Help! What’s wrong with my big cactus!

[PantyGiraffe]

Attention @nanthewiser or anyone else who can help...

My beloved big cactus has got this stuff appearing all over it. It’s also looking a bit yellowy at the join between the two parts of the cactus. Is there anything I can do?

That doesn't look good, PantyGiraffe. While it looks similar to a mealy bug infestation, I don't think that's what it is. It looks more like scale insects, specifically cochineal scale (google it). This isn't usually a pest in the UK, but if your plant originally came from a nursery in the EU (as so many do these days), I suppose it is possible that's where it came from (unless you've had it a long time). I don't really know what to suggest as treatment because there are very few insecticides available in the UK now that are effective systemics. Bug Clear Ultra is probably the best, but scale insects are difficult to treat as they hide under a sort of shell. In the first instance I would give it a strong blast of diluted washing up liquid, then see if your local garden centre has the above mentioned spray, and give that a go!

Good luck!

Thank you @nanthewiser It was from a garden centre who I’m certain get their plants from the EU. It was v cheap- £18- and it’s about 45cm tall. Such a shame as I adore it! Don’t want bugs to infest my other cacti.

Ah, that probably explains it - the scale was already on it, I guess. Shame. Another method I have seen advised is to spray or brush with diluted isopropyl alcohol ( aka rubbing alcohol or surgical spirit) which dissolves the hard outer shell. Disclaimer: I have never used it, but it's a well known method especially in the USA, you could google it, and maybe try that method - it's also good for mealy bugs.

Thank you!

Keep it away from your other cacti until the weather warms up, after which you have more options.

Things I'd try, as a last resort (they work on mealy bug but I've never had cochineal scale) are:

Hold the pads under a running tap and scrub off with an old toothbrush. Not too hard. You risk physically damaging the pads and being left with discolouration. You can't do this in winter as you'll always wet the soil, and cacti don't like the combination of cold and wet.

Park the whole plant outside in the sun, and hope the birds do their bit. You run the risk of slug damage.

Both these are things to do when the only alternative is binning the plant.

Apparently there are beneficial insects available to attack the scale insects, according to DH. He seconds the alcohol, too.