Where does it come from?
How can I prevent it?

Virus 'X' is a serious disease, which has adversely impacted on the profitability of the U.K. mushroom industry. It has recently been diagnosed on a number of farms in Northern Ireland and the Republic.
Although the consequences of Virus X may be significant, it is possible to overcome the problem by adopting strict procedures.
Infected compost may build a level of resistance to the Virus and yield at a poor to reasonable level.

 However Virus X free compost coming in contact with an infected crop or crops may yield disastrously.
It is important, should it be detected on your farm, that you do not aggravate the position by the spread of air borne contaminants i.e. mushroom spores or mycelial fragments.
On one of Monaghan Middlebrook’s U.K. farms it is believed that Virus X resulted from purchasing infected Phase III bulk compost.
The problem has been overcome eventually by changing to spawn run compost from a different source (Phase II½ Irish compost in blocks). Current yields are in excess of 650 lbs./tonne.
If compost quality is excellent the problem will be less pronounced and may not impact commercially either in terms of yield or margin.

However if the compost is slightly off target it may impact dramatically.

Research into the disease is ongoing at Horticulture Research International in the U.K. Diagnosis is based on detecting a high presence of double stranded RNA bands from mushroom tissue using a molecular test.
Current research suggests that there is a strong correlation between the high presence of these double stranded RNA bands and the Virus symptoms.

Symptoms:

Symptoms, which can occur separately or independently:
 Brown coloured mushrooms occurring in white crops, particularly in first flushes.
  First flush is later than normal.
  Premature opening of mushrooms particularly in the second flush. 
Loss of quality i.e. poor keeping quality.
 Patching – areas of the crop that do not pin, yet have strong mycelial growth.
 Yield losses of up to 80%.

Note: If you are experiencing an ongoing problem with non-run bags/blocks, it is advisable to forward samples to test for the presence of double stranded RNA bands.






Spread through:  Mushroom spores (arising from open mushrooms)
Mushroom mycelium.


Critical Potential Times of Infection on Growing Units
Filling of spawn run compost into growing houses especially bulk compost.
 Bulk Phase III is particularly vulnerable since the product is not contained in plastic, plus doors are open for extended periods of time to facilitate filling, ruffling and casing.
 Adding spawn run compost to casing.
 Picking open mushrooms/breakfast flats.
 Emptying – innoculum can get into adjacent cropping houses.

 Control

1. Avoid using a second source of virus free compost with a known infected compost (i.e. compost testing positive for high presence of double stranded RNA bands).
Experience to date has shown that the Virus free compost becomes contaminated in the presence of infected compost (i.e. compost testing positive for high presence of double stranded RNA bands).
Virus free compost in the presence of infected compost has no resistance and consequently yields are disastrous.

 Therefore, ensure, as far as possible, that all compost on your growing unit at any given time is virus free.
 2. Compost Source
Bulk Compost is much more susceptible to contamination as it is exposed for much longer periods thereby increasing the risk of spore and mycelial infection.  Growers with infected compost (i.e. compost testing positive for the presence of double stranded RNA bands) are advised to produce compost in blocks or sealed bags.


3. Consideration should be given to not growing Breakfast Flats/Open Cup mushrooms for those growers who are experiencing Virus X symptoms.


4. Exclusion of both mushroom spores and mycelial particles i.e. filter air exhausts and air inlets.
Best Practice: Air Inlet Filter: EU4.
Air Exhaust Filter: EU6.

Notes:

a) Growers using air filtration will have reduced air movement.
Consequently it is important that growers increase their fan speeds and may need to increase their fan capacity.
b) Arising from increased air filtration, it is imperative that growing houses are properly sealed in order to avoid the air inlet being by-passed by air entering the growing house through ‘cracks’ e.g. badly sealed doors.








5. End of Crop

• Formaldehyde assists in limiting the spread of spores or mycelial propagules. Therefore before emptying the compost, prill with Formaldehyde ensuring that the: -
a) Air temperature is greater than 15°C
b) R.H. is greater than 50%
Repeat prilling with Formaldehyde prior to compost filling.
• Live steam cook out.


6. Hygiene
Good hygiene is critical to avoid potential contamination of new crops with material from older crops.