At Cashman Field, Alvarado allowed just one run on four hits in 6 2/3 innings as the 51s (49-46) earned a split of the four-game series against the Southern Division-leading Padres (53-45).
Alvarado has a 0.89 ERA in four starts since being signed to a minor league contract in late June. He has allowed two runs in 20 1/3 innings.
Alvarado retired 13 of the final 14 batters he faced.
Wilmer Flores’ RBI groundout in the first inning plated Ruben Tejada with the first run and put the 51s ahead for good.
Flores, nursing an ankle injury, left the game after the first but is expected to play in Wednesday’s Class AAA All-Star Game.
WINNING/LOSING PITCHER: 51s, Giancarlo Alvarado (3-0); Padres, Matt Andriese (1-2).
NEXT: All-Star break; Class AAA All-Star Game at Reno, 6:05 p.m. Wednesday, KBAD-AM (920)