PORT ST. LUCIE, Fla. — Noah Syndergaard has been battling what manager Terry Collins called bronchitis during his last two spring training starts. That could be why the Mets’ Opening Day starter ran out of gas during Monday’s outing in the Mets’ 6-4 win over the Marlins.

Syndergaard allowed three runs in 3 1⁄3 innings. The righthander didn’t specify the ailment he has but said it has bothered him for the entirety of spring training.

“I’ve tried to get over it,” Syndergaard said. “But it is what it is. I just want to be ready for Opening Day.”

Collins said a flu-like bug has been going through the clubhouse, with infielder Matt Reynolds the latest victim.

“Sickness runs through a clubhouse, and when it gets in there, it can run rampant,” Collins said. “[Syndergaard] is fighting that. He’s on medication. He is getting better, but he’s just running out of gas, which comes with the territory not only of being sick.”

Syndergaard gave up three hits, including a two-run home run by Justin Bour in the fourth. He walked one and struck out four.

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“He’s been throwing great,” Collins said. “He’s 97, 98 miles an hour. Everything’s fine. He’s sick. That comes with the territory.”

Extra bases

Four Mets homered: Asdrubal Cabrera, Lucas Duda, Curtis Granderson and Neil Walker . . . Mike Piazza began a stint as a guest instructor and Al Leiter visited to do a segment for MLB Network . . . Jay Bruce took ground balls and pop-ups at first base during batting practice but started the game in rightfield . . . David Wright returned to take ground balls (without throwing) and is running while he waits for his shoulder injury to heal . . . Kevin Plawecki was the designated hitter for the second straight game. He left Sunday’s game after three at-bats with stiffness in his left knee.