Fusicoccum amygdali Del., the pathogen of almond (Prunus amygdalus St.) and peach (Prunus persica (L.) St.) canker, causes leaves of an infected shoot to wilt and dry, besides causing cankers around infected buds and nodes. These foliar symptoms have been attributed to toxins which are produced by the pathogen at the site of infection and transported in the vascular system1. A toxin, produced in culture by the actively growing mycelium of the pathogen2 has been extracted, purified, and called fusicoccin3. It is a glucoside of a tricarbocyclic terpene with a molecular weight of 680 (ref. 4). When introduced into the xylem, fusicoccin produces symptoms in almond and peach shoots closely resembling those found after an infection of Fusicoccum amygdali2. Furthermore, if tested on cuttings or detached leaves, the toxin affects cuttings from a wide range of plants2.