My plant was blooming beautifully,(I have it in a pot) I would deadhead it and kept it watered, ferilized with a time release fertilizer. At first I thought it was getting to much sun so I moved it so that it would not get the hot Florida afternoon sun but it just continues to decline what is the problem?
Otherwise you can Google "dahlia problems" and see if you find an answer there. I do think cutting it back and destroying all you remove is in order. It's obviously developed some kind of a serious problem. Maybe the new growth will be normal or maybe it's a lost cause.
Web usually = spider mites. Spray it with a jet of water daily for awhile, or as frequently as possible. That will shock the little jerks to death. You may find later that new growth is healthier. I use Spray n Grow foliar feed, or you could use some other foliar feed, sometimes it really helps the leaves.