Abstract
SUMMARY: Warning systems for fireblight Erwinia amylovora developed in New York, Illinois and California, USA, and in south‐east England are compared. General principles which might be applicable in the different climates were sought. The consequences of applying threshold temperature values chosen for one area in a different climatic area were examined using Sacramento, California; Rochester, New York; Vlissingen, The Netherlands; Kent, England as examples. A graded system for assessing fireblight risks, derived from all the systems, is suggested. It takes into account both risks of infection and risks of high insect activity and it is best used in conjunction with Billing's incubation period assessment system.