HI
IT could be bunchy top virus caused by aphids. CLosely observe if plants are attacked by aphids. Control of banana bunchy top is achieved by killing the banana aphids then destroying all infected material. First, the aphids should be killed on the infected banana mat and then proceed to kill the mat. Banana mats usually consist of several plants, some mature and some developing suckers, all coming from one underground root. Infected banana plants can be first sprayed with an insecticide like Sevin, or simply soapy water. Only the whorl and upper part of the trunk are sprayed to kill the aphids, which like to hide deep in the central whorl.
REgards
Ashwini