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In plant-pathogenic fungi, the pmk1 mitogen-activated protein kinase (MAPK) signalling pathway plays an essential role in regulating the development of penetration structures and the sensing of host-derived cues, but its role in other pathosystems such as fungal-fungal interactions is less clear. We report the use of a gene disruption strategy to investigate the pmk1-like MAPK, Lf pmk1 in the development of Lecanicillium fungicola (formerly Verticillium fungicola) infection on the cultivated mushroom Agaricus bisporus. Lf pmk1 was isolated using a degenerate PCR-based approach and was shown to be present in a single copy by Southern blot analysis. Quantitative RT-PCR showed the transcript to be fivefold upregulated in cap lesions compared with pure culture. Agrobacterium-mediated targeted disruption was used to delete a central portion of the Lf pmk1 gene. The resulting mutants showed normal symptom development as assessed by A. bisporus mushroom cap assays, sporulation patterns were normal and there were no apparent changes in overall growth rates. Our results indicate that, unlike the situation in fungal-plant pathogens, the pmk1-like MAPK pathway is not required for virulence in the fungal-fungal interaction between the L. fungicola pathogen and A. bisporus host. This observation may be of wider significance in other fungal-fungal and/or fungal-invertebrate interactions. INTRODUCTIONLecanicillium fungicola (Pruess) Hassebrauk is the causal agent of dry bubble disease of the cultivated button mushroom Agaricus bisporus (Lange) Imbach. It is one of the most serious pathogens of this crop and is worldwide in distribution. The disease manifests itself in three different symptoms: dry bubble, stipe blow-out and cap-spotting diseases (Mills et al., 2008). Disease outbreaks remain a serious threat to the mushroom industry and can devastate crops where successful management of this disease mainly relies on good cultural practices and strict sanitation. The broad-spectrum azole fungicide prochloraz-manganese has sometimes been used to control infections; however, resistance is becoming more common within the pathogen population, which limits the usefulness of this product (Bonnen and Hopkins, 1997). Other fungicides are less effective or impact on the host fungus (ChrysayiTokousbalides et al., 2007) and may give concerns regarding residues in the crop or in the spent mushroom compost. To date, no other acceptable means of chemical control for L. fungicola have been marketed, probably due to prohibitive development and licensing costs, whilst difficulties involved with breeding of A. bisporus have prevented disease-resistant strains from being developed. Although formerly known as Verticillium fungicola, the taxonomy of these fungi has recently been revised, with the group renamed Lecanicillium (Zare & Gams, 2001Amey et al., 2007), showing it to be more closely related to the insect pathogenic fungi, such as L. longisporum (formerly V. lecanii), and nematode pathogens, such as those within the genus Haptocillium (e.g....