anything on the back of the leaf ...

check for mites.. which cant be seen very easily .... see link for discussion

i suspect something is sucking the vital juices out of it ...

OR????

any chance.. you just moved it to full sun ... the alternative is sunburn ..

OR????

could be a fert burn.. if you have been spraying a lot of fert on it ...

OR!!!

do you see any salt accumulation on the soil... or the edge of the pot .... that is what you might find .. if you think its might be a water issue .... when was the last time it was repotted, into fresh media????

pic of the whole plant might help .... just reply to this post with more pix...

ken

Here is a link that might be useful: llink

Are you growing indoors, outdoors? Whats your feeding schedule, and whats the NPK ratio of the feed? Are the white line veins erratic or are they uniform to the veins in your plant? Can you post a picture of the white veins on the other leaves? That information is needed when trying to diagnose a problem. By just looking at the leaf I could think its 5 different things. If you can put a picture of the leaves with the white veins up that would help cause I don't think they are related.

Hi,

thanks for the follow ups and possible reasons!

The plant grows outdoors on a small terrace that gets intermittent sun but the climate is very mild, hot in summer, mild winters (Barcelona).
I am not sure what NPK means but the plant could very well have got too much fertilizer over the summer. I gave the plant the fertilizer that comes in small balls 2-3x (couple of tablespoons) over the summer. Watering 2-3x a week less now that the weather has cooled. I haven't noticed any salt accumulation on their pots however I did see this on another pot....the plant that was in there is now deceased! This plant is still growing very well and has been potted since early spring (6 months)
Really appreciate all your help.
Attached new pic of the worste effected area.
Thanks