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The Pittsburgh Pirates' season has unraveled quickly.

Chase Headley's first-pitch grand slam highlighted San Diego's first four-homer game of the season, and the Padres cruised past Pittsburgh 15-5 Friday night, handing the Pirates their eighth straight loss.

The Pirates were tied for first place before losing on a controversial call in the 19th inning in Atlanta on July 26. They are 1-10 since and have fallen eight games behind Milwaukee.

"We need to step up — we know it. We've talked about it," Pittsburgh manager Clint Hurdle said. "We need action now, and we need to find a way to fight through the hard part of the season."

The defeat to the last-place Padres was the seventh consecutive and 12th in their last 14 against them at PNC Park. The crowd was a sellout, the Pirates' 13th of the season. But the 37,766 on hand saw Pittsburgh lose for the eighth time in its past nine home games

The Pirates left a season-high 16 men on base. Pedro Alvarez was a primary culprit in that — he tied a Pirates record by grounding into three double plays. He also struck out and committed an error.

In the midst of a record for North American major professional sports streak with 18 consecutive losing seasons, the Pirates were as high as seven games over .500 only 2½ weeks ago. Now, they've fallen to 54-57.

"I don't hang the word 'disappointment' around my shoulders," Hurdle said. "The season's too long and there's too many other things going on. Our record is important, but our level of play, our commitment to one another, the energy we take out there and the way we want to perform in front of our fans and our fanbase and rebound this city with its baseball team, it's hard right now, but that's still our plan. We'll stick to it, and we'll fight through it."

The starting pitching that had been the Pirates' biggest strength all season has deserted them during a homestand in which they are allowing an average of 7.8 runs per game. Pittsburgh has allowed 14 home runs in falling to 0-5 on this homestand.

"It's the big leagues: If you pitch the ball down the middle, they're doing what they're supposed to do with it," starter Jeff Karstens said. "I just think we've got to be better than what we are, and we're not doing that right now. We've got to make adjustments, because if not, it's not going to be fun the next two months."

Karstens (8-6) entered the game with the fifth-lowest ERA in the majors at 2.49, but he was charged with more earned runs (nine) in 3 1-3 innings Friday than he had allowed in his past five starts combined.

"I couldn't pinpoint what I was doing wrong, but I put it all on my shoulders," Karstens said. "I expect to be better than that."

Kyle Blanks, Aaron Cunningham and Jesus Guzman also homered for the Padres.

Will Venable had three hits, two runs and two RBIs, Cameron Maybin added three hits and scored two runs and Luis Martinez had three RBIs for San Diego, which came in ranked last in the NL in runs and homers and hadn't hit four home runs in a game since July 17, 2010.

"You ask anybody, they'll tell you hitting's contagious," Blanks said.

"It's definitely something everyone wants to do, and it really became a case tonight where everyone did get involved."

The 15 runs were a season high for the Padres and the most Pittsburgh has allowed this season.

Headley's homer — his fourth of the season and San Diego's first grand slam since Adrian Gonzalez Aug. 26, 2010 — gave San Diego the lead for good, allowing Aaron Haring (10-3) to get the win despite allowing 13 hits in five innings.

Headley had a career-high five RBIs. Blanks and Cunningham each hit his first home run of the season, both solo shots. Guzman added a two-run homer in the eighth. The Padres hadn't homered three times in a game since May 3 against the Pirates.

It all added up to more than enough offense for Harang to win for the sixth time in seven decisions and rebound from his only loss since May 2. Only once in 252 previous starts, however, had Harang allowed more hits.

"Sometimes, that's how it works," he said. "When I was making good pitches, they were getting the ball to fall in at times. Obviously, our offense came out swinging and those guys were ready to play and helped me out."

Five days after being acquired in a trade from the Padres, Ryan Ludwick went 4 for 4 for the Pirates. Garrett Jones reached base five times as Pittsburgh lost by 10 despite having as many hits as San Diego and coming up one hit short of a season high.

NOTES: Venable (back spasms) was in the lineup for the first time in a week. ... Padres OF Chris Denorfia (strained right hamstring) worked out before the game and is expected to return soon after not playing since Sunday. ... Pittsburgh 1B Derrek Lee did not play for the second consecutive game after being hit by a pitch in the left hand Wednesday night. ... Padres INF-OF Brad Hawpe underwent season-ending Tommy John surgery on his left elbow Friday in San Diego. ... LHP Paul Maholm is 0-3 lifetime against the Padres, whom he will start against Saturday.