In the UK, blackish powder on plants is sooty mould, usually caused because the plant is infested with aphids or scale insect. Treatment is to spray the plant with whatever insecticide kills off the bugs present, and often with a combined fungicide to clear the sooty mould too. Because you're in Uganda, I've no idea what kind of pests you may have there that cause this, but can only suggest you inspect the plants thoroughly, leaves and stems, for signs of insects.