Dyson had moved to six under par before dropping two shots over his closing three holes on the Centenary course.
It left the 32-year old player two shots behind compatriot and early leader Richard Finch, who chipped in from off the green at the last hole in a score of 66.
Three other Englishmen – Robert Rock, David Lynn and Richard Bland – were well placed at five under par.
Dyson arrived in the Scottish Highlands in 13th place in the Ryder Cup standings, four places shy of automatically qualifying for the European team that will take on the United States in Wales in October.
The European lineup will be decided at the close of the Scottish event, and Dyson knows he must capture the ?282,772 ($360,000) winner’s check at Gleneagles to have any chance of inclusion.
“It?s amazing what your attitude can be like when you know you have to win,” he said.
Another Englishman, Ross McGowan, also harbored hopes of making the European team, but his chances receded on Thursday when a shoulder injury hampered him during a five-over-par 77.
Spain?s Alvaro Quiros, the only other player in the Johnnie Walker Championship who stands a mathematical chance of qualifying for the Ryder Cup, carded an even-par 72.
Colin Montgomerie, the European team captain who will name his three Ryder Cup wildcard selections on Sunday night, shot a one-under par 71.