Well, perhaps you got a poorly treated wood table. Looking at the picture, it sure looks like the wood is deteriorating at a rapid rate. It could be the lighting that makes it look like that. But regardless, this is what I would do:
First, scrape off all the fungus you can. Then make up a solution of bleach, and I would use the kind of bleach meant to be used outdoors, and spray on the bleach until the wood and fungus (mushrooms) are complete wetted. Leave it out in the sun to speed up the process of killing the fungus and spores. After drying, scrape/wire brush off the fungus as best you can. I would then re-apply the bleach solution a week later, and then again remove all the bits of mushrooms still there. Wash the wood well and allow to completely dry. Apply two coats of deck sealer to all the wood to prevent re-growth of the fungus and re-apply the deck sealant every year.
