Most mushrooms that we encounter in our local supermarket or delicatessen, like the button mushroom, are grown on dead plant material. However, more than 1000 species of edible mushrooms such as the truffles (below left), chanterelles (top right) and porcini (below right) are formed by ectomycorrhizal fungi and some of these have well established worldwide markets measured in billions of dollars.
Combined French Périgord black truffle and winter truffle production (tonnes) 1903 - 2001 (from Michel Courvoisier).
Possible reasons for this decline include deforestation, the loss of host plants within forests due to pests or disease, changed forest management practices such as planting more densely than occurs in natural forests, the replacement of natural forests with plantations of species that are poor hosts for edible mycorrhizal mushrooms, global warming since the last ice age, soil compaction by hordes of pickers, acid rain and, for truffles, the loss of expertise during two World Wars as to where and how to find them.