AbstractThe ideal warning system should indicate conditions favouring inoculum production and dissemination, infection and subsequent symptom expression. It should give guidance on the best times for inspections to be made and for chemical control measures to be applied. The choice of system is governed by the crop, the climate and economic considerations. New systems must be developed to deal with new situations. Most previous systems have been concerned with the spring blossom periods of pear and apple, but systems for assessing fireblight activity at other times in the growing season are being developed. This is especially important in Europe where the blossom periods of susceptible ornamental hosts extend into July and where, in some countries, summer blossom on pear may be present throughout the season. Monitoring of inoculum levels on cankers, blossoms or shoots has proved valuable where economy in the use of sprays has justified its use; sprays are usually withheld until the pathogen is detected.Warning systems based on weather data are broadly in agreement but differ in detail. Sprays during bloom are often withheld until maximum temperatures exceed 18° C (or mean temperatures reach 14.5 —‐ 16.5° C) and there is precipitation or high humidity (>60 % RH) at the same time. Some workers would spray immediately after moderate rain (e.g. 2.5 – 5.0 mm) when inoculum was known to be present. Spray applications immediately after damaging storms are the general rule. Proper evaluation of warning systems is difficult and is only possible in areas where fireblight is endemic and where adequate field records can be o