Department of Plant Pathology, Pennsylvania State University, University Park, PA 16802, USA

Background and objectives
In 1950, Sinden and Hauser [1] described a serious infectious disorder of the button mushroom Agaricus bisporus occurring at a commercial farm in the USA. Today this disease, commonly referred to as La France disease or virus disease, is known to occur virtually worldwide and is recognized as an important limiting factor in commercial mushroom production. Hollings [2] proposed a viral aetiology for the disease following the observation of a complex of three different virus-like particles (VLPs) in affected tissues. In the ensuing years, however, a plethora of morphologically distinguishable VLPs were detected in mushrooms and mycelium of A. bisporus. The presence of presumably the same VLPs in both healthy and diseased tissues, confounded by the lack of molecular evidence supporting the viral nature of the VLPs and of a reliable infectivity assay, created confusion and scepticism regarding the viral aetiology of the La France pathology.

Research carried out during the past two decades has significantly advanced our understanding of the viral complex in commercial strains of A. bisporus. Several RNA viruses have been isolated and described, and information has begun to emerge on genome organization and pathological importance. This presentation will serve as a forum to review the status of knowledge of the Agaricus viruses, particularly as it relates to La France disease, and to address the possible direction and constraints of future virus research.

Results and conclusions
Among the numerous viruses reported in A. bisporus, three viruses prevailing in commercial hybrid mushroom strains have been studied most extensively. One virus, La France isometric virus (LIV), consists of nine dsRNAs ranging in size from 0.8 to 3.8 kbp that are encapsidated in 36-nm isometric particles. All lines of evidence implicate LIV as the primary aetiologic agent of La France disease. Typical of dsRNA-genome viruses, virions of LIV contain an RNA-dependent RNA polymerase (RdRp), which synthesizes single-stranded RNA (ssRNA) transcripts corresponding to each of the dsRNAs. Sequence information is available for several of the dsRNAs. The 3.8-kbp dsRNA encodes the RdRp, the 3.0-kbp dsRNA encodes a major virion-associated protein, and the 1.3-kbp dsRNA encodes a cytoplasmic protein.

A second virus, mushroom bacilliform virus (MBV), has a genome composed of a single, linear, positive-sense ssRNA molecule (4 kb) that is packaged in a 19x50-nm bacilliform particle. MBV is the sole member of a new family of ssRNA fungal viruses known as the Barniviridae. The MBV genome has been sequenced and contains four major open reading frames, one of which encodes the capsid protein and another a putative RdRp. A survey of mushroom isolates by reverse-transcription PCR has suggested that MBV is not essential for pathogenesis, but accompanies LIV in ca 60% of the episodes of La France disease. Also, LIV and MBV are not co-dependent for replication, a conclusion based on the discovery of naturally occurring, singly infected mushroom isolates. Vesicle virus (VV), a third virus, is composed of three dsRNAs of 2.4, 5.2 and >13 kbp contained within ca 75-nm membrane vesicles. VV is widespread in commercial hybrid mushroom strains, but has not been linked to a specific phenotype; it can be detected in normal and abnormal mushrooms. Progress in gaining a precise understanding of the biological consequences of viral infection and in genetically engineering disease resistance hinges on the availability of a robust genetic transformation system for A. bisporus.

1. Sinden JW, Hauser E, 1950. Mushroom Science 1, 96-100.
2. Hollings M, 1962. Nature 196, 962-965.