I have a Shinseiki that has developed black spots on the leaves. There are some aphids on the new growth. However, on the whole, the tree has been lagging with respect to a Nijiseiki I purchased at the same time. The spots look like Fabraea, according to a quick Google search. Should I prune back the diseased growth in fall and wait until spring to see what happens? Or should I get a replacement?
Note: These trees are not yet planted, but are in pots.