Posted: 18/08/2013 at 18:58
Hugo, it may be nutrient deficiency, but first have a close look at the back of the leaves in case there are spider mite or thrips. You will need a magnifying glass as they are tiny. If no sign of pests, it could be chlorosis which means one of several nutrients isn't being taken up. Ironically, over-feeding can cause this - are you feeding, what with and how often?