It has come to my attention recently, on visits to mushroom growers, that bacterial blotch, and in some cases pit, have been appearing with some frequency.

This is somewhat surprising as blotch is a disease, which is more common in the autumn time. In particular it is common during the months of August and September, which tend to be quite hot and humid (giving ideal conditions for bacterial blotch spread). Indeed over the winter period I also came across a number of incidences of blotch particularly with a number of growers growing on Dutch shelving.

As to why blotch has become somewhat more common recently - I don’t have an explanation. However just for ‘revision purposes’ if you like, I have put together some of the key points which growers should reconsider if they are having blotch problems.

The first thing which should be said about blotch is, that it is caused by the bacteria pseudomonas tolaasii. However in fact, the underlying cause of blotch is dampness! For an outbreak to occur a certain population threshold must occur of the causative bacteria.

For this to happen, moist conditions must prevail, as the bacteria can only increase in numbers (thus reaching this threshold population) when damp conditions exist. If you have a blotch problem on your farm, then somewhere along the line your mushrooms have become damp!

(Please note as this article goes to print the weather is very humid resulting in a considerable blotch outbreak.)

The following are some of the key points that you should consider if you are experiencing blotch problems:

Key points for consideration

• Check RH levels settings. Check equipment  (Wet & Dry) and circulation speeds.
  
  
• Check for dampness on mushrooms.
  
  
• Dry off sufficiently. Don’t just rely on a set time period for drying off mushrooms. Check that mushrooms are completely dry before returning to ‘normal’ conditions. If possible return to ‘normal’ settings’ gradually after drying off or condensation on the mushrooms may result.
  
  
• If you are not confident of maintaining optimum conditions and are concerned about blotch, consider using a chlorine source when watering mushrooms. Ideally use chlorine on all watering (except chemical applications). This will prevent bacterial numbers reaching such a high level and thus reduce the likelihood of a blotch outbreak occurring. Some people suggest adding chlorine to the case watering and this is advocated in certain publications on the subject. Others suggest that chlorine will be of little benefit in these circumstances.
  
  
• When present, blotch is infectious and can be spread. Wash trolleys before moving to other crops. Warn pickers that if they touch blotch on mushrooms they could spread the disease.
  
  
• If a problem exists run at lower RH levels for a period. Remember scale despite being undesirable is better than blotch.
  
  
• A blotch outbreak may occur if casing becomes very dry. Approx. 1 week after this occurrence, the outbreak can occur as a negative flow of nutrients and moisture exists causing a total breakdown in evaporation.
  
  
• If the casing is very dry and becomes hard, blotch problems can occur. When the casing is watered the water it is not absorbed properly. Surface moisture can occur as a result, giving rise to high RH at the casing surface, even when equipment is showing low levels.
  
  
• With very inactive compost you are more likely to get a problem, as natural evaporation is far less, which may lead to blotch. This may occur particularly in latter flushes when compost temperature drops.
  
  
• Blotch can occur when days are warm but night temperatures are somewhat cooler. This can result in air reaching its dew point (when air temperature drops) thus giving rise to condensation on mushrooms.
  
  
• It is worth noting that when the same settings are used in fresh and humid conditions, the same conditions will not be achieved in the cropping house. For example if the house is set to humidify at 89% and dehumidify at 91%, in fresh conditions the RH will be 89% at maximum and probably lower. In humid conditions the very lowest RH level will be 91% and probably higher. So for the same settings, depending on the weather, actual RH levels could vary by 4-5%. The setting of conditions should take this into account.
  
  
• Blotch may also be more common in situations where the grower has a very large volume of compost in his/her house. Fans may not be of sufficient capacity to remove all moisture available on the crop surface.
  
  
• In the case of Dutch shelving growers, one explanation put forward for the predominance of blotch was that the beds may have been overly tamped, reducing evaporation and thus causing blotch. Slightly less tamping and perhaps more compost in the casing will help to keep the casing open and may reduce blotch.