I was out on a fungal foray recently with Andy Overall, at Hampstead Heath, and we came across an interesting little site: some boletes parasitising the common earthball (Scleroderma citrinum).
Almost exclusively, one would associate the boletes (now comprising of various genera, after the genus Boletus became near defunct thanks to molecular analysis) as being ectomycorrhizal (associating with the roots of trees), though that is not always the case. The parasitic bolete, Pseudoboletus parasiticus, in fact is both ecotmycorrhizal and parasitic.
According to Binder & Hibbett (2006), whilst this particular bolete is capable of ectomycorrhizal associations with the roots of particular tree species, this symbiosis is not sufficient enough for the fungus to acquire the nutrients it demands. Thus, it attacks the common earthball, in a bid to satiate itself of the desire for more resources (likely carbon, most notably).
In terms of its frequency, a quick search suggests it isn’t too abundant. However, Spooner & Roberts (2005) do note, in their offering to the quite wonderful New Naturalist series, that which species of mycorrhizal fungi end up producing fruiting bodies is not necessarily relative to their below-ground abundance. In fact, the fungal species with greater abundance may fruit less than those that have more limited distribution within the soil environment. The mechanics of this are generally beyond the scope of this post, though in essence resource availability is one means of driving sporophore production, and thus fungi with limited resources may look to create fruiting bodies more readily than those that have locked down vast swathes of an area, so that their genetic lineage has a better chance of continuing.
And thus, here are some cool (but rather poor!) images of the parasitism of this particular bolete. Poor earthball…
Binder, M & Hibbett, D. (2006) Molecular systematics and biological diversification of Boletales. Mycologia. 98 (6). p971-981.
Spooner, B. & Roberts, P. (2005) Fungi. Thailand: HarperCollins.