Always start by looking at the soil & roots!
Too wet, too dry, root growth restricted, not ideal fertilising, pests attacking under ground?
To me, from a photo, it looks like lack of nitrogen uptake. Try adding half a teaspoon of epsom salts to a gallon or so of your usual fertiliser

I'd suggest the Epsom salts treatment as the leaves look very yellow and this is usually due to lack of magnesium [hence the Epsom salts] that is a constituent of the green pigment chlorophyll.

Looks more like manganese deficiency than magnesium, though a little Epsom salts probably won't hurt it. Micronutrient deficiencies like this have some common causes, however. Poor drainage, alkaline soil, excess sodium in soil and/or water, and cold, wet weather are the most likely candidates.

on my larger laptop image it may indeed be manganese as tug says, as you can see green veins and it appears to be on younger[smaller tip] leaves which is more in keeping with manganese. look for a foliar feed that has manganese n it often as manganese sulphate.