Eocronartium muscicola is a parasitic bryophilous basidiomycete that has been found on at least 21 moss species, scattered across a number of families. The fungal fruiting bodies are simple, white stalks and this link will take you to a photograph that shows about ten such fruiting bodies growing from a species of trailing moss. Eocronartium muscicola is widespread in the northern hemisphere.
In the earliest detailed study, published in 1918 and involving the moss Climacium americanum, the fungus was found to parasitise the gametophyte and form its fruiting bodies at the apices of the gametophyte's branches. There had been two reports (in 1948 and 1981) of this fungus on moss sporophytes but the fungus was generally thought to be a gametophyte parasite. The 1980s saw more detailed investigations of the interactions between Eocronartium muscicola and six moss species – Climacium dendroides, Drepanocladus uncinatus, Eurhynchium hyans, Fissidens viridulus, Haplocladium microphyllum and Pylaisiella polyantha. Of those six, gametophyte parasitism was found only in Climacium dendroides. Otherwise the fungus infected mosses only where embryonic sporophytes have started developing. The fungus appears to initially develop around the foot of the sporophyte and so gets access to the nutrients passing from the moss gametophyte to the sporophyte. In its continued development the fungus stops the development of the sporophyte and grows in its place. Eventually, instead of a mature sporophyte appearing, what you see is the stalk-like fungal fruiting body - as shown in the photo-link above.
A moss sporophyte develops from a fertilized egg in an archegonium. After fertilization the embryonic sporophyte takes on an elongated form and at an early stage the tissue in the embryonic sporophyte starts to differentiate, with the upper and lower parts assuming different functions in the mature sporophyte. The upper section will develop into the spore capsule and the lower section will become the stalk (if the species produces stalked spore capsules) and a foot that penetrates the gametophyte. The young sporophyte depends on the gametophyte for nutrients and water. At the junction of the gametophyte and sporophyte's foot there are transfer cells. These are specialized cells which help with the efficient transfer of water and nutrients from the gametophyte to the young sporophyte. Transfer cells may develop on either the gametophyte or sporophyte - or both – depending on species. The development of transfer cells is triggered after the fertilization of egg in the archegonium.
Often a group of archegonia may also be enveloped by a cluster of protective leaves (called perichaetial leaves). After infecting a fertilized archegonium the fungus invades the developing sporophyte and supplants it. In some mosses the embryonic sporophyte is overgrown before it has developed beyond the perichaetial leaves. In other mosses (with more transfer cells) the sporophyte may not be supplanted until it has extended beyond the perichaetial leaves. In either case the fungal hyphae eventually hide all (or most) traces of the moss sporophyte. This makes it easy to understand why Eocronartium muscicola was commonly thought to parasitise the gametophyte.
By and large Eocronartium muscicola appears to make use of existing transfer cells but the fungus may have a limited ability to induce the development of additional transfer cells. It does not invade the moss gametophyte cells but hyphae may sometimes penetrate between the cells in the gametophyte, and then induce transfer cell development. This is however a very limited interaction with the gametophyte and the Eocronartium strategy means that the fungus taps directly into the nutrients being transferred to the sporophyte, with no harm done to the gametophyte. The fact that the fungus exploits the nutrients that pass through the transfer cells would explain the absence of the fungus from non-fertilized archegonia.
You've seen that Eocronartium exploits the rich nutrient flow across the gametophyte-spoprophyte junction in at least five moss species. Growing gametophyte apices are also major recipients of nutrients and this may explain Eocronartium's attachment to branch apices in the two Climacium species, for which sporophytes are rarely reported.
The genus Jola, related to Eocronartium, is widespread in the tropics (but not yet reported from Australia) and is also parasitic on moss sporophytes. Jola fruiting bodies develop on moss spore capsules and do not supplant the whole sporophyte. You could say that Jola also exploits the transfer cells indirectly, with the sporophyte as an intermediary, in contrast to the direct exploitation of transfer cells by Eocronartium muscicola.