It's incredibly hard to diagnose from photos - it's not always easy to ID things, either! - but there are a couple of quite straightforward issues with this tree, so I'll say what I think, and you can all do your own research if you think I am wrong.
There seemed to be three main problems: orange spots, curling leaves, and skeletonised leaves.
Firstly, here are the pictures:
Orange spots = a disease called rust: it is caused by a fungus, and is often specific to the plant, by which I mean that plums get plum rust, willows get willow rust etc.
Rust does not usually kill a tree, but it reduces the vigour - as the leaves have less green surface area available for photosynthesis - and it looks pretty horrible, so it is not something we want to encourage.
Treatment: if there are only a few leaves involved, pick them off as soon as you see them. This can help to slow down an even halt the infestation. If there are a lot, then removing the leaves can do more harm than good, so just leave them in place.
Clear up all fallen leaves as they fall - either burn them (which is preferable) or send them off to the council recycling in your wheelie bins: don't compost them, whatever you do. Commercial composting reaches much higher temperatures than our home composting does, which can kill the spores, but if you put them in your home compost bins then you are likely to end up with compost which is full of rust spores. Not good.
Prevention: spray with a fungicide. Pick one that includes the word "rust" on the pack: I won't give you the technical names of the chemicals involved, as they keep changing, but if you pick one which is good for "rust" you should be ok.
At the end of the season, make sure to clear up every single fallen leaf, diseased or not, and at the same time, check the tree for any dead or diseased-looking parts. Prune any such off with sharp secateurs, and burn or dispose of all this material: don't make leaf mold from them, just in case.
Next is the curling leaf edges - this is usually caused by aphids: they suck the sap and distort the leaves. It can occasionally be a virus, but that would be carried by aphids, so the treatment is the same. To prevent aphids, you need to spray with a Bug fighter: again, I won't give chemical names, just pick a spray with a picture of an aphid on the pack, it's entirely up to you if you go full-on chemical, or try the "organic" fatty acids ones.
Again, aphid damage won't kill a tree - usually - but it looks quite unpleasant. If some leaves are so distorted that they are completely crumpled up, pick them off, but otherwise just leave them.
The treatment for this is again, a bug spray, but you can help the tree by checking all the leaves on the tree (remembering to look underneath the leaves as well as on top) and picking off any caterpillars that you find. I would squish them, but I suppose it's ok to fling them away into a far corner of the garden... and keep on checking, as they do tend to keep on coming.
The fact that this tree has healthy new buds is a really good sign: all of the above problems are, in effect, only cosmetic, but of course we want ornamental plants in pots for their appearance, don't we!
So, what regime would I suggest?
Firstly, get the hose out and "gently sandblast" the tree, if you see what I mean. Wash all the remaining leaves, both the top and bottom surfaces, to literally wash off aphids and anyone else living there. As you run the water over them, use your fingers to gently massage each leaf, ensuring the water gets into all the nooks and crannies. This should remove the majority of the current population, giving the tree a short breathing space in which to recover, before the little buggers (that is a technical term) move back in.
When it's dry, and you have removed any really damaged leaves (into the bin or onto the bonfire heap, remember?) spray it with any of the fungus fighters and the bug fighters: Bayer do a very good one which treats both. (No, I'm not being paid by Bayer!)
Most of these treatments are "systemic" which means you spray it on, the plant absorbs it, and when the critters come back and start to eat the tissue, they absorb the poison, and die. So, once you have sprayed the tree and the chemicals have infiltrated it, it is protected from future attacks. But not for ever! Read the pack and check when you should re-apply it. And make a note that, next year, you will apply it earlier in the year, before the problem starts...
Meanwhile, you can help the tree to be healthy enough to resist attack, Here is the overall tree:
Pretty little thing, isn't it? You can see why these kilmarnock are so popular.
Now, Mike (the worried owner), I hate to sound critical, but the pot is just a little bit small for that size of tree, so if you can repot to a bigger one, so much the better. Choose a straight-sided pot if you can, rather than a tapered one: it's more stable, and gives a larger volume of soil for the tree to enjoy.
Give it a balanced feed (something like Growmore, it doesn't matter if it is pellet or liquid) and ensure it's well watered for the next few weeks, but at the same time you don't want it waterlogged, so raise the pot up off the patio on "feet" to allow better drainage.
This four-star pampering should help the tree to produce a new flush of leaves, and by spraying it now, those leaves should be much more resistant to damage by rust, aphids and caterpillars.
Hopefully, this will sort out the problems!
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