

This pieris began to drop its leaves last year when the black powdery substance started to cover the leaves. I nurtured it ( well I thought I'd nurtured it!) and tried wiping the black stuff off but was never able to completely remove it all. I thought everything would be ok this spring but as you can see it isn't . Any ideas what it is and how I can treat it please? Thank you
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Lukewarm water, a damp sponge and patience are the best way as any chemicals or soaps may burn the leaves. After that I would suggest a thorough weeding of the pot and a good drink laced with some liquid rose or tomato fertiliser and/or a feed for ericaceous plants. Do not use hard tap water as pieris don't like the calcium.
Then you need to deal with the source of the problem. Have a read of this info and follow the links for more - https://www.rhs.org.uk/advice/profile?PID=770
If it's scale insect, you can wipe those off with a cloth dipped in methylated spirits. Check out that link I gave you and se what else it can be and how to treat it.
https://www.rhs.org.uk/advice/profile?pid=224