Curled or distorted leaves

Spike leaf

Cause: A temporary shortage of calcium caused by rapid growth. Most common in spring and early summer.
Solution: Avoid sudden bursts of growth by using regular, light fertiliser applications. A foliar application of calcium nitrate at 10 g/L in early spring will reduce the problem.  
Left: damage to leaf. Note twists in leaf tip. Right: close-up of damage. Note the cracked and raised veins.

2,4-D damage

Cause: Spray drift of 2,4-D onto green parts of the plant.
Solution: Take efforts to avoid spray drift onto leaves. Plants should grow out of it. 

Tissue-cultured off-type

Cause: A genetic abnormality that occurs in tissue-cultured plants.
Solution: Destroy affected plants and set additional following suckers on nearby plants.

Banana streak virus

Cause: The banana streak virus (BSV). The streaks or flecks on leaves become progressively darker with age and the leaves may die. Symptom expression is sporadic. Symptoms of cucumber mosaic are similar.

Solution: Do not take planting material from infected plants. Eradicate infected plants where symptoms are observed.