Ok, so I look up some info on Cercospora Leaf Spot from this page...

Link

...and it seems to fit my problem.

Here are a couple of pics of the spotting on my Aji Norteno plants where it first appeared. All five of my Nortenos are suffering from this. At first I thought it was a bug eating holes in the leaves but then these brownish spots developed and it seems that this is responsible. I've noticed a couple of spots on my other varieties and as mentioned, one of my Asian chiles had quite a few leaves infected, but I pulled those off and the rest of the plant looks good.

I've got nightmares of this problem spreading to all my plants.

The linked page suggest copper sprays. Any thoughts?

Click images for full sized version:

Although, this page here...

Link

...lists Thrips and a pic which looks suspiciously like my Norteno leaves above. But I can't see any bugs on the plants.

You should be able to see thrips. I would look at the underside of the leaves with a magnifier ( some pests are very small) but it looks more like a disease. Is it upper or lower leaves? Or random? Here we have a government service that identifies problems with plants. Do You have anything like that or perhaps you could take a few leaves ( in a plastic bag) to a local nursery for an opinion? In the meantime can you isolate the suspect plants?

Thanks for your reply, fiedlermeister.

I don't see any bugs on the leaves. I did have an aphid problem a couple of weeks back but I've gone after them ruthlessly and they're not a problem anymore. They could have transmitted a disease, though.

The problem appears to be random on the plants, upper and lower leaves. However, on all five of my Nortenos, it seems not to be obvious on the newer top leaves. On my other varieties it's random.

I've moved the Nortenos off to the side of my growing area. If I have to kill them, I will, to save my other 20 plants. We don't have a disease recognition service here but I can always take the leaves to a nursery.

Here are a couple more pics from two leaves I pulled off today. The pic with two leaves in it is showing the underside of the leaves.

Quality not so hot, from a cellphone camera.

Your first two photos are definitely *not* thrips.

To determine if it's cercospora leaf spot, you need more than a description.

Here are pictures. http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/pp1244w.htm
To my eye, similar but not the same as on your leaves.

The final two photos (just above this post) are due to an environmental glitch -- a mishap in water relationships.

Where are the plants growing? Indoors or out? In pots or the ground?

Here is a link that might be useful: pictures & text re cercopsora leaf spot

Jean, yup, it's cercospora. I checked out some pics yesterday and determined that.

The plants are outside, in pots. They are all grown from seed and were in perfect health as seedlings. They have been in their outside pots for about a month and the problem showed itself about two weeks ago.

I guess I'll need a spray. I've been removing the affected leaves as I see them.

I had a small aphid problem that I corrected through aggressive seek and destroy tactics and they all appear to have gone now. I wonder if the aphids could have introduced the disease.