This fungus is known only from artificially prepared mushroom compost, where it is treated as a pest with a co-ordinated effort to eliminate it. The species is unkown in the wild. As a result of new and more effective control measures, populations of this fungus in mushroom farms have declined sharply since the 1980s.

ASIA: China, India (Jammu & Kashmir), Korea, Japan, Taiwan. AUSTRALASIA: Australia (New South Wales), New Zealand (introduced). EUROPE: Netherlands, UK. NORTH AMERICA: USA (Delaware, Minnesota, New York, Ohio, Pennsylvania). SOUTH AMERICA: Brazil (Parana, São Paulo).

MINTER (2007) reports visiting a mushroom farm in eastern England in 1988, and finding the fungus clearly common and familiar to people working in production. Visiting the same farm in July 2007, he found that most employees were completely unaware of the fungus, and the farm’s production manager reported that it had not been seen for many years. Its disappearance was attributed to three factors: better pasteurization of the compost (3 hours at 60°C), a change from traditional composts to wheat straw composts, and harvesting only two flushes of mushrooms compared to the four flush system used earlier. PEGLER, SPOONER & YOUNG (1993) reported that it appears to have become uncommon in Britain. Minter considered that they were probably correct. In most mushroom farms surveyed in north India, this species was present as a weed (SHARMA & VIJAY, 1996), suggesting that it was widespread and common there at the time. The survey was, however, accompanied by advice on control, so the situation may well have changed. Almost all published work on the species (notably in Brazil, China, India, Korea and the Netherlands) treats it as a pest to be eliminated. The species is thus clearly threatened by very effective control techniques eliminating it from its only known habitat.

Using IUCN Categories and criteria, the species cannot be satisfactorily assessed, because the available criteria are incompatible with what is known about it: the categories extinct, extinct in the wild, critically endangered, endangered, vulnerable and near threatened all relate to the status of a species in its natural habitat, while least concern is reserved for widespread and abundant species which qualify for none of the higher categories. All that remains is data deficient. The present species is neither widespread nor abundant. No natural habitat is known. It is clearly at least vulnerable in the only artificial habitat in which it is known to occur. With such provisos, MINTER (2007) assessed Diehliomyces microsporus globally as data deficient (vulnerable).

This fungus has never been observed in a natural habitat, only ever in artificially prepared cultivated mushroom beds. This species is well-adapted for the warm conditions of mushroom compost, with optimum temperatures for growth and spore germination at 22–30°C. The thermal death point of ascospores is generally accepted to be 60°C (BISSET, COLHOUN & GANDY, 1982), although GLASSCOCK & WARE (1941) reported that ascospores can survive 3 hours at 93°C. Heat treatment of compost at 60°C for 3 hours is commonly used for control, though such treatment, even for 10–12 hours, may fail if some areas within the compost do not reach this temperature (BISSET, COLHOUN & GANDY, 1982). Mycelium and ascocarps can be grown on several nutrient media: optimum temperature for mycelial growth ranges from 26°C to 32°C, with a slight depression at 30°C (VAN ZAAYEN & VAN DER POL-LUITEN, 1977). Ascomata form most extensively between 25°C and 30°C but not at 15°C or 37°C (WOOD & FLETCHER, 1991). Associated organisms: Agaricus bisporus; A. blazei; A. brasiliensis; A. bitorquis; Agaricus sp.

