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.
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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