Bacteria overwinter in orchard trees and in hedgerow hawthorn and susceptible ornamentals in adjacent gardens in bark tissue along the edges of cankers formed from previous year’s infections. In spring these bacteria multiply and produce ooze.
- In wet weather ooze spread by wind blown rain and insects infects flowers in spring or shoot tips.
- E. amylovora can also invade via wounds (especially young shoots damaged by hail or wind) and natural openings such as nectathodes, hydathodes, lenticels, leaf scars.
- The bacteria then invade the tissues giving rise to typical symptoms.
- Fireblight is favoured by warm moist conditions that promote rapid tree growth and high tissue water potential.
- Warm moist conditions favour inoculum spread, establishment of infection and rapid multiplication of bacteria in tissues.
- Disease risk is greatest when temperatures exceed 18oC and there is rain. Disease development occurs between 5-30oC with an optimum temperature of 27oC.
- Free water on the host surface is essential for infection, but early in the season, during primary blossom, rain is not essential as dew may be sufficient to provide the surface wetness.
- A high level of soil moisture (rain or irrigation) is necessary to maintain high plant-tissue water potential to aid disease progression in the host.
- Storms with strong winds or hail are important as they cause tissue damage and provide access points for bacteria and increase the risk of infection and rapid disease establishment.
- Stem blight (branches and trunks) usually follows blossom or shoot blight when stems are expanding rapidly in summer, especially on young trees. It is favoured by warm weather and high soil moisture and nitrogen.
- Every year there will be fireblight on some trees of all hosts that is not noticed. These can be a dangerous source of inoculum in later years if they are not located and destroyed.
- On hawthorns, individual trees sometimes have severe blossom blight though most trees remain disease-free. Infected trees can continue to harbour the disease in subsequent years and remain symptomless until weather conditions are favourable. Such trees can be important sources of the disease.