Curling leaves on a banana plant may indicate the presence of a disease or the need for an amendment to the soil. "Bunchy top" is the name of a virus that---like a deficiency of phosphorus or boron---may cause curling of leaves.
Bunchy Top Virus
Banana bunchy top virus causes the leaves to become twisted and bunched up. They may turn inward and become curled or develop yellow, wavy edges. Symptoms of the virus may not show up right away, so any plants that are connected to infected ones must be destroyed when the diseased plant is removed.
Banana plants that are suffering from phosphorus deficiency are stunted. Old leaves on the plant develop yellow leaf margins due to a lack of chlorophyll. Younger leaves look bluish-green. In addition, the leaves curl and the stems supporting the leaves can break.
Boron deficiency causes banana leaves to curl. In addition, the leaf blade will become deformed and white stripes will appear on young leaves. Root and flower formation will be suppressed.