phytotoxicant
/ˌfʌɪtəʊˈtɒksɪk(ə)nt/noun
A substance poisonous or injurious to plants; especially one present in the air. Also attributive or as adjective: phytotoxic.
Origin
1950s; earliest use found in Scientific Monthly. From phyto- + toxicant, after phytotoxic.