The present invention provides a tree trunk injection agent whereby pathogenic fungi and/or pests in trees can be effectively controlled merely by a short-time application in a low chemical dose, and a method for controlling tree diseases using the tree trunk injection agent. According to one embodiment of the present invention, the tree trunk injection agent is obtained by adding, to an aqueous solution of an insecticidal component such as acetamiprid, a hardly soluble fungicidal component such as thiophanate-methyl so as to suspend the hardly soluble fungicidal component, wherein a particle size based on 50% volume of the fungicidal component is 4 µm or less, or adding thereto at least one fungicidal component selected from a group consisting of triflumizole and metconazole.
