Abstract
Products containing prohexadione-Ca represent a considerable alternative to antibiotics for the control of secondary fire blight (Erwinia amylovora) in pears as well as in apples. Furthermore, less pathogen incidence is also induced by this compound in apples against scab (Venturia inaequalis), in grapevines against grey mould (Botrytis cinerea), and in several other host-pathogen systems. Clear evidence is available that these effects are primarily due to induced physiological resistance. However, it should not be ruled out that morphological and histological changes caused by prohexadione-Ca might support this effect.