Colours on leaves
Juvenile plants (red-purple blotches on leaves)
Cause: The juvenile stage of young Cavendish bananas and some other varieties usually has red-purple blotches on the leaves. This is normal.
Solution: No control is needed and plants grow out of this symptom.
Mite damage (bronzed leaves)
Cause: Feeding on the banana spider mite, Tetranychus lambi, or the two-spotted mite, Tetranychus urticae, on the underside of leaves.
Solution: Treatment is usually not necessary because predatory beetles Stethorus spp. maintain adequate natural control. If these beetles are not present, and monitoring results show the level of damage is severe, treat with an appropriate miticide. Ensure adequate coverage to the underside of leaves.
More info:
Better Bananas – Mite summary
Queensland Department of Primary Industries
Plant Health Australia
Sooty mould (blackened leaves)
Cause: A fungus growing on the honeydew secretions produced by insects such as aphids, scales and bugs.
Solution: Natural control by parasites and predators usually provides adequate control of these pests. Chemical sprays specifically to reduce insects which promote honeydew and sooty mould are seldom required.
More info: