Certified GKH Gardening Expert
Pruning will not correct this issue, unfortunately.
The first thing to do will be to test the nutrient contents and pH of your soil. My first guess is a deficiency of phosphorus and magnesium, but this can also be caused by a pH that is way too low, as well.
These articles will help:
If your soil tests are normal, then you may have a moisture issue. Wet soils that do not have a chance to dry out between watering will suffocate roots. This will prevent roots from taking up nutrients.
This article will help you to test for soil moisture: