I had a look at your diary. I'd say this might be a air circulation issue or water sitting on the leaves after watering, or leaves touching the soil. It looks like a disease more than a nutrient deficiency. Is it isolated on the lower leaves? or is it evenly distributed through the plant? I'd say its some kind of rot and I'd clip the infected leaves off and monitor to see if it spreads. If it does then it could be a soil borne disease or nutrient issue. If it doesn't then problem solved.