I'm having trouble identifying a disease/infection which is slowly killing all my indoor herb plants. The picture shows 2 stages of the infection - one on a normal basil plant at the early stages,showing areas of whitening/yellowing with black spots - the black spots can be removed by gently scraping the surface of the leaf, exposing the damaged leaf surface below.

Eventually the infection spreads to the whole leaf (and the rest of the plant), as shown in the picture on a purple basil leaf, causing the leaf to eventually wither and die.

I suspect this is some kind of fungal infection, but after extensive searching, I cannot find a set of symptoms which completely match what I'm seeing. There are no obvious signs of insects or creatures, and none of the greyish powder or mould which may be associated with powdery mildew or gray mould. Other key bits of information:

1. It has affected all my herb plants (basil, purple basil, greek basil, thyme), with the exception of my garlic chives and rosemary, which seem to be much more resistant.

2. In the early stages of a leaf being infected, only the top surface of the leaf is affected.

3. New leaf growth initially comes through unaffected, looking green and healthy, even if many of the surrounding leaves are showing symptoms. But eventually all new growth succumbs to the infection.

4. The stems of the plants appear completely unaffected.

5. I have had a lot of success growing these herbs, with many weeks of strong, healthy growth, but they all seem to get the infection eventually (leading me to believe that this is a fungal infection being spread by spores in the air), apart from the garlic chives and rosemary.

I realise I am not growing these herbs in ideal conditions (indoors on a windowsill, in a fairly humid apartment), but sadly I have no alternatives.

Any help much appreciated!