This looks like Banana Leaf Streak. Also called Leaf Spot or Sigatoka Disease. It is caused by a fungus, Mycosphaerella fijiensis.
Here is a video that shows many details of the disease https://www.youtube.com/watch?v=BQncj...
The disease affects photosynthesis and final yield. The fungal spores are dispersed by wind and rain. The seasons can be more pronounced after rains but it is also recognized (by CABI, see below) that symptoms can vary even in the same infected plant. So, some leaves look OK and others have the streaking.
The fact that you have infection in a cluster also suggest a fungal disease. It will spread outwards, especially if you have a lot of rain or dew. If you are watering, make sure you water directly into the base and not the leaves (as that will spread spores). If it is Black Leaf Streak the soil will be infected and you should not plant bananas next time in this area
"It is very important to maintain an adequate level of banana plant nutrition to minimize the effects of the disease. Undernourished plants are more susceptible to infection and also succumb more rapidly to disease. The objective is to have banana plants growing as quickly and
possible, to “outgrow” the effects of the disease as symptoms “climb up” the plant. In Hawai‘i, banana growers try to produce plants with 10 or more healthy (disease-free) leaves at time of flowering, because after flowering there are no more leaves produced on banana plants"
As for controlling the disease the recommended solution is fungicides. I have read here on an Indian site http://www.nhb.gov.in/fruits/banana/b... that
"Dithane M-45 WP (in oil-water emulsion) and Dithane M-45 (in water only) controlled
Mycosphaerella fjijiensis var. difformis in banana. Foliar spray of Copper Oxychloride (3 g/litre of
water) or Thiophanate Methyl 1 g/ litres of water) controls the disease effectively. "
This is similar to what Scott Nelson discusses on his excellent sheet http://www.ctahr.hawaii.edu/oc/freepu...
You can also remove diseased leaves and burn them.
Some additional information from CABI PlantWise http://www.plantwise.org/KnowledgeBan...
And here from Plant Health Australia
1) When did you plant?
2) How many leaves do you have per plant?
3) Do you want to use fungicides?