Wild strains of Agaricus bisporus: a source of tolerance to dry bubble disease

Abstract

The button mushroom Agaricus bisporus is susceptible to various pests and diseases. Dry bubble, caused by the Hyphomycete Verticillium fungicola, is currently the more serious disease and is worldwide in distribution. Cultivars are susceptible and the pathogen develops resistance towards the very few fungicides admitted at mushroom farms. Breeding for resistance is necessary and wild strains of A. bisporus are putative sources of tolerance to V. fungicola.We present results on the susceptibility of some wild strains of the INRA-CTC collection and the PPO MRU collection. Besides the severity of the disease, the strains were also compared for their ability to develop each of the symptoms induced by the pathogen: spotted mushrooms, stipe blow-out and spheroid masses (the bubbles) which are the typical symptom of the disease. Agaricus bisporus 2100, cultivated in numerous French mushroom farms, was used to assess the aggressiveness of various isolates of V. fungicola var. fungicola, the variety responsible for the disease in Europe at present. Significant variability in aggressiveness was observed. Isolate VCTC, which induced severe symptoms on A. bisporus 2100 (30-40% of diseased mushrooms), revealed interesting tolerance (10-18% of diseased mushrooms) among five wild A. bisporus strains and hybrids between wild strains. A cross test was performed with two cultivars and seven wild stains of A. bisporus contaminated with five V. fungicola isolates, two of var. fungicola and three of var. aleophilum, the latter identified as responsible for the disease in USA and Canada. The wild strains screened in this experiment were far more tolerant than the cultivars, exhibiting 3-9% of diseased mushrooms compared to 20-22%. All the strains were more susceptible to the pathogens of var. aleophilum than to those of var. fungicola. These experiments showed that very tolerant material exists in collection and can be used as parents to breed for resistance. The greater susceptibility of A. bisporus to V. fungicola var. aleophilum must be taken into consideration in breeding programmes, this variety being present in North America and being isolated in Europe in the past.