Summary. The differential value of 15 characteristics was studied for the determination of plant pathogens in the fluorescent group of the genus Pseudomonas. All but 2 of the 161 pathogenic cultures and the 15 nonpathogenic cultures examined could be placed in one of 5 groups on the basis of tests for: oxidase, potato soft rot, arginine dihydrolase, levan production and a hypersensitivity reaction in tobacco leaves. Tests for production of acid from sucrose, nitrate reductase and a lipase for margarine were useful as subsidiary determinants. Aesculin hydrolysis, gelatinase and tyrosinase tests, and the production of a blue fluorescent pigment were of little or no value at the group level, and hydrolysis of Tween 80 and the catalase reaction had no differential value. With the exception of Ps. tolaaii and two cultures of questionable pathogenicity, the pathogens studied could be separated readily from the few nonpathogens studied. A determinative scheme for plant pathogenic fluorescent pseudomonads is proposed to serve until the taxonomy of the group is better understood.

Summary The characters of 20 nonpigmented and 2 yellow pigmented isolates of Ervinia‐like organisms from plant material (primarily pome fruit) are described and compared with those of Erwinia amylovora. Cultures of some other Erwinia species were also included in the study for comparison. It is suggested, on the basis of their similarity to Erw. amylovora, that they should be included in the genus Erwinia, although they have no obvious pathogenic role. The relationship of these organisms with Erw. lathyri and Bacterium herbicola is discussed.

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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.