WASHINGTON (AP) — Republican Sen. Johnny Isakson of Georgia says he’ll retire at the end of 2019, citing what he calls “health challenges.” The three-term senator was diagnosed with Parkinson’s disease in 2013 and suffered a fall in July, according to his office. Georgia Gov. Brian Kemp will appoint someone to fill the seat when Isakson steps down in December. That’ll set up a rare situation in November 2020 when both of Georgia’s Senate seats will be on the ballot.