Please can anyone identify these black spots on my cactus

[Rich66]

Hi Simon
I normally use SB plant invigorator, will that normally do the trick and how often do I carry out the treatment process. Also what causes black/sooty mould?
Thanks

[SimonT]

I think it gives exact instructions on the SB site about using SB Invigorator against scale.
I don't really know how these different approaches would compare.
I guess just try and see if it works for you.

You can get sooty mould with lots of sap sucking insects and they excrete excess sugars that then encourages mould to grow on the surface of the plant. It is hard to tell if this is what you have from the photos but it seems a reasonable possibility.

[Paul in Essex]

A friend of mine swears by alcohol-based hand sanitising gel for treating scale - applied with a cotton bud. As it is a gel it kind of sits on the little devils and doesn't run off. I haven't had any since he recommended it so can't say first hand.

[Rich66]

Many Thanks for your advice everyone. I will try these different methods and see how I get on.

[Rich66]

Also do these pests scare the plant quite badly?

[habanerocat]

Also do these pests scare the plant quite badly?

I've heard of the petrified forest but I don't think it was in relation to pests...

Seriously though.
Damage tends to grow out of globular cactus after a few years, but unfortunately it's there to stay on columnar cacti. It may become less noticeable as the plant grows but it won't grow out.

[Rich66]

These things always seem to happen to your favourite cacti