I have no experience with plantain problems but it looks as if it is a below ground problem - possibly fungal pathogen or nematode. I suggest you carefully dig up the plant and look for signs and symptoms on the below ground parts. Take photographs of these and post them here for further evaluation.
Looking at the plant and taking a close look at the soil, the yellowing could be a lot of factors and i want to agree with Rony regarding the effect of weather. Some areas in southern Nigeria had issues with rain this year, areas which ordinarily had eight months of rain in the past had relatively shorter periods. So, probably insufficient rain coupled with intense sunlight might have contributed to what we are seeing in the picture. For the blighted portion i think it is pathogenic, symptoms like was observed last year though in the raining season and on young unfolding leaves.
I am re-posting this from colleagues at Bioversity. Please answer and we will try get more information for you
"As for the specific question by the Nigerian banana farmer, here I am not very sure (from just one photo) if this is related to a biotic constraint (however, the farmer would need to verify roots (nematodes?) and corm (weevils?)); the symptoms could possibly be caused by mineral/chemical effects (here the farmer would need to inform us of any possible input history); it would also be good to have additional information on soil fertility level/issues."