Actually a Google Scholar search for 'citrus postharvest antifungal treatments' does show that this is in fact the standard operating procedure. E.g. this paper starts out with a summary of the current state of conventional agriculture:

http://ucce.ucdavis.edu/files/datastore/234-1524.pdf

It's just that a more simple Google search like 'citrus antifungal' doesn't turn up anything.