Been doing some research myself on this, and you're right, its next to impossible to find an answer. I did discover that this type of fungus is often the cause of brown rot and other fungal diseases on vegetables and other plants - did you find a definitive answer elsewhere?

I have now been told it's because the horse manure was to fresh. It's harmless to plants and was told to dig it out as and when I saw it.

Thanks

thanks for the info.