I think one of the important clues is that the same thing is happening on several of your plants. This leads me to believe that it has something to do with your water. I do agree with the others in that it is turning into a fungal issue, but I think that is only secondary. It is not the cause of the problem. You indicate that your water is about 100 ppm TDS. Is there a chance that you are using RO water that has too much sodium? For example, an RO system that taps the soft water supply of the house. Is there a chance that the pots are not being flushed frequently enough? Even at 100 ppm, salts will accumulate eventually without a good flush and appropriate drainage. Is there a chance that you might have changed fertilizer shortly before this started happening? It is important to remember that things like this don't just happen. There is a good reason, but it is probably something you have overlooked because it didn't appear to be significant. Mike