In a disastrous but perhaps predictable result, Saints crashed out of the cup after losing the advantage to first division visitors Gretna early on. Steven Tosh stunned the home crowd in the 12th minute, the former Saint hammering home Kenny Deuchar's low cross beyond the outstretched hands of diving keeper Kevin Cuthbert.
The game had started fairly evenly, but from that point on there appeared to be only one winner possible.
The Black and Whites were posing a real threat, and David Nicholl's forced a save out of Cuthbert when the keeper had to scramble across his goalmouth to stop the former Falkirk man's effort from counting.
A Deuchar header in the 22nd minute flashed a yard wide with Cuthbert wrong footed, and it took a combination of the keeper and Steven Anderson to prevent the same player from slipping the ball into the net four minutes later.
All Saints had to show for the first half were two high and wide shots by young debutant Andy Jackson, and they were clearly second best in almost all departments.
It was more of the same after the restart as Gretna continued to dominate with Ryan McGuffie closest to adding to their tally when his low drive scraped past Cuthbert's far post in the 51st minute.
Deuchar almost exposed the home rearguard with two good second half runs but on both occasions his final shot was too weak to bother Cuthbert.
As for Saints, they looked more like the lower league club than the visitors, and with their league championship challenge fading this result looks like effectively putting paid to their challenge in all competitions this season.