It sure looks like a fungal infection to me. For comparison, scroll down this page and check out some images of Sansevieria leaves with a fungal infection:
http://download.springer.com/s...
I'd cut out the infected part of the leaf and discard it and then sprinkle cinnamon on the cut area (cinnamon is a natural fungicide.) The cut leaf won't look so pretty but at least the infection won't spread.
