sometimes its a magnesium deficiency. I would try a teaspoon of epsom salts dissolved into 5 litres of water (ive never tried this before, but it does cure Mg deficiency on most garden plants with this ration, even 2 1/2 to 5 litres.......) and then run through the roots and bark. Otherwise try a properly developed magnesium balancer.
Otherwise, it looks as though the lea is starting to die off. Unless this is a new growth, i wouldnt be too concerned.