Until about 1980, button mushrooms had been improved by selection: the discovery and preservation of mutants and of spore cultures that showed improvements relative to their parents.

Between 1970 and 1980 several laboratories learned techniques for cross-breeding two strains of A. bisporus to produce a hybrid strain.   The first successful commercial hybrid strains were the Horst 'U1' and 'U3' strains, developed by Dr. Gerda Fritsche at the Proefstation voor de Champignoncultuur in The Netherlands.  The U1 hybrid now forms the bloodstock of almost all commercial white button mushroom strains grown outside of Asia.

Dependence upon a single crop genotype is called 'monoculture'.  It raises the risk that a large fraction of the global crop could be susceptible to a new or mutated pathogen.  Serious pathogen outbreaks have affected the button mushroom crop previously: LaFrance virus disease in the 1960s, and aggressive Trichoderma harzianum strain types in the 1990s.

A basic goal of crop breeding is genetic diversification, to reduce and manage such risks.  Further goals of mushroom breeding often include some or all of the following improvements:

Breeding and diversification depend upon the availability of large numbers of genetically different individuals (= strains).  Until recently, major culture collections held as few as 12 (or fewer) genetically distinct strains of A. bisporus.  The Agaricus Resource Program is the first program designed to increase the amount of Agaricus germ plasm available to mushroom breeders worldwide.