In October I wrote an article, published in The Mushroom People Vol. 3a No. 140, Nov. 2002. In this I posed many questions, aimed at various aspects of the biology of brown plaster mould (BPM).

I called BPM the Cinderella of serious loss causing weed moulds because the only literature on it that I had been able to find suggested that it is ‘now’ uncommon and of trivial importance. I believe the losses it causes in Ireland in the autumn of most years, make it very worthy of further research, particularly so, if this illuminated its life cycle and those aspects of BPM biology that might result in reduced losses.

This article aims to give some control advice. BPM being mainly an autumn weed mould, such advice is certainly early!

BPM Experiments
In the months since writing my plea, I have performed about a dozen small experiments. These could be helpful to a potential researcher, or to a grower that has suffered crop losses from brown plaster mould (BPM) and wishes to minimise future loss.

Comparing and contrasting BPM and Trichoderma Th2

I have found BPM experiments are best performed using phase II compost that has not been spawned. The reason for this is that mushroom (Agaricus) mycelium appears to stop the growth of BPM mycelium dead. Agaricus mycelium may then overrun that of BPM. However, if BPM has formed its large brown ‘spores’ (bulbils), these will survive and can successfully transfer BPM growth to ‘fresh’ compost.

The ‘preference’ of BPM to grow best in unspawned compost, contrasts with the ‘preference’ of Trichoderma harzianum (Th2) for spawned compost. The latter appears to only grow strongly in compost alongside Agaricus (or else in phase II compost that has been artificially sterilised). Both fungi sometimes end up sporing luxuriantly on the casing surface, however, growers should be aware that the mycelium growth of both fungi has been in the compost below. Trichoderma (Th2) appears to ‘needAgaricus, whereas BPM is strongly inhibited by it.

BPM growth rate is much faster than Agaricus

A series of experiments were initiated using an inoculum consisting of either BPM mycelium, or BPM spores. These experiments indicate that BPM extension growth can be as high as 18mm per day. This is much faster than Agaricus’ growth.

BPM thrives in unspawned compost at ‘high’ spawn run temperatures

In experiments, one successful method of BPM inoculation was to use a strip of stiff paper about 2cm wide. The end of this had been brushed against a sporing BPM colony. It was then inserted down the side of either a large or a small plastic bag. This had been recently packed with unspawned phase II compost. In initial experiments, bags containing spawned compost gave variable results, sometimes BPM thrived, sometimes not. When bags were unspawned, BPM always thrived, provided the incubation temperatures were high enough.

Is BPM a decomposer of cellulose?

In experiments where a strip of stiff paper was used to transfer spores, it was noted that the paper ended up full of holes after a several weeks of BPM growth.

How do BPM bulbils germinate?

So far, I have not been able to get BPM ‘spores’ (bulbils) to germinate in a film of water on a microscope slide, even after several weeks, nor when bulbils are floated on the surface of jars of water, with or without a little compost present. Neither do the bulbils that sink to the bottom of a jar of water germinate. However, so far, these negative results have been obtained at ‘cool’ room-temperatures. These fluctuated daily between 10 and 15°C. Furthermore, there was no BPM growth in bags of either spawned, or unspawned compost that were incubated for four weeks at similar temperatures (10-13°C). Such results contrast with those in which BPM bulbils on paper strips, or in drops of water, were introduced into small, (or full size bags) of unspawned compost that were then ‘incubated’ at around 20°C or above. After 4 weeks, such bags were fully colonised by BPM and were literally golden brown in colour due to masses of BPM bulbils. At 27°C, even large, unspawned, bags of compost were almost fully colonised after only two weeks.

As to be expected, colonisation that was initiated using BPM bulbils always lagged behind that started by introducing a pinch of compost containing live BPM mycelium. However, the sizes of the latter colonies after incubation were not very much larger. This suggests that either BPM bulbils ‘germinate’ quite rapidly (provided temperatures are high enough). Alternatively, is it possible that BPM bulbils ‘germinate’ to produce motile spores?

Beware BPM spreading on farm

It is usually assumed, by both grower and compost maker alike that BPM always ‘arrives’ in the compost. However, it is very likely that this is not always the case.

