The Republican candidate for mayor of Tucson, Arizona has dropped out of the race due to a signature challenge on his nominating petitions. This is of interest to third partisans for two reasons. First, it is rare that a major party is pushed off the ballot due to signature challenges (meaning that he didn’t collect enough valid signatures on his nominating petitions in order to meet the legal requirement). It’s more common to see major party primary challengers and minor party or independent candidates pushed off the ballot in this way.
Second, this means that in the general election there will only be a Democrat and a Green. That Green will either be Mary DeCamp, a City Council candidate in the past, or Dave Croteau, a past mayoral candidate. There will be a primary to determine who will be on the ballot for the general election. Another candidate who was running for the Green nomination dropped out of the race in order to support and work on sustainability issues with the now-former Republican candidate.
EDIT: Apparently, according to Jeremy Young and Richard Winger in the comments, as well as the blog Jeremy Young recommended, the Republicans can still nominate a candidate by write-in through their primary. Also, in addition to the candidates mentioned, an independent was also thrown off the ballot.