Prohexadione-calcium (Regalis)
This is a growth regulator that reduces shoot growth on apples by inhibiting gibberellin biosynthesis.
- When used on apple to control shoot growth it also reduces the incidence and severity of fireblight shoot infection.
- Regalis has no direct bacterial action but increases host resistance by reducing plant vigour and also by altering phenylpropanoid biosynthesis pathways that enhance host resistance.
- Regalis is applied in late bloom to suppress shoot growth so its main effect is on secondary fireblight rather than on primary blossom infection.