Molecular and physiological diversity among Verticillium fungicola var. fungicola.

The genetic and physiological variability of Verticillium fungicola var. aleophilum responsible for Agaricus bisporus dry bubble disease in North America is well documented but little is known about the var. fungicola affecting European crops. Variability was assessed within this variety and compared with that reported for the var. aleophilum. Eighteen isolates of V. fungicola var. fungicola and four var. aleophilum isolates were analysed for DNA polymorphism, mycelial growth, response to biochemicals produced by A. bisporus, fungicide resistance, and pathogenicity assessed by direct inoculation on sporophore or casing contamination. RAPD and AFLP markers delineated three French isolates from a homogeneous group containing the other var. fungicola isolates, but no correlation could be drawn between DNA polymorphism and the various traits studied. The var. fungicola isolates were more susceptible than the var. aleophilum isolates to the antibiosis effect of A. bisporus. Only mycelial growth rate at 23 degrees C could explain the variability in aggressiveness among the European isolates. The putative effect of the post-incubation temperature on contamination during mushroom cultivation was discussed. This work emphasized that, like the American var. aleophilum, the var. fungicola in Europe is genetically homogeneous, but physiological diversity exists, especially in France where it could be related to less standardized cultural practices.