A harvesting machine such as a disc mower-conditioner (10) is provided comprising a disc cutterbar (26) with transversely spaced rotatable cutter units (28) operable to sever standing crop material by impact action. A flexible curtain (40) extends around the perimeter of the disc cutterbar (26). A transverse deflector panel (50) is supported by the mower-conditioner frame (14) between the disc cutterbar (26) and the flexible curtain (40) to restrict the movement of the forward transverse portion (42) of the flexible curtain (40) toward the cutterbar (26). The deflector panel (50) is pivotally mounted for fore-and-aft movement about a generally transverse axis and has sufficient mass to resist movement of the curtain (40) induced by an air flow created by rotation of the disc cutters (28). The deflector panel (50) includes a lower curved portion (54) to facilitate the passage of crop material therebeneath for presentation to the disc cutterbar (26).