Top down yellowing is a minor/micro nutrient issue, in this case and with these symptoms, it is a zinc deficiency.
If it were P or K, the symptoms would start from the bottom up, not from the top down.
Should be easy to solve with a dose or three of a soluble trace element/trace mineral mix applied to the root zone.
Soluble trace mineral mixes are available from any decent garden centre.
Avoid mixes designed purely for foliar feeding.......soil application is far more effective for the long term.