As you can see from the picture I have a handful of leafs affected by this. The one in the picture is the worst of all affected, it has been slowly spreading over the past 3 days. Have no idea what this can be any ideas?
It looks a lot like phosphous patches, but your picture didn't include the stems, which are usually purpling as that is happening. Do you have a goodly supply of p in your soil or nutes, and what stage is your plant in?..and what colour are the stems of the leaves from leaf to meristem?
Then it's a transient phosphorous uptake issue or plain old not enough. Plants need phosphorous and potassium when they reach a certain size, just feeding nitrogen will mean that the plant isn't getting anything else it needs. Find some balanced nutrition at a hydrostore if you are into nutes, or transplant into some stocked soil if you like to just organically grow. Both are options. Good luck.