The second half of baseball’s season has opened and we’re set for a heck of a stretch run. A total of 18 teams sit at or above the .500 mark heading into Tuesday night’s action, which means a ton of teams have a shot at reaching the postseason and making a run at a World Series title.

With that in mind, here is a look at those contenders and the one thing that could derail each of their chances at a World Series title.

San Francisco Giants (57-36): Rotation hole after Bumgarner and Cueto

Jeff Samardzija (4.05 ERA), Jake Peavy (5.09) and Matt Cain (5.34) have all struggled at times this year, leaving a huge hole behind Madison Bumgarner and Johnny Cueto at the top of San Francisco’s rotation.

Chicago Cubs (56-36): Jake Arrieta’s arm fatigue

Arrieta blew away his previous career-high in innings last season with 229.0, and has already thrown 114.1 this year. In his last three starts of the first half, Arrieta allowed 15 earned runs in 16.1 innings (8.27 ERA). If he’s tiring that’s bad news for the North Siders.

Washington Nationals (56-37): Lack of offensive balance

Anthony Rendon (.255), Jayson Werth (.244), Drew Espinosa (.236) and Ryan Zimmerman (.221) have helped to drag down baseball’s 19th-ranked team batting average (.250).

Cleveland Indians (54-38): Starting pitching regresses to mean

Starters Trevor Bauer (7-3, 3.36 ERA), Josh Tomlin (10-2, 3.34), Danny Salazar (10-3, 2.75) and Carlos Carrasco (6-3, 2.49 ERA) are all having career years. What are the odds all four guys keep it up as the innings mount?

Texas Rangers (55-39): Pitching, pitching, pitching

The Rangers currently rank 22nd in ERA (4.43), 21st in WHIP (1.40) and 20th in batting average against (.264) league-wide. They need help.

Baltimore Orioles (53-38): Awful starting pitching

Baltimore’s starting pitchers rank 28th in baseball with an ERA of 5.10. Ubaldo Jimenez (7.38), Tyler Wilson (5.22) and Mike Wright (5.97) have been particularly horrific. Offseason acquisition Yovanni Gallardo has been just as bad, with a 5.77 ERA and a 1.76 WHIP in nine starts.

Boston Red Sox (51-39): Brutal road schedule

The BoSox will play 44 of their final 75 games on the road, and just 22 of the last 63 contests will be at home. They will have a few long trips, including two West Coast swings. Boston will have just 10 home games in September while fighting through what is sure to be a tight AL East race.

Los Angeles Dodgers (52-42): Expensive, but weak offense

Despite spending lavishly on offense, the Dodgers currently rank 19th in baseball in runs (401), 25th in batting average (.244), 20th in on-base percentage (.318) and 25th in slugging percentage (.396).

Toronto Blue Jays (52-42): Contract distractions

Jose Bautista and Edwin Encarnacion are in the final years of their current contracts. If questions persist about the future of both guys, who are central to the core of the Blue Jays’ lineup, it will be a distraction and it will almost certainly impact the team negatively.

Miami Marlins (50-42): Starting pitching

Jose Fernandez has been outstanding and Adam Conley (6-5, 3.61 ERA) has been solid. Unfortunately, Tom Koehler (6-8, 4.68 ERA, 1.60 WHIP), Wei-Yin Chen (5-3, 4.90 ERA, 1.26 WHIP) and Justin Nicolino (2-5, 5.57 ERA, 1.53 WHIP) have all been terrible.

Houston Astros (50-43): Carlos Gomez’s slump

Gomez had a horrific first half, and is currently hitting .212 with five home runs, 27 RBI and an atrocious .609 OPS. His WAR of -0.1 (per Fangraphs) is awful. He needs to turn it around for the Astros to have a shot in October.

New York Mets (50-43): Young arms breaking down

Matt Harvey is out for the year and Noah Syndergaard needed a break at the end of the first half because of arm fatigue. Meanwhile, Jacob deGrom has thrown 293.0 innings in the last 18 months and Steven Matz has thrown 101.0 innings this year after never throwing more than 142.2 in a professional season.

Detroit Tigers (49-44): Justin Upton’s brutal year

Upton signed a six-year, $132.75 million deal this offseason and has not delivered. The 28-year-old has hit .236 with 10 home runs, 39 RBI, an OPS of .677 and a WAR of -0.5.

St. Louis Cardinals (48-44): Closer woes

In 31.1 innings, Trevor Rosenthal has allowed 38 hits, 24 walks, 19 earned runs and has four blown saves in 18 chances. His 5.64 ERA and 1.98 WHIP are horrendous, as is his 0.0 WAR.

Pittsburgh Pirates (47-45): Lack of power

The Pirates rank 25th in baseball in home runs (89) and 19th in slugging percentage (.408). They need to find more power if they want to make a run at a title. Andrew McCutchen’s .244 batting average needs to improve as well.

Kansas City Royals (47-45): Lack of an ace

The defending champs need to find an ace for the front of their rotation. The team’s starters boast an awful collective ERA of 4.90, which ranks 25th in baseball.

Seattle Mariners (47-46): Who helps King Felix?

Wade Miley (5.44 ERA), Nathan Karns (4.81) and James Paxton (4.56) have combined to make 40 starts for the Mariners this year. Seattle needs to find more reliable pitching if it wants to make a run.

New York Yankees (46-46): Teixeira and A-Rod

Mark Teixeira (-1.4 WAR) and Alex Rodriguez (-0.7) are currently having the worst seasons of their careers. There’s zero chance the Yankees make a run without both of them turning things around.