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Abstract: Mycogone perniciosa, which causes Wet Bubble Disease (WBD) of Agaricus bisporus, is very contagious and results in severe crop losses. The mycelium, phialoconidia and aleuriospores of Mycogone spp., all proven to be infectious, are easily spread by vectors. The pathogen adheres to and penetrates the host during any stage of fruit body development. The mycopathogen grows superficially but causes dramatic cytological changes inside the host. Microscopic features of the infected mushrooms are characterized mainly by a dedifferentiation process and an excessive, nodular, reactive or hyperplastic proliferation of hyphae. Exudation of accumulated extracellular fluid results in a tear-drop phenomenon (hence wet bubble). When young primordia (up to 6 mm) are infected, large, very irregular, nodular and tumorous fungal masses are formed. Despite the large size of these fungal masses, no signs of cell differentiation and organogenesis can be found. This phenomenon provides experimentalists with an easy and reproducible model to study a parasitic fungus-fungus interaction as well as fiingal developmental errors. The pathogenesis of WBD, and gross and structural changes occurring during this mycotic infection of A. bisporus, are presented from a morphologist's point of view.
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