Both of Guthrie's goals come from free-kicks - the first unintended, as it flew through a packed Birmingham penalty area, and the second an accurate set-piece that found the net via a post.
Those goals helped Reading make it 13 points from their last five league games and leave Birmingham unable to cap a memorable week with a third successive victory.
Lee Clark's men beat Sheffield Wednesday 4-1 in the league last time out and then followed that up with a 3-1 cup win over Swansea, and buoyed by those successes they started brightly.
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The family of a man found dead in a park in Basingstoke say he will be 'sadly missed'. Jason Williams was found on Tuesday morning.
Thursday evening's weather for the west of the region
Thursday evening's weather for the east of the region