Five year old Swiss Limbertwig apple - all leaves have dark green middles and light green outer edges. Weak growth. Leaf color new to me. Trying to ID nutrient deficiency. Somewhere there’s a thread showing many different deficiencies in leaf photos but I can’t find it.
Check for leaf hoppers ?
Cut into any twigs yet to make sure they look healthy? I’d sacrifice some hardwood and some fleshy growth, both.
Magnesium deficiency? Here is the chart: Simply Educational: Deficiency Chart of Micronutrients
Looks like Potash deficiency or lockout. Lockout can be caused by pH or excess nutrients.
I dug this up with a search.