We are now starting the season that Brown Plaster Mould shows up
By the time this article is printed we will be well into the ‘season’ that Brown Plaster Mould (Papulaspora byssina) makes its annual appearance, usually around week 20 (mid May) possibly as early as week 16. It then usually spontaneously disappears around week 46 (mid November). This may be due to the fact that fresh new straw is coming on stream for compost making or/and the weather is much cooler and brown plaster mould (BPM) is a somewhat ‘heat loving’ fungus.
The aim of this article is to outline what has been recently, experimentally found out about BPM and to suggest measures to be taken that will minimise its impact should it appear on farm (from whatever source).
Appearance of Brown Plaster Mould (BPM)
Its first appearance in compost is usually 3-4 weeks after filling a new house. It has strongly growing, white, mycelium that produces dense white patches. These first go ‘grainy’, and then quickly change through yellow to reddish brown. The edge of a growing colony is densely white. Further in, it is yellowish then reddish brown.
Brown ‘spores’, are mature and are relatively huge in size, so much so, they even have a texture. Between finger and thumb they feel like fine table salt.
BPM can be easily transferred to (cross contaminate) fresh compost
Just touching a colony with a single fingertip, and then touching phase II compost in a bag can lead to a large colony developing. In fact the number of spores that could be transferred by the tip of a pin have been shown to be a sufficient inoculum to lead to formation of a colony. Transfer of a straw of compost that has BPM mycelium growing on it is a particularly virulent inoculum and very rapidly leads to a visible colony (this is likely to be large if the temperature is in the range 25-30°C.).
Such points emphasise the need for careful hygiene if BPM appears on farm. If a sporing colony is touched, for example to feel its texture, as described above, this could easily cause considerable cross contamination. Similarly, machinery that picks up spores, or compost straws with BPM mycelium could spread contamination. This has actually been observed on farm, where casing machinery has moved along a shelf of compost.
Mushroom mycelium stops BPM growth dead
Experiments have shown that BPM growth in unspawned phase II compost is very much stronger than if the compost has been spawned. Where mushroom mycelium and BPM meet, BPM stops dead. The mushroom mycelium, on the other hand, continues growth, even covering the BPM spores and mycelium.
The significance of this observation is that ‘control’ of BPM could be best achieved by encouraging mushroom mycelium growth at the expense of BPM growth. Anything which hinders mushroom mycelium growth, such as poor aeration, will probably encourage BPM. Beware water lying on the folded-over plastic of a bag, weighing it down and ‘sealing’ it.
Best strategy to reduce the impact of BPM
Brown Plaster Mould is not nearly as damaging to yield as Trichoderma. Nevertheless, it is a potentially fast growing weed-mould that at ‘higher’ temperatures can rapidly produce a large colony on a bag or block. This is most likely to be on the outside, next to the plastic film, or, on the casing surface. In either situation the fungus assumes a ‘high profile’. It looks bad, because it’s so obvious and a patch on the casing surface could prevent pinning in that area for at least one flush. It uses compost nutrients that are then not available for mushroom growth.
To avoid BPM growth, keep spawn-run temperatures as low as practical. Incidentally, this will also help increase yield. A few days extra production-time could be very well rewarded.
It is very bad policy to set a high air temperature for the house in an effort to get spawn run off to a rapid start. It is suggested that anything over 22°C is excessive. Furthermore, an air temperature of 25°C or more is not going to help the mushroom spawn, as its first requirement is to take up moisture from the compost, before it can start growth. Furthermore, such a relatively high air temperature, as it first warms the compost just under the plastic, is more likely to stimulate the growth of weed moulds such as BPM or Trichoderma. It may even stimulate the natural heat loving fungi, leading them to degrade nutrients that would otherwise have been available for the mushroom.
BPM’s optimum temperature is near to 27°C; this stimulates it to grow several times as fast as mushroom mycelium. At 22°C BPM spores appear to take much longer to germinate. Experiments to try and germinate BPM spores at 12°C were not successful.
BPM appears unaffected by the fungicide ‘Bavistan’
It is hoped to test several disinfectants to find which is most effective in killing BPM. In an initial experiment, BPM spores were shaken in a solution of Bavistan (1 g/litre). Two mls of this solution was squirted into each of three bags of phase II compost (at a marked spot). Three control bags were similarly inoculated, but using BPM spores in water only. Bags were incubated in a chamber with an air temperature of approximately 25°C for three weeks. Very strong growth and sporing of BPM was observed, starting in the inoculation areas only.