Going to make a guess, was this deck built close to grade or over an old slab?
Reason I ask is it's rare to see pressure treat wood rot out like that unless there's no air flow under it so it can dry out.
No. It has 18 to 20 in crawl space under it. Deck wasn't being maintained when I bought the house. Not sure how many years ago it was built. I've owned the home 12 yrs. I have stained the deck every year since I have owned it. I am hearing people say replace whole boards. Guess that is what I will do