As far as I can tell from the photo, that is a mealybug infestation - there's a newish one becoming a problem in Tunisia (first reported in 2014) called Phenacoccus madeirensis, common name Cassava mealybug because it primarily affects cassava crops, but among the other plants it infests is Cestrum -that seems to be what you've got on yours. Since its quite an extreme infestation, I recommend you seek out a systemic insecticide which professes to treat mealybug and use that - but try not to spray open flowers and try to spray at dusk, after pollinating insects like bees have stopped working. If you're fertilizing the plant regularly, stop - high nitrogen feeds applied regularly make the plant more attractive to these pests. Alternatively, you can try an insecticidal soap spray to see if that does the job first.
The link below leads to a PDF with information about cassava mealybug - you might be interested to read it, but its mostly scientific info