noun
defoliant
A chemical agent used to cause leaves to fall off plants, often associated with military use (e.g., Agent Orange) or agricultural/forestry applications.
ベトナム戦争では、広範囲に落葉剤が散布された。
During the Vietnam War, defoliants were sprayed over wide areas.
除草剤 is a herbicide that kills weeds, while 落葉剤 specifically causes leaves to drop without necessarily killing the plant.
Compound of 落葉 (defoliation, leaf fall) and 剤 (agent, chemical).