what the hell is it ??, and how do i avoid it ..

[rowdy23]

hello again.
below is a pan i stitched of the gallery in melbourne library.
i noticed that it has what i think is called banding around the centre 3rd of the image.
just want to know what it is and how to avoid in the future .



any help would be great.
exif is-
f4.5
iso200 ( i wanted to make the light illuminate rather than camera settings )
taken at 18mm
at 1/50th
canon 550d
18-135mm lens

regards
josh