Pseudomonas gingeri.
Pseudomonas tolaasii.
Trick
A trick to discover whether you are dealing with a mould or a bacterium is to isolate and seal the infected mushroom in a sandwich bag and leave it in the growing room. If mould keeps growing on the mushroom - you will see mycelium on it - you can conclude that the casual agent was a mould. If the mushroom just rots, you can con- clude that it was infected by bacteria blotch. This remains a hit and miss test - it is not very accurate - but taking into consideration the other conditions in the growing room, you can still hazard a good guess as to whether it was a mould or a bacterium. Pinning down the cause to a bacterium is not the end of the story. Which strain of bacterium is it, and, more importantly, what caused the bacteria blotch? Once we know the cause, we can prevent the consequences in other growing rooms.
Bacterial blotch There are various types of bacterial blotch. Sometimes they are caused by growers’ mista- kes, while in other cases external infection is the cause. Here too recognising the differences is important.
P. Gingeri presents as yellowish spots, with an indistinct edge. The colour ranges from pale yellow to dark yellow/brown. The blotches start to appear before the first flush. If you don’t notice them then, you will during the first days
of harvest. These Gingeri blotches also keep growing after the mushrooms have been pic- ked. The infection will decline in later flushes. In general (see edition 102) peat seems to be one of the causes of the infection. The problem is often seen in places in the growing room where evaporation levels are lower, but even- tually you will ee it everywhere. It is important to notify the casing supplier, so they can try and analyse the problem as quickly as possible and take action. All casing soil suppliers have been faced with this issue, and there are no guarantees it will no longer occur in the future. That is why research into the problem is so important as it is not a problem restricted to farms, but a sector-wide problem. P. tolaasii, but perhaps also P. salomonii or maybe another strain, is pale to dark brown and usually presents as spots. However, if the bacteria ‘dries ‘a distinct edge can occur, and the discoloured patch will collapse into the cap. Mushrooms will not grow in the infected spot on the bed but will grow where infection is absent.
Preventing P. tolaasii Since P. tolaasii most commonly occurs at farm level, I will address this in more detail. Mould, such as green mould and dry bubble, can be prevented by good hygiene. P. Tolaasii is a pro- blem caused by evaporation at a certain point during the cropping cycle. Casing soil acts as a
MUSHROOM BUSINESS 9
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