Where do you live? It doesn't look like insect problems, it looks more fungal. Have you been getting an excessive amount of rain recently?

I live near Nashville TN. And Yes, we have had more rain this summer than normal. What should I try?

Well I don't know 100% what it is but as long as the plant is growing and continually flowering, I'd leave it alone. I would definitely not give it any supplemental water. Unless you're willing to dig the entire plant up and put it in different soil, leaving it alone would be the next best option. If Ma Nature would turn the spicket down, that'd help. I assume you are going to dig this plant in the fall and overwinter the tubers? How is your native soil? Dahlias do not like to be wet. We have heavy clay soil with rocks (S PA), which helps, but I still amend with sand, pea gravel, and vermiculite. All these things allow the moisture to pass through and dry out rather than stay wet. Even with that, I still almost lost a plant this past June to over watering by Ma Nature.

With all respect intended, that is not caused by water damage. I lost over twenty varieties and two hundred tubers to June rains in new beds that were too low, and none came close to looking like that. I wouldn't hold off watering an already stressed plant.

This is either insect, fungal or virus... It truly looks more like insect infestation to me, either mites or scale. I truly doubt it is fungal, but you could spray Neem oil or horticultural oil for good measure just in case (and that would help against insects, too).

Sometimes sprays take a while to start helping, and you have to examine NEW GROWTH to see if it has helped any. Carefully strip off all the damaged buds and leaves, throwing them away in your garbage to avoid spreading whatever you have. Watch for the new growth out of the 'armpits' of the old leaves to see if they are healthy. If you have been spraying as directed, the problem should be alleviated if insects are the problem.

If the new growth looks similar to what you are showing even after treatment, I'd dig and toss that plant in the garbage as soon as possible.

Actually, I'm not sure that it has bloomed since it got severe and it is about a foot shorter than the one next to it.

How close is the next plant? Does it look anything like this one?

I'd be treating all the plants you have, as it would be a real bummer if this spread. If you have another of this variety, I'd recommend pulling it and continue treating the rest. If this was the only dahlia plant you had, it might be worth the fight to keep it alive... Just my thoughts...

If it was an insect infestation, wouldn't there be evidence? I've done a little reading online and it sounds like it could be a virus. The Iowa ext says to pull and discard infected plants.

Iowa Ext

Yup, it could absolutely be a virus, and typical treatments for insects and fungus won't help in the slightest. But as she's already been treating, why not wait for new growth to see?

There are lots of little critters that are very hard to see, breeding and feeding under each leaf until colonies are established. There's different types of spider mites, too, with different damage to the foliage.

I've been gritting my teeth and pulling plants that have virus sign. It's no fun, especially when they will produce beautiful flowers with only slightly marred tell-tale leaves. I had marked two plants of Wildwood Marie for possible destruction, as it had oddly ribbed and very shiny leaves that didn't belong. I sprayed Neem Oil a couple times, and waited for the new growth. Perfect sprouts from the root, with no sign of ribs or shine. Glad I didn't sacrifice the plants because I couldn't identify the insect.

Here's two clearly virused plantsthat I gave plenty of preventative care to, and even fertilized to see if it would make a difference. Pulled and trashed.

thank you for advice.

Interesting Cici. Thanks for the help!

That's a good link. If only plants could talk, then we might be able to get ahead of these things..

Sure looks viral to me!

Get rid of it soon!

Just a thought: Do you smoke tobacco? Tobacco users are a primary vector for TMV.

Wanted to share this link on the subject of dahlia viruses.

http://www.dahlia.org/index.php?page=virus-research-w-wsu