Gnats feast on saguaro fungus

Q: A saguaro cactus in my parent’s yard has something moving into it. But I was wondering if you knew what the black stuff was. When I disturbed it, some gnats flew from it, which I thought was weird.

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A: The black stuff is a fungus that is growing on the excavated cactus parts that were kicked out of the hole and stuck to the spines. The fungus gnats you saw flying from the fungus are just feeding on that and not really a problem for the cactus.