As D. microsporus causes economic damage in commercial mushroom-growing beds, the species is very strongly threatened by persecution in the only environment from which it has been observed. The effects of fungicides on D. microsporus have been tested in vitro and in growing trials, without clear results. The fungicides tested could not prevent or control D. microsporus (VAN ZAAYEN & VAN DER POL-LUITEN, 1977). Various natural products, including water hyacinth (Eichhornia crassipes), neem cake and neem leaves have been found to result in significant inhibition of D. microsporus. When these were incorporated in the compost, water hyacinth in particular resulted in drastic reduction of D. microsporus, irrespective of the type of inoculum, and in significant yield increase of both A. bisporus and A. bitorquis (SHARMA & JARIAL, 2000). There has been some effort to find mushroom strains tolerant of D. microsporus (YADAV, DHAR & VERMA, 2000). Highly productive strains Horst K26 and Horst K32 are not be seriously affected by D. microsporus if the following precautions are taken: cooking out at the end of a crop (compost temperature 70°C for 12 hours) followed by treatment of the wood, and protection by covering of the compost by a thin plastic sheet during mycelial growth at an early stage in the crop (VAN ZAAYEN & VAN DER POL-LUITEN, 1977). Infestation of Agaricus brasiliensis mushroom compost and its control have been described by NASCIMENTO & EIRA (2003). Further controls discussed in internet sources include use of air filters, and, where there are only small areas of infestation, treatment with formalin. In Korea, D. microsporus has been controlled by temperature adjustment of the growing room and by fumigating the compost and the house with Basamid (http://www.cdms.net/ldat/ld7PI000.pdf) and Vapam (internet source). Various strains of bacteria and actinomycetes (LIN & CHEN, 1977) and pseudomonads (SINGH, CHAUBE & SINGH, 2000) have been tested as potential inhibitors of D. microsporus with some success. There have also been various experiments to determine which compost pasteurization technique is most appropriate against this fungus (NASCIMENTO & EIRA, 2004).

Ex situ conservation. Strains of this fungus are held in some culture collections. Some information about sequencing of this fungus is available on the Internet.

Surveying to discover this fungus in its natural habitat.

