The 2nd picture looks like aphids but is it residue left over from the bug that is eating the plant?
The 2nd picture looks like mildew? You will have better opinions than mine. I am so sorry this has happened to you. I would be miserable. Good luck

one of my plants, not the one in the picture does have powdery mildew, I have been spraying for that. Since the spot was yellow and brown maybe it is another type of mildew.
I always thought aphids were white, which is why I posted the photo. The bugs are small and black and all over this one plant but none of the others. I did spray already with a fungicide/insecticide but still wanted to know what it was.

I am also getting holes at the top of the plant after using Sluggo. The other day I saw a small worm scouting out lunch. I picked it with my bare hand (can't believe I did that) threw it on the ground and stepped on it! I try to stay organic.
Aphids come in many colors (red, white, green and black) and those look like aphids to me. I had a small colony sitting right under a bud just waiting for it to open. Spray at 10 day intervals for several weeks should do the trick. The intervals ensure that any offspring are taken care of.
Now you said that you planted 25. Were they tubers or plants? If they were plants my next question would be did you harden them off before planting? The second photo could be sunburn. This could also be how the Dahlia in the bottom photo was affected.
You also may want to think about amending your soil next year with a some peat and/or compost. It really helps in the health of the plants. I have a bed that I amended with peat and compost and several other areas that I did not. There is a marked difference in performance. The amended bed has 3-4 foot dahlias in bud. In the unnamended areas, the dahlias top at 15 inches.
At this point, I would continue with the Sluggo. Spray all with a baking soda and water solution for powdery mildew (just in case). Don't just spray the affected plants as it can spread. It's best to apply as a preventative because once it takes hold it can be hard to control.
Just as importantly water well and apply a good fertilizer - one that contains a bit of nitrogen as it will help the plants green up. Also, Epsom salts encourage greening - 1 Tsp. to a gallon of water or just spread around base of the plant and water in.
One more thought - certain Dahlias can be more finicky than others. I try to keep a journal of environmental (water/ heat) conditions and how each is performing. This will help in culling in future years in order to keep the best performers.

Google leafhoppers.

I did look up leafhoppers but I don't have much of the yellowing on my leaves. just holes in them. I did find a worm on one of my new dahlia's.
All of mine are tubers and new this year.
The Dahlia in the bottom photo gets sun all day and its hot. I have not planted in this area before and it may be too much sun.

April -

In the last photo you posted, the ground looks very dry to me. I don't know if you're in California (like me) but think you might be & if so, the sun is very strong. I have about 30 different varieties & have learned which varieties take more sun & which need afternoon shade (in my climate).

Here's what I would do, if I were you:
1) I would ensure that your plants are getting enough ground-soaking water on a regular basis.
2) You don't mention if you have fertilized any/all of these plants but if you have not, I would fertilize them all ONCE with a product like Miracle Gro (my preference) at half strength. If it helps, you would notice it in about a week.
3) I would spray with Ortho's "Insect & Disease Control" product which is a strong product but, for me, works very well on dahlias. I think this would clear up the black bug problem right away. I would do this after I fertilized.

Pat in Sacramento CA

Pat thank you so much for your post. I think you are right in that I need to learn which ones handle more sun. We are in Southern Cal and inland, usually very hot this time of year but had 3 weeks of marine layer, hence the powdery mildew. I broke down and called swan Island dahlias today, I have every pest known to dahlias right now, aphids, mites, caterpillars and borers as well as powdery mildew. I think I am on the right track, sluggo, sevin and fungicide.... hopefully I caught it all in time. my next choice will be your ortho selection. I do have jobes tomato spikes on each side of my plants and have watered with super bloom.
Thanks again.
April

Something is eat the dahlias bub before it is coming up. It look like a worm.

Did you dig up your tuber to review and see something that looks like a worm? Do you have pictures of the worm? Is the worm eating the whole tuber or just the sprout? What area of the country to you live in?

A neighbor told me that slugs bury their eggs under the ground, and when the slugglings hatch, if they find the dahlia sprout, it's toast. If that has happened to me, I never knew that was the problem. But several times now, I've planted a good tuber with a sprout and the sprout never made it up. When the tuber was dug, the sprout was long gone, so something got it.