Bordeaux extend Saint-Etienne's woes
The home side went ahead through Plasil after 14 minutes and Diabate struck a late second to sink a Saint-Etienne side whose European challenge has been hit by a run of just two wins in ten games at a crucial stage of the season.
The visitors started promisingly and almost took the lead after seven minutes as Sylvain Marchal's cross directed towards goal by Emmanuel Riviere went narrowly over Cedric Carrasso's bar. Bordeaux responded five minutes later with Alou Diarra freeing Anthony Modeste and his shot producing a fine save from Saint-Etienne keeper Jeremie Janot.
Plasil broke the deadlock with the next attack, tapping in after Janot could only parry Modeste's powerful shot. Modeste's long-range shooting was proving to be a highlight of the opening period and another 25-yard effort drew a fantastic save from Janot, who managed to claw the ball to safety as it arrowed towards the top corner.
The Saint-Etienne keeper excelled himself once more before the break with a fine save to keep out a Diarra effort. Riviere had another chance nine minutes into the second half, his header having been picked out by Blaise Matuidi was hacked off the line by Fernando with Carrasso beaten.
The visitors threatened again midway through the second half and again it was Riviere who was denied as his header came back off the bar.
Bordeaux, having ridden their luck, began to exert pressure as the game went on and substitute Evan Chevalier saw an effort kept out by Janot before the hosts grabbed their second with three minutes to go. Diarra released Diabate and he beat Marchal before firing past Janot.