Orange agents of decay

While checking a bee-log in our garden, I noticed one of the holes had a dead fly in it. I removed it and was about the throw it away when I saw tiny orange dots on the surface. Because I am a nosy ecologist I put it under the microscope to see if there was anything interesting going on, and saw what looked like a microfungus growing out of the joints and bristle-bases of the fly.

Fly (possibly in the family Muscidae) with orange fungal growths.

There are some very interesting fungi that parasitise insects and I've written about one of them before. However, this looks different - maybe something simply using an already-dead fly as a substrate.

Fungal growths showing the thin threads of hyphae between the orange fruiting bodies.

Fungal fruiting bodies at the bases of the fly's bristles.

The fungal structure is clear - hyphae and fruiting bodies - but with most microfungi, it is necessary to look at the spores, so I made a slide and zoomed in.

Fungal spores x100

The spores are clearly visible as slightly curved spindles, each split into several sections by cross-walls (septae). However, though a mycologist would probably know this fungus by sight, I couldn't find anything that matched it, so sent my pictures to the British Mycological Society's facebook group. It didn't take long for one of the BMS to tell me that this was a species of Fusarium. This at least explained why I hadn't worked out what it was - most Fusarium species are soil fungi involved in decomposition of dead organic matter but I hadn't been looking at soil-dwelling species. If so, the fungus was simply decomposing the fly - which is what it does, and very usefully too; no soil decomposers = dead soil. However, some Fusarium are insect (and plant) pathogens, so it could have killed the fly before decomposing it.

Fusarium are varied, diverse (in terms of both species and strains) and not all well understood or taxonomically clear, though some cause plant diseases and others can infect humans, while one is the main ingredient in Quorn! So, the identification stops at genus on this occasion, but has introduced me to a fungus that, although I'd heard of it, and it may be growing all around us in the ground, I've never stopped to look at before. Thanks BMS!