Should my deck be growing mushrooms?

Replace the board, possible the boards next to it and the joist under the boards. Then wash the deck with deck cleaner and seal the deck with a stain, every year.

Plus in winter climates remove snow from the deck.

fungus grows on rotten wood. replace the pieces that are rotting. try pouring bleach on the fungus, killing it, scraping out the rotten wood and fill it with a wood filler. think your deck could use a sealer.

Yeah, that's not good -- it means there's a lot of moisture in the wood. Can you replace just the particular boards that have this? Otherwise they're going to rot out quick and break apart. You don't have to replace the whole deck, just the trouble pieces.

Replace the boards. They are beyond rotting when mushroom fungus is feeding on the wood. wash the deck down with deck cleaner, stain it with deck stain or just a deck waterproofer. Decks need to be maintained about every four years.

Polly Fuller is correct. You need to clean out the decaying wood until you reach good wood. I have had that problem with a pergola. I used a rotary tool with a grinding head to clean out the debris. I used a epoxy wood filler and painted it. That was 10 years ago and it still looks good.

Your deck should be cleaned and sealed. A stain may help cover the patches. Check with a good hardware store to see what is best for your location. Even wood exposed to the weather will last a long time if it has a protective coat on it.

This might give you some more information. You can also search YouTube for “deck repair”.

https://homeguides.sfgate.com/repair-deck-wood-filler-40862.html

I'm not sure about the mushrooms but I must say that that is ONE CUTE DOG!!!!!!!