Here's a look at the top pitching performers in each league for the week ending April 21:

International League
Vidal Nuno, Scranton/Wilkes-Barre
(2-0, 0.77 ERA, 2 G, 2 GS, 11 2/3 IP, 7 H, 1 R, 1 ER, 0 HR, 0 HBP, 2 BB, 14 K)
A 48th-round pick by the Indians in 2009, Nuno has never really been considered a prospect despite having great numbers over the past three seasons. Last year he went 9-5 with a 2.45 ERA in 20 starts for Double-A Trenton. This spring he allowed one run over 14 2/3 innings and held Grapefruit League hitters to a .182 average. The 25-year-old lefty has been dominant with the RailRiders thus far in April, allowing 13 hits and two walks while fanning 26 in 23 1/3 frames. Nuno picked up his first two Triple-A victories this week with strong performances against Rochester and Syracuse and ranks second in the IL in strikeouts.

Pacific Coast League
John Gast, Memphis
(1-0, 0.00 ERA, 2 G, 2 GS, 13 2/3 IP, 10 H, 0 R, 0 ER, 0 HR, 0 HBP, 2 BB, 10 K)
Gast began the season as the Cardinals' No. 10 prospect after going 9-5 for the Redbirds last season, albeit with a 5.10 ERA. The left-hander's stock is soaring after pitching 24 2/3 scoreless innings in his first four starts of 2013. Despite the lack of an overwhelming fastball, Gast is holding PCL hitters to a .220 average, has fanned 22 while walking six and obviously has yet to give up a longball.
· Prospect Pitch: John Gast

Eastern League
Nathan Karns, Harrisburg
(1-0, 1.29 ERA, 1 G, 1 GS, 1 CG, 7 IP, 2 H, 1 R, 1 ER, 0 HR, 0 HBP, 1 BB, 13 K)
Karns' season got off to a very rough start on April 8 when he gave up seven runs on seven hits over 3 2/3 innings against New Britain in his Double-A debut. The Nationals' No. 6 prospect bounced back with five scoreless frames in his next start and was even more dominant Saturday despite allowing a run. Karns notched his third career complete game in the opening game of a doubleheader against Altoona, fanning a career-high 13 batters over seven innings in the Senators' 4-1 victory. The Nationals' reigning Minor League Pitcher of the Year, Karns is holding the Eastern League to a .212 average and ranks fourth on the circuit with 24 strikeouts.

Southern League
C.J. Riefenhauser, Montgomery
(1-0, 0.00 ERA, 3 G, 7 1/3 IP, 0 H, 0 R, 0 ER, 0 HR, 0 HBP, 1 BB, 5 K)
Riefenhauser was just shy of perfect this week, allowing one walk over 7 1/3 frames in three relief appearances. All five of the hits he's allowed this season came in back-to-back appearances at Jackson last week -- the other five outings, covering 9 1/3 innings, have been hitless. The 23-year-old left-hander, a 20th-round pick in 2010, has held Southern League hitters to a .119 average overall, with right-handers just 3-for-30 this season.

Texas League
Asher Wojciechowski, Corpus Christi
(1-0, 0.00 ERA, 2 G, 1 GS, 9 IP, 6 H, 0 R, 0 ER, 0 HR, 0 HBP, 0 BB, 8 K)
Acquired from the Blue Jays organization last summer, Wojciechowski has been masterful since making his Double-A debut with the Hooks on July 24. The Astros' No. 15 prospect closed out the 2012 campaign with a pair of scoreless outings and has begun the 2013 season with four more, setting a club record with 31 consecutive shutout frames along the way. Working in relief for the Hooks on Saturday after starter David Martinez tossed five perfect innings, Wojciechowski did allow a pair of hits over four innings to complete the two-hitter but continued to keep Texas League opponents off the scoreboard as the Hooks edged Frisco, 1-0.

California League
Austin Fleet, San Jose
(2-0, 0.00 ERA, 2 G, 6 1/3 IP, 1 H, 0 R, 0 ER, 0 HR, 0 HBP, 0 BB, 9 K)
Fleet is used to working in relief -- he made 41 appearances out of the bullpen for Double-A Richmond last season -- but normally not so early in games. When top Giants prospect Kyle Crick left Thursday's game with a strained oblique after two frames, Fleet came on to pitch four perfect frames, fanning six, as San Jose topped Inland Empire, 5-1. Fleet, who turned 26 on Wednesday, also had a 2 1/3-inning scoreless outing Monday and has now allowed three hits and one walk to the 30 batters he's faced this season.

Carolina League
Taylor Hill, Potomac
(2-0, 0.69 ERA, 2 G, 2 GS, 1 CG, 1 SHO, 13 IP, 5 H, 1 R, 1 ER, 0 HR, 0 HBP, 1 BB, 11 K)
Vanderbilt product Taylor Hill is off to a flying start in 2013, having won all three of his starts. The 24-year-old right-hander's most recent outing was his best: after allowing one hit over six frames at Salem on Monday, he tossed a seven-inning one-hit shutout against visiting Frederick on Saturday night. Hill has fanned 16 while walking two over 19 innings this month and is holding Carolina League batters to a .191 average.

Florida State League
Kyle Ryan, Lakeland
(0-0, 0.00 ERA, 1 G, 1 GS, 8 IP, 1 H, 0 R, 0 ER, 0 HR, 0 HBP, 4 BB, 3 K)
After the Dunedin Blue Jays hit Ryan around for seven runs on 10 hits in six frames on April 13, it didn't take long for the 6-foot-6 lefty to exact revenge. Back home in Lakeland on Thursday, Ryan stifled the Jays for eight frames, allowing one hit over eight shutout innings. The Flying Tigers were unable to take advantage, falling 1-0 in 13 innings, but Ryan's rebound performance bodes well for a Lakeland team that sits in last place in the North at 5-12.

Midwest League
Jeff Ames, Bowling Green
(0-0, 1 G, 1 GS, 5 IP, 1 H, 0 R, 0 ER, 0 HR, 0 HBP, 0 BB, 8 K)
Nearly the only thing Ames did wrong in his first two starts with the Hot Rods was to allow one homer in each game. On Tuesday the Rays' No. 13 prospect avoided that problem and nearly every other, fanning eight over five one-hit frames and combining with a pair of Bowling Green relievers on a one-hit shutout at Great Lakes. A first-round pick in 2011, Ames was dominant for Class A Short-Season Hudson Valley last summer and has been even better in his first Midwest League outings: in 15 innings he's fanned 15 while allowing five hits and one walk.

South Atlantic League
Alexander Claudio, Hickory
(2-0, 0.00 ERA, 2 G, 7 IP, 1 H, 0 R, 0 ER, 0 HR, 0 HBP, 0 BB, 11 K)
A 27th-round pick out of Juncos, Puerto Rico, in 2010, Claudio went 4-0 and was named a postseason All-Star in the Arizona Rookie League last season. Apart from a rough outing at Kannapolis last week, the 21-year-old lefty has been even more effective with full-season Hickory. Claudio was nearly perfect in two relief appearances this week, allowing one hit over four frames at Asheville on Wednesday and tossing three perfect innings against Charleston on Sunday. Claudio struck out 11 without issuing a walk and earned the victory in both games as Hickory reeled off seven straight wins.