To provide a chemical agent for plants, which has a high epidemic-preventing effect and is rich in safety.

This plant chemical agent is obtained by subjecting the tuberose roots of an Aconitum plant (for example, YAMA aconite, Aconitum grossedentatum (Nakai) Nakai., Aconitum japonicum Thunb. var. eizanense (Nakai) Tamura., or Tanna aconite) in Ranunculaceae to an ingredient extraction such as supercritical extraction or solvent extraction to obtain an extract holding active ingredients such as aconitine, thermally treating the extract at 100 to 130°C to remove toxicity, subjecting the product to an attenuation treatment, and then compounding the treated extract with a surfactant in a weight rate of ≤10%.