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I have some herbs growing in a couple of pots on my balcony. Today I noticed that the aloysia leaves are all covered in this stuff, it looks as if somebody emptied an ashtray on it. It mostly on the underside of the leaves and on the stem itself, especially near the top. When the plant is touched or shaken, the stuff drops off.

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What is this? Can it be helped? Is it contagious? I also have some lemongrass and some salvia in the same pot - might they be infected by this? What about other types of plants in adjacent pots (peppermint, basil, oregano)?

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up vote 2 down vote accepted

It appears to be an aphid infestation - when you say the deposit drops off when you move the plant, do the black specks remain? Does anything fly off when you shake the branches? If it is aphid, you need to spray with an insecticide suitable for edible plants, which is not systemic. Systemic treatments remain in the tissues of the plant for up to a fortnight, and plants sprayed with those should not be consumed in that time.

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soap and water works remarkably well on aphids but a good rinse afterwards is recommended for anything you might eat –  kevinsky Jan 4 '13 at 18:16
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