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The characteristics of 59 pathogenic pseudomonads isolated from stone fruit trees in south east England, 30 from cherry and 29 from plum, were compared with 41 isolates from pear trees in a wide range of biochemical, physiological and lesion tests. Many characteristics were common, but several consistent and stable differences were found between the stone fruit and pear groups, each of which exhibited a high level of homogeneity. The few atypical isolates in each group deviated from the majority in one or two respects only. Ten citrus and 3 lilac isolates were biochemically and physiologically indistinguishable from the pear isolates but had distinctive phage and bacteriocin sensitivities. The stone fruit isolates correspond to PaeUd0m1m.a.q moraprunorum and the pear isolates to Pa. ayn'ngae. The relationship between these two 'species' is discussed.

Pseudomonas mors‐prunorum Wormald, the organism causing bacterial canker of stone‐fruits, was present on healthy cherry leaves during the autumn in numbers sufficient to suggest that they were the main source of inoculum for the infection of stems and branches. Quantitative estimates of the pathogen could be obtained by shaking leaves in water and plating out the washings. This method was examined in detail as a possible means of comparing the inoculum potentials on different cherry varieties.The numbers of the pathogen varied considerably between leaves and between branches, but not between trees of the same variety. It was estimated that a sample of 192 leaves, eight from each of twenty‐four different branches would be adequate to show differences between varieties if sufficiently replicated in time. The accuracy of a twenty‐four branch sample was tested by comparing parallel samples from two separate groups of trees of the same variety. There was a highly significant correlation between the numbers of bacteria obtained from the two series of samples. From the error variance in an analysis of variance, it was calculated that five successive samples would show a significant difference between two such groups of trees if one exceeded the other by 54%.It was necessary to wash leaves for 4 hr. or more, depending on temperature, to recover all the bacteria from the leaf surfaces. At temperatures lower than 27° C. however, bacterial growth intervened before this stage was reached. At laboratory temperatures a period of 4 hr. washing was found to be most suitable for comparing varieties and gave 80–90% recovery of bacteria without errors due to growth.A characteristic flora of non‐parasitic bacteria was observed on cherry trees, coexisting with the pathogen on the leaf surfaces. The fungus Pullaria pullulans was also extremely abundant.

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In a plot of sweet-cherry trees (Prunus asium), leaf spot infections caused by Pseudomonas morsprunmum were more abundant on the cv. Roundel than on the normally more susceptible cv. Napoleon. Roundel also supported significantly higher leaf surface populations of the pathogen. This reversal in the relative field susceptibility of the two cultivars was associated with the presence on the trees of a variant form of P. morsprunorum that differed * Present address : Hudeiba Research Station, Ed-Darner, Sudan.