It is surprising that a fungus that causes as much loss to the mushroom industry as brown plaster mould (scientific name Papulaspora bysinna) has not been more thoroughly researched.

In the autumn, most years, it is a familiar sight on spawn run compost, many small (or large) dense white patches that then become dusty looking, before turning reddish brown. Growing colonies may retain a white fringe. Later, the mycelium of the fungus may move up through the casing to further ‘sporulate’ and where dense patches of brown ‘spores’ cover the casing there will be no first flush on such areas. On shelves, patches can cover over a square metre of bed, probably having been enlarged by virtue of movement of casing-machinery (ruffler/leveller) spreading contamination. However, after watering the casing several times and after the densely packed ‘spores’ have been washed away, a second or third flush of mushrooms may ‘miraculously’ appear. Compost known to be contaminated by brown plaster mould (BPM for short) appears to have reduced yield. Is this due to straight competition between BPM and Agaricus for compost nutrients? Or is there a connection between BPM and older, poorer quality straw?

BPM ‘spores’ are round, variable in size but relative to other spores enormous and composed of hundreds of cells, each one many times larger than, for example, a Trichoderma spore. BPM ‘spores’ are referred to as bulbils. The literature states that they have a waxy feel, however I would say a ‘pinch’ of bulbils rubbed between finger and thumb feels almost exactly like fine table salt! If such a method is used to check ‘identification’ please remember to wash them off before contaminating anything that is subsequently touched!


Some of the questions the mushroom industry needs to have answered.

The scientific literature on BPM appears sketchy at best. It suggests that if modern composting methods are used, the occurrence of BPM is uncommon. This does not appear to be true. If it was true then most compost makers with modern equipment in Ireland would not have problems with BPM most years. Where does the initial contamination come from? Is the fungus commonly found in soil that wheat has grown on and therefore BPM comes into the compost making cycle via the wheat straw used? The problem weeks appear to be 26 to 46. This suggests that maybe BPM multiplies up in the bales as they age in warm damp summer conditions. It may require some sort of moist fermentation to cause spores to germinate and start growth? A specific culture media that only supported BPM and thus excluded other, more rapidly growing moulds would be helpful in isolating BPM from soil or straw. A physical method of isolation of BPM bulbils, with which I have experimented, is to shake material that is known to be contaminated with BPM in clean water in a glass container, both being free from traces of detergent. If left for an hour, many bulbils float to the surface of the water. In the right light conditions (coming from the side) they can be seen glinting on the surface. The bulbils can then be easily transferred to a microscope slide by dipping it into the water. They are then seen as tiny granules on its surface and positively identified at even as low a magnification as x 100.

How does BPM survive a normal phase II peak heat cycle? Are BPM bulbils, more heat resistant than most other mould spores due to their relatively great size? They even appear to have a coating of air filled cells on the outside. Do these comprise an insulating layer? Or do they aid flotation? It would be helpful to know what temperature x time regime is needed to kill them? However, in a phase two peak heat tunnel there is a high concentration of ammonia gas (after maximum temperature is reached). Does this help kill BPM bulbils or not?

The literature suggests BPM is "stimulated by unconverted ammonia containing compounds and amines in the (phase II) compost". No experiment is described, so it is not clear, if such chemicals were added to compost and stimulated BPM or whether such chemicals normally appear in compost made from old straw, during the weeks when BPM is most common. Ammonia is known to be so toxic to mushroom spawn that care is taken to virtually eliminate it and this is checked to be so, well before the end of phase II conditioning. Nevertheless, patches of BPM repeatedly turn up in spawn running compost from late summer until just after the changeover to new straw.

When not found in compost what is the natural habitat for BPM .

Much needs to be discovered about the biology of BPM. For example, it would be very helpful to know what stimulates BPM bulbils to germinate and start a fresh cycle of new growth. Is a period of raised temperature, or the availability of specific ‘compost type’ nutrients required? What is BPM’s optimum temperature for growth? If this was found to be higher than the optimum for Agaricus (24°C) then carefully controlling temperature during spawn run would be advisable to reduce its impact during the BPM danger weeks (26-46). How fast is the extension rate of growth at different temperatures? BPM appears initially to grow faster than Agaricus. Later Agaricus mycelium can sometimes limit further growth. Does BPM grow better in spawn run conditions with ‘high’ temperatures or ‘low’, where there is good or poor aeration? Does the fact that a high proportion of BPM bulbils float, mean that bulbils are ‘designed’ to be water dispersed? Why are bulbils so large, could this be a survival mechanism to resist heat during the initial stages of a natural fermentation of wet grass or straw? Maybe ‘in nature’ they have to survive passage through the gut of a grass eating animal, before germination and growth in the animal’s dung. This idea is pure speculation but would be interesting to research. It is also important not to rule out poultry litter as a source of BPM. However, the prevalence of BPM problems in weeks 26-46 strongly suggests BPM multiplies in aging straw.

An old saying

There is an old saying: a fool can ask questions that a wise man cannot answer! Where is that researcher that could by experimentation answer some of the above questions? Besides being a subject that the industry would appreciate more information on one can’t help thinking BPM biology would make an excellent PhD study. Such research costs money, so it is likely that the industry would be asked to pay, or to organise a grant.

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ed.
for more on BPM:
Integrated Disease Management and Plant Health/edited by V.K. Gupta and R.C. Sharma. 1995, 318 p., tables,

Mushrooms: 59. Bacterial blotch of Agaricus bisporus: occurrence, incidence and host range/Rajeev Raina and C.L. Jandaik. 60. Influence of some mushroom cultivation factors on the development of brown plaster mould (Populaspora byssina) on mushroom beds in Kashmir/G.M. Dar, G.M. Beigh and N.A. Mir. 61. Effect of compost supplementation with oil seed cakes on the yield of Agaricus bitorquis (Quel) Sacc./M.S. Saharan and D.S. Guleria. 62. Some observations on epidemiology and management of brown rot (Gliocladium deliquescens Sopp) of Pleurotus sajor-caju/V.P. Sharma and C.L. Jandaik.