BISSET, P.G., COLHOUN, J. & GANDY, D.G. Germination of Diehliomyces microsporus ascospores and determination of their thermal death point. Transactions of the British Mycological Society 78 (3): 540-542 (1982). DIEHL, W.W. & LAMBERT, E.B. A new truffle in beds of cultivated mushrooms. Mycologia 22: 223-226 (1930). GEA, F.J., PARDO, A., NAVARRO, M.J. & PARDO, J. Mycoflora related to mushroom spawn. In T.J. ELLIOTT [ed.] Science and Cultivation of Edible Fungi, volume 2. Proceedings of the 14th International Congress, Oxford, 17-22 September 1995: pp. 557-562 (1995). GILKEY, H.M. Taxonomic notes on Tuberales. Mycologia 46: 783-793 (1954). GLASSCOCK, H.H. & WARE, W.M. Investigation on the invasion of mushroom beds by Pseudobalsamia microspora. Annals of Applied Biology 28: 85-90 (1941). HAWKER, L.E. Revised annotated list of British hypogeous fungi. Transactions of the British Mycological Society 63: 67-76 (1974). KERNI, P.M. & GUPTA, A. Competitor moulds and pathogenic fungi of cultivated mushroom, Agaricus bisporus (Lange) Sing. in subtropical Jammu (J & K) during spawn run and case run - a survey report. Research and Development Reporter 14 (1-2): 1-7 (1997). KHANNA, P.K., SODHI, H.S. & KAPOOR, S. Diseases of Agaricus bisporus and their management. Annual Review of Plant Pathology 2: 163-205 (2003). KLIGMAN, A.M. Control of the truffle in beds of the cultivated mushroom. Phytopathology 34: 376-384 (1944). LIN, C.-Y. & CHEN, D.-W. Actinomycetes as biological control agents for calves’ brains fungus, Diehliomyces microsporus, in mushroom culture. Mushroom Science 9: 125-143 (1977). LIN, L.-P. Prevalence of truffle disease in relation to microbial flora of mushroom bed. Memoirs of the College of Agriculture, National Taiwan University 13: 153-159 (1972). MASZKIEWICZ, J. & SZUDYGA, K. Occurrence of false truffle (Diehliomyces microsporus Gilkey) and the efficacy of bulk pasteurization. Vegetable Crops Research Bulletin 50: 107-133 (1999). MINTER, D.W. Diehliomyces microspores. IMI Descriptions of Fungi and Bacteria No. 1714 (2007). NASCIMENTO, J.S. Etiologia, Controla y Demanda de Energia na Prevenção da Falsa Trufa (Diehliomyces microsporus) em Cultivos de Agaricus blazei. Brasil, Botucatu: 115 pp., PhD Thesis, Facultade de Ciências Agronômicas, UNESP (2003). NASCIMENTO, J.S. & EIRA, A.F. DA. Occurrence of the false truffle (Diehliomyces microsporus Gilkey) and damage on the himematsutake medicinal mushroom (Agaricus brasiliensis S. Wasser et al.). International Journal of Medicinal Mushrooms 5 (1): 87-94 (2003) [available on-line through http://www.theliterature.org/bio/The_Fifth_Kingdom/International Journal of Medicinal Mushrooms/10372099.pdf]. NASCIMENTO, J.S. & EIRA, A.F. DA. Sistemas de pasteurização do composto e demanda de energia para controle preventivo da falsa trufa (Diehliomyces microsporus) em cultivo de Agaricus blazei [Compost pasteurization systems and energy demand in the preventive control of the false truffle (Diehliomyces microsporus) disease on Agaricus blazei mushroom cultivation]. Energia na Agricultura 19 (3): 47-53 (2004). PEGLER, D.N., SPOONER, B.M. & YOUNG, T.W.K. British Truffles a Revision of British Hypogeous Fungi (Kew, UK: Royal Botanic Gardens): [i-viii], 216 pp., 26 plates (1993). SHARMA, S.R. & VIJAY, B. Prevalence and interaction of competitor moulds in Agaricus bisporus. Mushroom Research 5 (1): 13-18 (1996). SHARMA, V.P. Biology and management of false truffle (Diehliomyces microsporus) during cultivation of Agaricus spp. Mushroom Research 7 (1): 1-12 (1998). SHARMA, V.P. & JARIAL, R.S. Efficacy of different fungicides and botanicals against false truffle (Diehliomyces microsporus) and yield of Agaricus species. Journal of Mycology and Plant Pathology 30 (2): 184-187 (2000). SINGH, M., CHAUBE, H.S. & SINGH, R.P. Effect of fluorescent pseudomonads on primordia formation, yield and control of pathogenic fungi of Agaricus bisporus (Lange) Imbach. Journal of Mycology and Plant Pathology 30 (3): 313-326 (2000). VAN ZAAYEN, A. & VAN DER POL-LUITEN, B. Heat resistance, biology and prevention of Diehliomyces microsporus in crops of Agaricus species. European Journal of Plant Pathology 83 (6): 221-240 (1977). VAN ZAAYEN, A. & VAN DER POL-LUITEN, B. Heat resistance, biology, some biological aspects and prevention of false truffle (Diehliomyces microsporus). In M.J. Maher [ed.] Science and Cultivation of Edible Fungi. Rotterdam, Balkema 2: 319-336 (1978). WOOD, M.W. & FLETCHER, J.T. The occurrence of ascocarps of Diehliomyces microsporus, the cause of false truffle disease. Proceedings of the 13th International Congress on the Science and Cultivation of Edible Fungi, Dublin, Irish Republic. Mushroom Science 13 (1): 379-384 (1991). YADAV, M.C., DHAR, B.L. & VERMA, R.N. Breeding studies on development of high yielding and quality hybrids of Agaricus bitorquis. In L.D. VAN GRIENSVEN [ed.] Science and Cultivation of Edible Fungi. Proceedings of the 15th International Congress on the Science and Cultivation of Edible Fungi, Maastricht, Netherlands, 15-19 May 2000: pp. 299-304 (2000).

See also the following internet pages:

http://archives.eppo.org/EPPOStandards/PP2_GPP/pp2-20-e.doc (EPPO standards for good plant protection practice)
http://ftp.dna.affrc.go.jp/pub/dna_all/A/Y8/39/84/AY839842/AY839842 (sequencing data);
http://kmbase.medric.or.kr/Main.aspx?d=KMBASE&m=VIEW&i=0379119790070010013 (occurrence in Korea);
http://loco.biosci.arizona.edu/cgi-bin/sql_getcluster.cgi?ti=129384&cl=53&ntype=1&db=GB159 (sequencing data);
{www.freepatentsonline.com/5149715.html (patent for control of mushroom weeds including D. microsporus);
http://www.nederlandsesoorten.nl/get?site=nlsr&view=nlsr&page_alias=conceptcard&cid=0FHCYFAXRNLM (status in Netherlands);
http://www.nbrc.nite.go.jp/jscc/idb/strains?Snt=0&Sn=Diehliomyces+microsporus (sequencing data);
http://www.nbrc.nite.go.jp/NBRC2/SequencSearchServlet?ID=NBRC&CAT=00030003&DNA=6 (sequencing data).