Two thoughts come to mind:

First, this June's soggy weather had rotted many tubers in our region (I'm in NW PA), and that might have happened to your Edna C. The varieties right next to it might have weathered the wetness better, or after losing their tubers had recovered by rooting from the existing stem. I was able to save many varieties by digging and replanting the plant (with rotted tubers) deeper in a mounded bed. This allowed more stem for rooting.

The second thought is about the variety itself that you're having issues with. Edna C is known to be a fickle grower, and even very experienced growers have a challenging time with it. This variety will grow great one year, then spectacularly fail the next with seemingly no rhyme or reason. Good stock is hard to find, and protected when obtained. When grown to its full potential, it is a spectacular bloom that often wins at shows. So even though it is not an easy variety, it will be grown until a better form with similar color comes along. This is an opinion based on talking to many growers, and growing it myself.

Oh, gosh... After visiting a public dahlia garden with lots of virused plants, I am chagrined at not suggesting virus. Is there any streakiness in the yellowed leaves, or twisting of the sides of the leaves? If so, yank it and don't look back.

We too have had a mix of extremely dry weather followed by wet weather in June and early July, and dry almost ever since. I water 1-2 times a week thoroughly. I have had what you describe as streaky leaves with yellow splotches mixed with the surrounding green. Also some leaves curling at the ends. I see some of the curling on a plant or 3 every year, but often it goes away. There is a lot of streakyness on my Embrace dahlia plant, combined with extremely slow growth all season. That plant is worse off than Edna C. Edna C has not seen much streakiness in its leaves. Its just shorter and slow growing, with Browning leaves at the bottom 1/3 of the plant.

One important note here: Since having an issue with powdery mildew in my zinnias, I've been doing multiple preventative sprayings of Bayer systemic fungicide every 3-4 weeks since about mid June.