(Sports Network) - Randy Johnson will try to run his recent unbeaten streak to six straight starts today, when he leads the Arizona Diamondbacks against the Pittsburgh Pirates in the finale of a three-game series at Chase Field.

Johnson is 5-0 in his last five trips to the mound and has recorded a 1.39 earned run average over that span. He last toed the rubber on Friday against the Los Angeles Dodgers and permitted just one unearned run and five hits in six innings of a 2-1 victory. The Big Unit also fanned six batters and issued no walks, and has walked just two batters during his five-game surge.

The left-handed Johnson, who is 5-3 in 10 home starts this season, owns a 9-6 record and a 2.43 ERA over 15 career matchups with Pittsburgh. Johnson lost to the Pirates on June 9 at PNC Park, where he was reached for five runs -- four earned -- in 5 2/3 innings of his team's 5-3 defeat.

Arizona pushed its lead in the NL West to 2 1/2 games over the Dodgers with Tuesday's 3-1 victory over the Pirates. Brandon Webb took a shutout into the ninth inning and became the first 16-game winner in the majors. He allowed one run on nine hits, while walking none and striking out four batters.

Chris Young provided most of the offense with a two-run homer for Arizona, which tries for a series sweep on Wednesday.

Pirates starter Zach Duke remains winless in his last 10 starts despite a solid seven-inning start last night. The lefty was credited with two runs allowed on four hits to go with a walk and six strikeouts over seven innings for the Pirates, who have dropped four in a row.

Doug Mientkiewicz went 2-for-4 and drove in the lone Pirates run.

Jeff Karstens will make his second start in a Pittsburgh uniform on Wednesday, and he is coming off a solid performance in his 2008 debut.

Karstens, who was recently acquired from the New York Yankees in a deal that sent outfielder Xavier Nady to the Bronx, took the hill at Chicago's Wrigley Field on Friday. He tossed six scoreless frames of five-hit ball to back a 3-0 victory by the Pirates.

The right-handed Texas Tech product will face Arizona for the first time.

The Diamondbacks and Pirates split four games in Pittsburgh earlier this year.