Effect of the fungal pathogen Verticillium fungicola 1 on fruiting initiation of its host, Agaricus bisporus Miche `le L. LARGETEAU*, Jean-Michel SAVOIE INRA, UR1264, Mycologie et Se ´curite ´ des Aliments, BP81, F-33883 Villenave d’Ornon, France article info Article history: Received 6 September 2007 Received in revised form 18 January 2008 Accepted 24 January 2008 Corresponding Editor: Daniel C. Eastwood Keywords: Fruit body initiation Fungicolous fungi Mushroom Mycoparasites Lecanicillium fungicola abstract Dry bubble disease caused by the fungal pathogen Verticillium fungicola 1 is responsible for large losses to the mushroom (Agaricus bisporus) industry. The pathogen induces various symptoms on the host, bubbles (undifferentiated spherical masses), bent and/or split sti- pes (blowout) and spotty caps. Inoculation of A. bisporus crops with isolates of V. fungicola var. fungicola of various degrees of aggressiveness showed that the more aggressive isolates induced higher numbers of bubbles. The production of other symptoms did not vary with the isolate of pathogen. The total weight of the crop (healthy and diseased mushrooms) was not significantly affected by the disease, but inoculation with highly aggressive iso- lates resulted in a significant increase in the total numbers of mushrooms. Two hypotheses are proposed to explain the effect of the pathogen on fruiting initiation in relation to aggressiveness. ª 2008 The British Mycological Society. Published by Elsevier Ltd. All rights reserved. Introduction The common cultivated mushroom Agaricus bisporus is sus- ceptible to various competitors and pathogens, among which Verticillium fungicola, 1 responsible for dry bubble disease, causes severe losses worldwide in mushrooms farms. Two varieties of the pathogen have been identified. Currently, V. fungicola var. aleophilum affects mushroom crops in Canada and USA (Collopy et al. 2001) and the variety fungicola is re- sponsible for the disease in Britain, the Netherlands, Spain, and France (Muthumeenakshi & Mills 1995; Foster et al. 2004; Desrumeaux & Sedeyn 2001; Gea et al. 2003; Largeteau et al. 2006). Both varieties induce similar symptoms on their host. The pathogen is unable to attack the vegetative myce- lium of A. bisporus (Calonje et al. 2000), but when the casing soil covering the growing substrate is inoculated with conidia of V. fungicola the pathogen is detected inside very small primordia of 1–2 mm diam (Largeteau et al. 2007). Infection at this stage of development results in the development of an undifferentiated, non-necrotic spherical mass (dry bubble) in place of the sporophore. Bubble is the main symptom of the disease, but later infection, when the mushroom morphogen- esis is achieved, is manifested in two other types of symp- toms, mushrooms with a bent and/or split stipe (blowout), and superficial cinnamon-brown lesions of the mushroom cap (spotty cap), (North & Wuest 1993; Rinker & Wuest 1994; van de Geijn 1982). Largeteau et al. (2005, 2006) detected vari- ability in aggressiveness among a group of V. fungicola var. fungicola, but observed that the total weight of the inoculated crops did not differ significantly from that of the uninoculated control. Further observations revealed that the pathogen might modify the level of fruit body initiation. The present work reports complementary studies on the effect of the pathogen on fruit body initiation in relation to aggressiveness * Corresponding author. E-mail address: largeteau@bordeaux.inra.fr 1 Now classified as Lecanicillium fungicola; see pp. 811–824 in this issue of Mycological Research. journal homepage: www.elsevier.com/locate/mycres mycological research 112 (2008) 825–828 0953-7562/$ – see front matter ª 2008 The British Mycological Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.mycres.2008.01.018