"I made a homemade insecticide using a tiny bit of meyer’s pure soap and a lot of warm water. That knocked off some of the black spots, as well as most of the white residue."

That would tend to knock back thrips, although a heavier soap mix certainly won't hurt a dahlia. You may want to try some Neem oil if the problem continues post a soap spray. With my usual gentle warning regarding Neem. http://npic.orst.edu/factsheets/neemgen.html

Wipeable removal indicates a surface issue, however. Thrips slash leaves to lay eggs, which will leave very visible and obvious damage that can't be wiped off. The bugs themselves are usually pretty obvious on plants as well.

If it is powdery mildew (it doesn't seem to be and PM doesn't pattern like that, but...) it'll take a while to develop, doesn't damage plants very much in smaller amounts, and can be controlled at any time with any good mild fungicide, including homemade ones (1 qt water, 1 tsp any soap [or 3-10 drops dish detergent], and 1 tsp baking soda being one)

"I’ve been growing this dahlia indoors (to KEEP indoors, not move outside)"

That's going to be a challenge as dahlia are full sun, but it should be a fun challenge. Keep us updated and send photos. :-) You'll either need lots of natural light or artificial lighting, but this sounds like a fun experiment.

You may have to point out where you see a problem. Or get much closer detailed pictures.

Try one of these USB microscopes - they do work after you play with them for a while.

@hc mcdole let me try this. See the top leaf and the leaf just to its left. Sorry for the quality - best picture I can get with my equipment (phone)!

@User THANK YOU! Super helpful info.

And yes, I know I’m a little bit crazy!! I live in an apartment but I LOVE dahlias so trying to see if I can swing it. I have the pot near my brightest window and I even tilt it slightly toward the sun for part of the day (just for now while it’s small and may not reach the light as easily). So far, other than these white spots, it seems to be doing extremely well. I planted the tuber (with a small sprouted eye already) 11 days ago. It took about a week to get established/grow the first inch out of the ground. But in the last four days it has grown an average of 0.75 inches per day! I think that’s a good sign? Hah I hope it is!

Oh yeah, those dahlias can put on a lot of growth in no time. Outdoors is best but if you can grow it indoors, kudos to you!

I put dahlias in the ground and let them do their thing. If they winter over, great; and if they don't, then that is okay too. I always seem to buy some more each spring in hopes of growing some beauties. One year I bought 3 large inexpensive (cheap) plastic pots and put mixed dahlias in each one. Here are some of the results. The pots were heavy and flimsy after filling so I would get a more rigid pot if I did this again.

a few over the years.

Multiple sprouts per tuber are expected and happy dahlia can grow into pretty hefty bushes according to their sizes. Dwarf dahlia like the Harlequin just make tiny bushes. Thomas Edison become six foot bushes with multiple stems from the ground. By the end of the growing season (yours will clock out even in an eternally warm apartment environment, given indications by the sun and daylight lengths) you could have twenty, thirty stems off that thing.

Do I have...I do, but these are not great photos. The purple flowers are Thomas Edison dahlia in flower, in August. To the right is another dahlia, slightly shorter, the name of which is escaping me at the moment. I'll remember the instant I post the image, I'm sure.

Both are in tomato cages as they tend to fall over if they aren't (height is around 5' or so). Behind me are two more large ones and there are three out front. There are usually 36 smaller dahlia through the gardens.

You can see the multiple stem nature of the dahlia pretty clearly, though, even though I don't have a specific closeup of just the single bush anywhere I can find.

Morpheuspa, That is one nice bed of flowers there!

I hate staking dahlias but that is the best way to get them to grow and show. I never thought of tomato cages for them.

Longwood Gardens September, 2017 - wished I could have a smidgen of these.

I have my tubers ready to plant for the last 3 or 4 weeks. This is what I usually buy each year from Home Depot - 3 bags - either mixed or one color. Some are smaller than what they look while some do quite well.

"I never thought of tomato cages for them."

If you ever want to find the easiest and simplest way to get a job done, give it to the laziest person you know. I came up with that idea some years ago and have not moved the tomato cages to this day.

And I adore your dahlia gardens. I do a frenetic mix of every color I love as well, mixed with every other color I love. Which is every color.

Alright, folks. Here’s an update. white stuff is still there and appearing on more leaves. AND the bigger leaves are starting to curl outward?!?!? Any idea what’s going on And how to help it??

Here’s a picture of the white stuff

and of the curled leaves:

Here’s also a picture under another one of the leaves. Maybe spider mites?? the edges of the leaves are quickly becoming brittle.

Maybe bacterial or mineral related? Hard to see if it is insects or not - need a really good close up of the damage if possible. A good dose of sunshine should help the plant grow and flourish if and when you can. If you are able to put it in sun, remember to start out for 15 minutes or so and increase each day just like getting a tan.

Spider mites would be pretty visible under a magnifying glass or as moving dots in webs under decent light, even to my poor vision. Those are easy to get rid of (a bit of soap in water, sprayed, kills them, any mild insecticide, or Neem). Damage usually isn't directly visible, it's subtle.

I'm thinking viral or bacterial at this point. But take a magnifying glass and go over the plant, looking closely and see if you see anything moving.

The leaves started RAPDILY wilting this afternoon, and it got me wondering whether perhaps there was poor soil drainage. I don’t think I’ve been overwatering, but i Decided to go a few inches deep in the soil. It was VERY damp deep down, despite being relatively dry for the first inch or two. So, I immediately got rid of the old, moist dirt (in case it had developed any kind of fungus), drilled extra holes in the bottom of the pot, and re-potted with fresh soil. No water. I gave the plant a few minutes of very direct (outdoor) sun and then brought it in the shade inside to recover. It’s slow going but it seems to be at least marginally better.

tonight I also added a homemade fungicide (baking soda base) to see If that helps with the mystery white stuff.

crossing my fingers this isn’t viral.

Here's hoping you have solved the mystery. Water logged soil is a problem for sure. Exposing it to full sun will be therapeutic too (just set a timer as it is easy to forget - been there, done that).

Quite! If it persists, you can sterilize the soil with hydrogen peroxide (don't do this if it's recovering on its own) by watering with 1 part hydrogen peroxide (3%) and 3 parts water.

Dahlia, like any other indoor-kept plant, aren't going to reward you for being kept in too heavy a soil. Lots of Perlite and pumice and sand is going to help you out here to keep it airier. Not too much as they also really hate going completely dry...so this is going to be an interesting and challenging balancing act.

I believe you're witnessing guttation, a physiological process which occurs when the plant's internal water pressure forces sap through tiny openings (often stomata) in leaf and sometimes stem surfaces. In this case, sap might be leaking from the feeding sights of insects with rasping/piercing mouth parts. Guttation is common in some plants, a normal part of their physiology, and not as common in other plants. Conditions that limit respiration, like closed stomata during the dark cycle, over-watering, humid conditions, cool conditions, low light, defoliation/pruning) are all potential causes or contributors. When the water in sap on leaf surfaces evaporates, whatever was dissolved in the sap is left behind in a thickened or even crystalline form which often goes right back into solution when rubbed gently w/a damp cloth.

Ultimately, it's a culturally induced issue, even if insect herbivory is also in play (keep after getting that controlled if determined to be an issue). More light, a better-aerated medium, a lighter hand on the watering can, and more air movement will likely go some distance toward limiting the issue

Al