A re-examination of fire blight epidemiology in England.
Modes of entry of fire blight (Erwinia amylovora) into a new country and its subsequent early spread often remain a matter for speculation. Available evidence may be largely circumstantial but all possibilities need to be explored. For this, adequate field and weather records need to be in place. Spread of disease may precede detection by several years so, in a re-examination of the early history of fire blight in England, Billing's integrated system (BIS) for risk assessment was used to identify seasons when conditions favoured detection on one or more hosts. Such conditions were highly favourable in 1957 when fire blight was first noticed in 'Kent' pear orchards. New evidence suggests the possibility that there was a greater risk than usual in 1955 of blighted pears (and hence, contaminated fruit boxes) being imported from the USA. There was circumstantial evidence that migrant birds spread fire blight from England to coastal hawthorns in continental Europe in the mid 1960s. New evidence suggests that 1964 was the most likely year of spread. That year, migrant starlings were grounded in coastal areas of England because of adverse weather and the same was true in 1958. This may account for some puzzling features of the early rapid spread of disease on hawthorns and ornamental hosts in the Thames Estuary area and the southern London suburbs. Much of the later, slower spread across southern England might have been due to sale of diseased ornamental trees and shrubs. Insects and wind as agents of spread over the longer distances cannot be ruled out but, supporting evidence is lacking. During storms, strong winds sometimes spread inoculum to a distance of 100 m or more from hawthorns to orchard trees. Other potential effects of storms have been studied experimentally.