Well, thank you Doug Caldwell and University of Florida!
I have been agonising for months about the tiny white dots on the underside of many of the leaves of my plumbago. They have a raspy, almost sand-paper feel to them and are most unpleasant to touch. I was convinced they were the tiny eggs of some pest bug.
So I sprayed the plant with insecticide. One hapless praying mantis died, despite my efforts to resuscitate it by taking it away from the sprayed area and by giving it a quick bath in clean water. Alas, nothing helped. Normally, I would never kill a praying mantis, because it devours “bad” insects in the garden. If a praying mantis gets lost and wanders into my house, I scoop it carefully with a tea towel and take it outside.
The dots remained on the plumbago leaves. Most annoyingly, every single resource dealing with “pests” and “plumbago” on the internet assured me that this is a hardy, pest-free plant. Even if some caterpillar or a few aphids munch on plumbago leaves, says the internet, they do not cause big damage to the plant.
And then, when I almost gave up hope – bingo! I stumbled upon this article. In the article Doug Caldwell talks about a new danger for otherwise pest-resistant plumbago in Florida in the guise of some beastly insect called chili thrips. More importantly for me in Johannesburg, South Africa, he also mentions this:
“A feature that sometimes confuses even the experts trying to diagnose declining plumbago hedges, is the whitish undersides of the leaves. This white material resembles a powdery mildew disease or a chemical spray deposit, it is neither. This white deposit is the natural exudate from “chalk” glands that are found on the plumbago species.”
There is also a picture of the underside of a plumbago leaf covered with the chalky dots – and yes, it looks exactly like my “affected” plumbago leaves!
Here is the image, so that none mistakes the whitish dots for pests, sprays the plant with insecticide and murders another innocent praying mantis: