Total annihilation

Look, I'm no expert on this kind of thing so take it with a grain of salt...but that doesn't look like slug damage.

Slugs tend to eat low growth, and eat anything they can get their...er, hands...on. Young dahlia, when they're short, are particularly susceptible. I've always scattered Sluggo around mine until they get larger, after which the slugs don't seem to be much of a problem (probably because they're dead).

I don't see evidence of leaf damage on the shots (the damage in the second shot not being slugs), so I think something else is going on here.

A fungus? Bacteria? Virus? I have no idea. Hopefully somebody has a clue and can help you.

Well the bottom of the plant is pretty bare and a little crisp. Related?

Very related, thank you. That's definitely not slug damage--the little *^%&%@ tend to take semi-circles out of the leaves, put holes in leaves, or just eat the whole leaf!

Also, while slugs can get up that high without a problem, they generally don't bother unless there's no other food source. I see another food source right in that shot at a lower level.

Just to rule it out, you can try a beer trap from a low (and disposable) dish. Put it into the soil with the lip relatively even with the soil and leave overnight. In the morning, if you have a slug problem you'll find dead slugs. Lots of hung-over dead slugs.

I'm still saying disease, but I'll be durned if I can ID it for you. Otherwise, the plant seems a bit spindly, perhaps. At least it doesn't look like DMV offhand.

Not directly bug related. Might be a fungus, invited by the overwatering, but not sure.

The foliage looks healthy, though sparse. Just pull off the brown leaves and black buds so the plant knows to grow out more, and discard waste away from your plants to avoid giving bad bugs or fungus places to hide/grow.

This occasionally happens, and it seems random with no clear cause. The plant will most likely shake it off, and start producing normally in a week or so.

A bit of fertilizer sprayed on the leaves might help it shake off whatever is ailing it. Think of it as a vitamin supplement to help you over a cold.

Well, I installed the watering system halfway through the growing season, so most of the early growth didn't get enough water. Now I have watered deeply twice a week and fertilized twice in August, and the top growth is much better. Now I know for next year! I'll have to see what I can do about the fungus potential, though. That's in a low spot along the foundation plantings. I will have to pull them up this winter and put in some new soil, or maybe just put the soil right over the top of them since I can overwinter in the ground.