A botanical plague was a disease that affected plant life.

In 2269 Merak II suffered from such a plague and the USS Enterprise had to go to Ardana to retrieve zenite a mineral that caused temporary mental degeneration for the Troglyte laborers who mined it until Captain Kirk convinced Plasus of the leader of the Stratos city-dwellers, as well as the rest the planet, of the dangers by having the ship beam him to the mine an forcing him to mine zenite with Vanna the leader of the Disrupter rebels demanding social and political equality for them and their fellow Troglytes. (TOS: "The Cloud Minders")