what is the reason for black spots on Weigela bush
It has been replanted in a larger pot at present before planting in the garden.
The most common cause of black spots on leaves is a bacterial disease called black spot. This article will give you information on it: https://www.gardeningknowhow.com/plant-problems/disease/black-spot-fungus.htm
Also, you should probably get the weigela into the ground as soon as possible. The stress of being in a pot could be contributing to its vulnerability. Here's some info on growing these beauties: https://www.gardeningknowhow.com/ornamental/shrubs/weigela/growing-weigela.htm