Hi,

Are these holes caused by a disease or is it an insect? Either way, how should I treat it? Almost all the leaves have these holes. Should I cut the leaves before applying the treatment?

Thanks

A symptom of a disease is not to create holes. You might have a fungal disease that causes brown spots on a plant but not holes.
Chances are that you have an insect problem. Perhaps a slug or snail problem as they might nibble the edges of the leaves. Beetles, grasshoppers and a few other insects can nibble leaves.
But without knowing where you live, in what climate zone, makes it more of a guess to name a cause.
Is this a shrub or a bush that drops all of its leaves before winter. If it is, you can treat next year when you first find the damage starting. If the insects are there now, you could treat but if the leaves are going to drop for the winter, why treat something that is going to drop anyway.

Thanks for the answer @BigBill. I'm in London which is, according to Google, "humid temperate oceanic climate".

This plant never drops its leaves. It's been having these holes for a long time but I think it's increased over time.

It could be a slug/snail problem yes, but am I right in thinking that the lower leaves would have been damaged more because slugs/snails could reach them more easily? In my case, almost all parts of the plant are affected similarly.

The plants on either side seem relatively unaffected, is that true?
This plant reminds me of a flowering azalea of some other shrub. Maybe a dwarf rhododendron or a Spirea of sorts. But some on else might know.
If the holes have been there for a while and have gradually increased, the problem doesn't sound immediate or catastrophic. More likely just building up over time.

Those "bites" are characteristic of leaf cutter bees. Damage is usually only cosmetic. I believe there are product that repel—not kill—them.
https://www.rhs.org.uk/biodive...

@BigBill yes, the plants on either side are unaffected. I have never seen a flower on it. I wish there was a name tag so we wouldn't play a guessing game.

@NMoasis if that's the case, the link you shared suggests "not to control it".

Leaf cutter bees never occurred to me. But whatever it is, it seems odd to me that they attack only one plant.
Maybe they like the taste as opposed to the other two.

Yeah, apparently they do prefer certain plants over others. I used to have a Cestrum 'Orange Peel' they loved—and nothing else nearby, but only for a few weeks out of the year. I just got used to the weird fringed leaves. This article says they prefer softer leaves
https://crownbees.com/providin....

Well, we are never too old to learn something new.