DMR reports that bacterial blotch spots spread when temperature is at least 20 C and a film of free water sits on the cap, and that a film lasting more than about three hours is the dangerous case. NHB adds that humidity above 85 per cent with temperature above 20 C should be avoided.
That gives a practical rule needing no chemical: after watering, run air movement until caps are dry to the touch. Never leave a wet crop overnight in a warm room.