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.
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)?