Much of it appears to be caused by leaf miners, with the black specks being the frass in the larval mines.

From what I can understand, leaf miners leave a line-like trail like this one: https://s3.amazonaws.com/plantvillage-production/images/pics/000/002/018/large/Leaf_mining.jpg?1384281096

I don't see such a thing on my plant.

Plus, it's a bit odd that the same thai basil (supermarket purchased) is doing well outside, but the one inside has leaf miners. As did the thyme and the other basil before that.

You are right, that it is not leaf miners.

Where in the world are you? If in the USA have you contacted your local office of your state universities Cooperative Extension Service?

I was actually thinking about the blotch leaf miner, much more common on basil than the typical serpentine pests. But, after a good night's sleep (lol), I'm going to suggest basil downy mildew for you to consider, advanced.

What is the black stuff on your fingers? I do see an aphid in one of the pictures, by the way. They never travel alone.

Yeah, one aphid on the basil, one on the thyme. I don't see any on my new thai basil, by the way.

The black stuff on my finger is from the leaf. See all that black stuff? Well, yesterday I was at a Thai Basil leaf with a razor and trying to see if I can find a worm or something. I didn't. But those black dots, they come off.

The thai basil came about 45 days ago. The ones outside are thriving, but the one indoors is not.

===>> which begs the question.. why are the ones in the house stressed ... stressed plants have magical ways of attracting all kinds of secondary problems ... but its hard to solve those problems on a stressed plant ....

did i see clay pots ... i have never had a lot of success with clay pots indoors.. but that might be a location issue.. where are you ...

how old is the media ... do they get proper light .. how do you water ... on a schedule.. or when they need it ... etc ... did you transplant the new one.. when you brought it home from the store???

also ... if you take the plants in and out of doors.. you may be bringing outdoor problems indoors ...

so many questions.. so little time ...

ken

ps: sometimes its easier.. and cheaper.. just to throw away a bad plant .. rather than spend 20 dollars.. trying to save a 3 dollar plant ... always keep that in the front of your mind ....

It's not about the money. At this point I just want to know *WHY* this keeps happening, whereas it never did before.

They get plenty of light by the kitchen windowsill.

I usually water from the bottom. No schedule--just water it when I think it's too dry.

I bought the soil last year, kept it in the garage. But the ones outside are also in the same soil as well.

The thai basil, they came in a bunch in a tiny container, so I thinned them out. Put 5 or 6 in a larger pot outside, and kept two in this clay pot indoors.

And I'm in Toronto.

The only thing that I can think of at the moment is that somehow the proximity to the kitchen sink/faucet is causing something. Maybe it is mildew after all.

I seldom disagree with rhizo, but I do this time. I don't see any evidence of leafminers, neither serpentine nor blotch.

I do see an aphid -- in the 2 images of a critter.

I'm uncertain what is causing the irregular whitish blotches. Reminds me of leafhopper damage. But not a good fit because they don't leave black specks.

Also reminds me of thrips but, again, not a precise fit.

All gardeners MUST understand that a sunny window is insufficient light for plants such as herbs, which do best with full sun or, at the very least, 6 hours of sunlight daily.

Finally, all gardeners need to understand that indoor plants are susceptible to numerous problems because they are stressed. Among those stresses are minimal air circulation; too little light; & lack of beneficial insects which help limit most of the common insect pests,

Sepandee I can't offer an answer but this might still help - I had exactly the same problem. Indoor basil, threw the first out and bought a new plant from a different place - same issue. I did keep it on a sunny windowsill but I guess that's not enough then.

I have to agree with Jean.......not about what the exact problem is but with the fact that anyone should be surprised that growing herbs indoors results in problems. Herbs are NOT indoor plants and keeping them indoors when they are completely hardy outside is bound to put them under a great deal of stress. And stressed plants are inclined to develop a lot of problems, both with insects and diseases. The fact that the outdoor basil is thriving while the indoor basil is not is pretty clear evidence!

I do realize that there is a high convenience factor to grow herbs indoors but it is critical to understand that this is not a desirable environment for them - too dry (very low humidity), too dark (even a sunny window sill is no comparison to growing outside in direct sunlight) and too even a temperature. They really would prefer to be outside.

I have grown herbs indoors for many, many years and never had this problem. I understand that growing them indoors is far from *ideal*, but that doesn't mean that it can't be done or that this disease, whatever it is, is a normal thing. It's not. I've done this for more than a decade in both apartments as well as in a house. In both cases, they were next to the window and getting at least 4 hours of sun.