Nerine Oleander

Sounds like sooty mould. It grows on the sticky honeydew which is excreted by sap-sucking insects like aphids, mealybugs, scale insects, and whiteflies.

Treat the infestation first then you can wash off the mould with soapy water.

Thanks, I'll start treating it straight away, it's a lovely plant with pale pink flowers. In France they formed high hedges in pale and dark pink and also white.

Oleander almost all its parts are poisonous - flowers, sap and leaves.

I always wash sooty moulds off with a half strength Milton solution .
If I don't have any I use bleach .
But Milton is better .
I keep disposable surgical gloves at hand for this kind of task .
Oleander is very poisonous like so many other very lovely plants .

If I wash it with Milton, will I still need to treat it with a mould chemical?

I've been using Milton on plants for a very long time-about 50 years .
I would leave it on and then wash it off after an hour or so -longer if you feel confident .
I wouldn't use a mould chemical .
It's up to you .
I'd certainly wash the Milton off if I were going to use a mould chemical as well .
I have to admit to not having a mould problem with Oleander .
Good luck with your plant.

Here's a really good remedy for sooty mould which I forgot !
Epsom salts in water .
In theory a tablespoon in a gallon of water .
But I used a far stronger solution basically I threw a packet in a full bucket of water .
The sick plant was a huge old white Camellia which was smothered in unsightly sooty mould .
I sponged it on the leaves .
It took hours and hours !
The Camellia was saved and in fact years on it's doing well . I found that pruning back the tree next to it prevented excessive shade and water continually dripping on it from rain or dew .

My camelias get sooty mould, I've never had it on anything else. It does seem to happen to plants in deep shade.
I spray the scale insects on the underside with insecticide, then later rub them off & spray the top of the leaves with mild washing-up liquid solution - & pamper the plant a bit with a feed & good watering, to help it fight back.

I have some Epsom salts for my achey legs, will try that next time!

Thanks for your comments on my problem, I shall try the Milton first I think and pamper it a little as well!