Provided incubation temperature is around 20°C (or better still for BPM 25°C+) a drop of water containing BPM spores leads to vigorous growth. Similar growth arises from a 2cm wide strip of paper with a dusting of spores visible on the tip of it.
In these experiments, spores were placed at the bottom of a compost bag, or at each side, or on the surface of the compost (a minimum of four replicates were used for each placement type).

Growth was greatest when BPM spores were placed at the bottom of the bag. Here, it is assumed, the weight of the compost gave best contact between it and the BPM spores, thus ensuring their rapid ‘germination’.

To emphasise the point that a grower, if careless about farm hygiene, could transfer BPM on farm, a simple direct experiment was conducted.
A sporing colony was touched using a fist that was then pressed onto the surface of each of 4 large bags. This produced a single, central, dent. BPM growth was observed, starting at the dent and progressing vigorously down into the bag.

After two weeks, about half of each bag was colonised. In more than one case, BPM transferred, via a small tear in a bag, to an uninoculated bag that had been placed below it. (To create greater warmth). It is thus presumed, BPM growth could go in the other direction from a contaminated floor up into a ‘clean’ bag of compost.

Inoculation by fingertip?
A next step would be to ‘infect’ a bag using a single fingertip to transfer spores. Previous experience with Trichoderma (and cobweb) suggests that almost anything that moves can transfer fungal colonies that sporulate on the surface of compost. For example, contact by humans, dogs, cats, mice, mushroom flies, mites, even water splashes etc have been shown to transfer such fungi.

The difference between Trichoderma and BPM is the bulbils of BPM are vastly greater in size than Trichoderma spores.

However, a BPM bulbil is still very small compared to a fly. Furthermore a single bulbil would be a very much more powerful inoculum than a group of Trichoderma  spores, because of its relatively very large size.

A bulbil feels like a grain of table salt if rubbed between finger and thumb. The strong and unpleasant smell of a disturbed BPM colony may attract a ‘natural’ animal vector. Whatever that may eventually prove to be?

The vital importance to Agaricus of adequate aeration
With Trichoderma, high levels of CO2 appear to favour it when growing alongside and in some ways in competition with Agaricus for compost nutrients.

In two experiments, using BPM, bags of spawned compost were inoculated and placed one on top of the other. In the upper bags, both Agaricus and BPM grew more rapidly and BPM sporulated sooner than in the lower bags. Eventually, however, the lower bags ended up with the larger BPM colonies. It is assumed this is because Agaricus had been so severely hampered in its growth, by lack of ‘air’ (oxygen) that it had less ability to stop BPM growth.

On several occasions in experiments, over the years, where aeration was deliberately hampered, Agaricus growth was relatively very poor. This is an important point that growers should keep in mind, high CO2 may benefit Agaricus growth, but only if there is also adequate oxygen needed for its respiration.

The vital importance to Agaricus of correct growing temperatures
With Agaricus, optimum growth rate is around 24°C, above this growth rate rapidly declines and mycelium is heat damaged. Furthermore, above 24°C the natural heat loving fungi (thermophils) in the compost become active and start to destroy the compost.

BPM could not be classed as a heat loving fungus, however, its optimum growth temperature appears to be several degrees higher than for Agaricus.

Furthermore its minimum appears to be somewhere between 5 and 10°C above that for Agaricus. Consequently, the best strategy to combat BPM is to restrict air temperature during spawn run to no more than 15°C. Possibly allow a couple of extra days for spawn run. This gives Agaricus its best chance to outgrow and overrun BPM (if by chance BPM is present in the compost).

Pick BPM colonies off the surface of compost prior to casing

Often on farms, BPM is seen growing mainly at the interface between compost and the plastic of bags/blocks. Such colonies often do not penetrate deeply, having been restricted by Agaricus. BPM may thus look much more extensive than it is and may thus have a relatively small effect on yield.

For example, if BPM is found sporing on the upper compost surface colonies can be picked off. They will then not physically restrict Agaricus movement into the casing. When such colonies are found frequently, it is assumed that a high initial air temperature, plus plentiful condensation on the internal, plastic surfaces, have aided germination of BPM bulbils but mycelium growth from them has soon stopped by growth of Agaricus mycelium.

I should not have to mention the importance of hand hygiene and of safely disposing of such BPM contaminated material after removal!