cox pipin tree, planted in a large barrel. the leaves have small brown spots and the leaf turns yellow and falls off. as you can see in the photo, any ideas
Cox apples are very challenging as they are prone to many things. Unfortunately many people want to grow them as most people know the name!
Look on a good site such as the RHS site to find out what is wrong.
Poor choice for a restricted growth environment, it really needs plenty of room. And as Welshonion says not the best variety for pest and disease resistance. Even though I love the taste it would be far better to choose a more effective modern variety for your situation - Sunset on M27 rootstock would be my recommendation.