Discoloured leaf petioles

Note vein flecking in the leaf petiole caused by banana streak virus.

Banana streak virus

Cause: The banana streak virus (BSV).
Solution: Do not take planting material from infected plants. Eradicate infected plants where symptoms are observed.
Note vein flecking in the leaf petiole caused by banana bunchy top virus.

Bunchy top

Cause: The banana bunchy top virus (BBTV) which is spread on infected planting material and by the banana aphid, Pentalonia nigronervosa. BBTV is not present in tropical Australia, and movement of planting material is controlled by legislation.

Solution: There is no cure for this disease and all infected plants must be eradicated. Outbreaks must be reported to your state primary industry authority.
More info:
Pink discolouration of the petioles first appears on the older leaves but all leaves quickly become affected.

Nitrogen deficiency

Cause: Insufficient nitrogen available for the plant due to inadequate fertiliser applications or leaching and gaseous loss of the nitrogen fertiliser applied.
Solution: Appropriate application of nitrogen fertiliser. You need to know your soil type, climate, past fertiliser program, and soil and leaf analysis results when determining the fertiliser rate.