Study of pseudomonas, with special reference to species pathogenic to stone-fruit trees

The object of the work reported here was threefold, the aim was, firstly, to examine the relationship of two species of bacteria pathogenic to stone-fruit trees, namely, Pseudomonas mors-prunorum Wormald and Pseudomonas syringae van Hall; secondly, to compare these organisms and some other pathogens with a series of pseudomonads isolated from plants and soil, with a view to improving the methods by which the pathogens may be isolated and identified; and thirdly, to investigate a possible means of controlling the disease on stone-fruit trees. A brief account is given of the nature and incidence of bacterial canker of plum trees in Scotland. . A procedure for chelating heavy metals in culture media was found to promote the appearance of the fluorescent pigment produced by the pseudomonads. Traces of iron in the media were shown to interfere with the production of the pigment. A medium was described whereby the pigment was more readily detected. The action of the organisms on various sugars was investigated. No significant differentiation was found on the basis of acid production from sugars but the utilisation of sucrose as the sole source of carbon was considered to be a significant differential character. Levan production from sucrose and the copper-reducing properties of sucrose cultures were studied. The two properties, although both were often found in one organism, were shown to be independent of each other. An investigation was made of the production of 2 -keto-gluconate from glucose and related compounds. The majority, of the plant pathogens tested did not produce the keto-acid which was, however, commonly a product of the other organisms. The lipolytic activity of the organisms was examined using an improved method for its detection. This property was found to be less common among the plant pathogens than the other organisms tested. Observations on the stability of the colonial characters led to the conclusion that little reliance can be placed on the features of the colony as an aid to the identification of the organisms. The strains of Ps. mors-prunorum examined in this work could not be distinguished, by the laboratory methods applied, from the strains of Ps. syringae. The grounds on which these species are separated appear, therefore, to be questionable. In the whole series of pseudomonads examined the laboratory examination did bring out one line of division which could be related to the sources of the organisms: the strains isolated from plants (including the pathogens) could be separated from those from the soil by the possession of the following characters by the plant organisms, viz., (a) The utilisation of sucrose with or without the production of acid. (b) The formation of levan from sucrose. (c) No copper- reducing action from gluconate. In view of this finding it may be suggested that the plant pathogens in this genus may be regarded as more or less specialised members of a larger group of Pseudomonas, which normally live in association with plants. Work was done on methods of isolating pseudomonads from diseased plant tissue. Of the techniques tried the most promising was one involving the use of sodium gluconate as the sole source of carbon. The information derived from the above studies was of assistance in the conduct of two field experiments. These were designed to test the value of streptomycin in the control of bacterial canker of stone-fruit trees. In one the streptomycin is applied as a spray with the object of destroying organisms on the external surfaces of the trees. In the other experiment, in which the streptomycin is applied as a paint, the object was to prevent infection where the bark was subject to abrasion