compost from a virus affected batch, has consequences that are graver than at an infection by spores. As a result of anastomosis (the intergrowth of affected fragments of mycelium with healthy tissues) the virus particles enter the hyphae much faster and spread through the whole mycelium. The technological processes that lead to the mycelium tearing with its subsequent anastomosis, like phase 3 compost filling, that is, the compost fully grown with mycelium, loosening the casing soil and cacing can also cause the spread of disease if a virus infection is present on the farm.

In order to escape all problems which have to do with the consequences of a virus disease, the mushroom grower should organize the production in a way that excludes any possibility for the virus to enter his farm. A strict observance of the sanitation and hygiene regulations and job discipline are the basic conditions of a successful work of a farm. If the infection did happen, then only strict sanitation and methodical job in preventing the spread of the virus disease can help overcome the hard situation.

You must make sure that you:

• harvest mushrooms in proper time, not allowing them to open;
  
  
• use filtered air during the pasteurization process, spawning and spawn run;
  
  
• when the farm is infected by virus, use filters in order to keep the spores inside all growing rooms;
  
  
• change filters after each batch, and steam them after treating the growing room;
  
  
• since the most critical moments for the infection to occur are the unloading of pasteurization tunnels, spawning and filling the growing rooms, these operations must be carried out right after the equipment, machinery and the room have been cleaned and disinfected;
  
  
• after spawning, it’s best to cover the beds with a paper, which must be sprinkled with 2% formalin solution every 3-4 days, in order to kill all settling spores;
  
  
• to prepare the casing soil, use only «clean» supplements, that are free of pathogenic organisms;
  
  
• apply the casing layer using disinfected equipment;
  
  
• when loosening the casing soil, the equipment must be disinfected after each bed is loosened;
  
  
• use disposable crates or containers when harvesting mushrooms;
  
  
• the use of one crate many times increases the risk of infection and the spreading of virus, that is why all crate must be thoroughly cleaned and disinfected before use;
  
  
• steam the growing room at a compost temperature of 70ºC for 12 hours, after the end of the growing cycle;
  
  
• after steaming and an immediate removal of spent compost from the farm’s territory, the growing room and all equipment is thoroughly cleaned and disinfected;

You must remember that all Agaricus Bisporus strains are susceptible to virus. It doesn’t only concern white strains, but also the hybrid and brown strains. The infected mycelium of one strain can intergrow with all other strains to a smaller or greater extent.

Research has shown that Agaricus Bitorquis is resistant to virus disease. That is why during a severe virus infection of a farm, it is advised to cultivate Agaricus Bitorquis for a specific amount of time.