try foliar-feeding it with a dose of potassium ferts (wood ashes and water is a quick fix) for a few days - foliar-feeding won't over-fertilize, so you're safe on that - I once had some leaves that looked like an early stage of what you have - the leaves that are already affected will not clear up, but the new growth will be corrected - IF that's a potassium deficiency, it tends to be just 'cosmetic', and usually won't kill the plant - it's an easy fix, and hurts nothing if I'm wrong :jointsmile: