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PESTS AND DISEASES


Smokey Mould: the smoke screen lifts


Smokey Mould is a relatively unknown and very difficult to recognise competitor mould that oc- curs in Agaricus cultivation. This sooty assassin wreaks havoc under cover of a smoke screen. New research has shed light on the origin and life cycle of the mould and has made a valuable contri- bution to the identification and control of this disease.


By Con Hermans, AdVisie (hermans@mushroomconsulting.nl) and Jos Houbraken, CBS-KNAW Fungal Biodiversity Centre (j.houbraken@cbs.knaw.nl)


A number of very aggressive competitor moulds raise their ugly heads in mushroom growing. The best known and easiest to recognise of these is Trichoderma. In re- cent years, research has focused entirely on this mould. A less well-known mould but the same catastrophic consequences, that has seldom been the subject of any studies, is Smokey Mould. This mould can cause a drop in production of up to 80%. This mould goes to work stealthily and is undetected in many cases, so it is not identified as the real culprit behind the problems with quality and production. And once the mould has been identified, it has usually left a trail of destruction in the production figures. It is a very inconspicuous com- petitor mould, able even to escape the eagle eye of a professional. So finding it involves some real detective work.


AdVisie and CBS-KNAW in the Netherlands have done extensive research into the matter the last few years, and have come up with new insights ánd a new name.


Confusing nomenclature Smokey Mould thanks it name to the profuse clouds of spores that resemble rising smoke that infected com- post releases when disturbed. The first time Smokey Mould was signalled in the Netherlands was 20 years ago. At the time, the mould was grown as a culture on a medium in a laboratory and then determined as Pe-


Large bare patches


devoid of mushroom growth caused by Smo- key Mould infection.


nicillium citreonigrum. If you consult today’s literature, you will notice many different names linked to Smokey Mould, including Penicillium chermesinum, Penicillium paxilli, Penicillium viridicatum, Penicillium implicatum and Penicillium fellutanum. Smokey Mould therefore belongs to the Penicillium genus. The species that fall under this genus all bear a strong resemblance and are very difficult to distinguish from each other based on exterior traits. So it is quite possible that all the Penicil- lium names linked to Smokey Mould are actually one and the same genus of mould, as these strains have not been characterised properly. The first step in this study was determining the precise identity of Smokey Mould. To do this, we compared Smokey Mould isolates taken from six different out- breaks that occurred over a period of more than 15 years. Four of these outbreaks occurred in the Nether- lands, one in Canada and one in the United Kingdom. To accurately identify these strains, DNA techniques and characterisation based on external traits - the phenotype- were combined. This analysis revealed that all the outbreaks, in all three countries, were caused by the same strain of mould. To some degree this result is comparable with Trichoderma, where it is predo- minantly one strain (Trichoderma aggressivum) that causes all the problems in mushroom cultivation. Many moulds can form massive amounts of spores that can be released from compost. We are all familiar with the ‘cloud’ of actinomycetes at the moment of inocula- tion. Assuming the diagnosis ‘it must be Smokey Mould because of the cloud of spores’ would often have been an erroneous decision as Smokey Mould is caused by a very specific strain of mould. If Smokey Mould is compared with other, previously described Penicillium strains, it appears we are dea- ling with a new strain. We have called this new strain Penicillium hermansii, after one of the authors of this article. The official description of this strain will ap- pear in another publication. Another notable aspect of the Smokey Mould isolates is the slow growth rate on nutrient mediums. This can sometimes complicate the diagnosis in the laboratory, as Smokey Mould can be easily overrun by other moulds.


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