Bitter gourd witches broom is caused by

Bitter gourd witches broom is caused by Virus.
Witches' broom is a deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest. It is sometimes caused by pathogens.
The normal hierarchy of buds is interrupted, and apices grow indiscriminately. This can be caused by cytokinin, a phytohormone that interferes with growth regulation.