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Physiological characteristics After detailed characterisation of the Smokey Mould strains, the relationship between mould growth and the temperature was investigated. For this purpose we inoculated Penicillium hermansii (= Smokey Mould) cultures onto a nutrient medium (Czapek yeast extract agar) and incubated the cultures for two weeks at dif- ferent temperatures. Following this incubation period, the extent of growth was determined. This experiment revealed that the minimum temperature for Smokey Mould growth is around 15 degrees Celsius, and the maximum temperature for growth is between 33 and 36 degrees. The optimal temperature for growth of this mould strain is between 27 and 30 degrees Celsius. Other experiments reveal that Penicillium hermansii is not heat resistant. To test this we transferred spores of Smokey Mould to a buffer (ACES) and heated them for different durations (2, 5,10, 20 and 30 minutes) and at different temperatures (50, 52, 54, 56, 58 and 60 °C). This experiment showed that some strains of Smokey Mould can survive a treatment of 54 °C for two minutes (the majority of the spores have already been destroyed at this juncture); after five minutes no signs of life were observed. This study could lead to the conclusion that with an outbreak of Smokey Mould the infection occurred in the compost after pasteurisation. However, there is a caveat here. These experiments were performed in a fluid (buffer). It is known that heat treatment under dry conditions is less efficient and that mould spores are then able to withstand much higher temperatures. So if there are dry patches, with e.g. straw, present in the compost during pasteurisation, this mould can easily survive the pasteurisation process.
Observations in practice The first signs observed by growers if the mushroom crop is infected with Smokey Mould are elevated com- post temperatures that are impossible to control even using extremely low air temperatures. The compost temperature increases after ruffling and will not decre- ase during cool down. With a mild degree of infection you will not notice anything particularly striking on the bed; the mushrooms are a little paler, mature slightly earlier and are more thinly distributed on the bed. These symptoms can of course be traced to many other causes. In the event of a serious infection there are large bare patches, or even complete beds devoid of any mushroom growth. In the patches that have pro- duced mushrooms they are clearly fewer in number. The mushrooms that do develop have small caps that quickly tend to split open, and have thin, spongy stalks.
[Figure X2]. Growth of Penicillium herman- sii mould (= Smokey Mould) on nutrient me- dium after two weeks of incubation at different temperatures.
The general impression is of a weak, poorly developed mushroom that has clearly not been supplied with sufficient nutrients by the compost. If you dig into the compost in one of these bare patches, clouds of dust will be formed by the spores of Smokey Mould esca- ping from the compost. This can be observed with even more clarity in the beam of a torch. The appearance of the compost, even to experienced eyes, is actually rela- tively normal for the stage of development; however, a stale odour is often noticed. With a serious infection, the upper layer of the compost is marginally darker and the mycelium is gradually disappearing from the com- post. If the infected compost is not cooked out, clouds of spores will be released when the growing room is emptied. Anyone with a specific or general allergy to spores will notice this immediately.
Microscopic observations Contrary to what can be found in the sporadic literature that deals with Smokey Mould, we can demonstrate that incubated compost can be infected by Smokey Mould, whereas it does not grow on spawnable com- post. Under a microscope you can see that Smokey Mould mycelium is unable to grow independently on spawnable compost, even after three weeks. Smokey Mould only starts to develop when it hitches a ride at the moment the mushroom mycelium starts to grow. It develops along with the mushroom mycelium and at a later stage can gain the upper hand in the compost. Microscope images of infected incubated compost will show Smokey Mould mycelium growing between Agaricus mycelium. At a certain moment, Smokey Mould triumphs and slowly but effectively displaces the mushroom mycelium.
The infected patches where growth is seen show mushrooms with elongated stems and small caps.
Moment of infection Smokey Mould is incapable of growing independently in spawnable compost, but after spawn has been ad- ded it can grow along with the developing mushroom mycelium. Spawning is therefore a critical moment for potential infection. Smokey Mould can grow with the mushroom mycelium for its entire period of develop- ment, so that it has already extensively colonised the compost before delivery. However, the spawned com- post will have a seemingly normal appearance. Even under a microscope many people will fail to recognise the mould. You may suppose the compost is nicely colo- nised with mycelium – which it is, but with an undesira-
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