The invention relates to an antiseptic which is composed of compound antiseptic agent (A) and an active osmotic agent (B), aiming at solving the difficulties that all the current medicaments for curing a fruit tree canker apply drugs after rossing and nicking but the curing effect is still unfavorable. The invention adopts the compound antiseptic agent (A) component to be matched with the active osmotic agent (B) component; after the two components of (A)(B) are mixed, a compound antiseptic with extremely high germicidal and osmotic forces and taking a chlorine dioxide as a main component can be generated in a plurality of minutes; the compound antiseptic not only has extremely high osmotic force that can permeate a complete pathological organization, the extremely high germicidal force of the compound antiseptic can kill all the diseases caused by virus, bacteria and epiphyte; therefore, the compound antiseptic is called a mushroom poison killer. The invention can omit rossing and nicking when curing the fruit tree canker and the cure rate in a shorter time can reach by 100 percent. The invention has the advantages of simple producing technique, convenient storage and use, security and innocuity and no drug fastness. The invention is suitable for the fruit tree canker and various diseases for tree crops caused by the virus, the bacteria and the epiphyte.