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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Plenty of sunshine for some, but a little more cloud in the east and feeling cold
A dry and sunny afternoon for most, although staying cold.
Most places will have a sunny day, but feeling chilly, with the chance of a little more cloud in northern areas.