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Last year around end of summer our two blueberries growing on our balkony got some illnes. On the branches grew a lot of bulges (a few mm width and height, but around 1cm long), first sporadic and later in dense packs. It was possible to remove them with a simple touch, they did not grew inside the branch, just outside. A gentle pinch on the bulge revealed a lot of white fluffy stuff. At the same time the leaves and everything else got covered by some black smut. The leaves itself did not turn black since it was possible to remove it with a sponge and some water. Not only on the plant, the pot and tiles below got covered in it as well.

I removed the worst leaves and the plants got its usual cut before the winter, they are both having new buds but still the black stuff is around on the branches. We have cold winters (was around -15C for a week or two). All bulges got removed. Sadly I do not have any pictures of it.

Does someone have an idea what it is and how to treat it?

Edit Living in Europe - Slovakia.

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It sounds as if the blueberries got infested with scale - you don't say which part of the world you are in, so its hard to know for sure as well as knowing which type of scale.

These insects have two stages - hard shield shaped objects may form on the woody parts, but there will be crawlers which look quite different on the soft parts, like the leaves and green stems. It may be that some crawlers have survived, so you will need to inspect regularly and use a horticultural oil spray to keep them at bay, see here https://pnwhandbooks.org/insect/small-fruit/blueberry/blueberry-scale-insects

The black deposits are sooty mould from the frass created by the scale - this can be washed off and should not recur unless you get another heavy scale infestation.

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