During the last three years research work has been carried
out on certain plant diseases which are carried by seed. The lines
followed in this work were: Seed testing with a view to the detection of the presence
of parasitic fungi; the testing of fungicides which are used in the control of
,seed -borne diseases; the preparation of descriptions, illustrated by photographs, of
'`'the appearance of these diseases on the-seed and seedlings.
The first line of work has been studied on account of the fact
that seed testers in this country do not generally take into consideration the sanitary condition of the seed while this is a routine practise on the Continent. The methods employed in each case have
been studied and also, to a certain extent, practised. The seeds
used in these tests were supplied by various growers in England
and Scotland to whom reports on the condition of the seed were sent.
In many of the tests the technique employed was that devised by
Wilson.(1). Seeds were sterilised superficially in a solution of
bleaching powder, then planted in Petri dishes of oat agar medium
such as is commonly used for the cultivation of fungi. Any fungi
which were present either in or on the seed grew out into the
medium, then they were easily subcultured and identified. Figures
illustrate the use of this technique. The fungus Ascochyta Pisi,
the cause of the disease known as "Spot" has grown out into the agar
(from almost every seed.
Seeds were also germinated on damp filter paper in Petri dishes,
method similar to that commonly practised in seed-testing stations,
but although it is often possible to identify seed-borne fungi in
such tests, the previous method is much more efficient in this respect.
A considerable number of sterilisers, between thirty and forty,
have been tested with regard to their efficiency in controlling
seed-borne diseases. These sterilisers were tested first in the
Laboratory and then the most efficient were further tested under
more natural conditions, e.g. two sterilisers "A" and "B" were found
to be very efficient in laboratory tests so they were further tested
in the following manner. Three samples of pea seed known to be
heavily infected with Ascochyta Pisi were used, each sample consisting
of 50 seeds. One sample was treated with "A", one with "B" and the
third remained untreated as a control, the seeds were then sown
singly in pots of sterile sand, and put into a greenhouse until they
had germinated. The percentage which germinated in each sample was;
"A", 86% germinated of which 76% were healthy, in "B" 68% germinated
of which 58% were healthy, and in the untreated sample 68% germinated
of which 28% were healthy.
When the seedlings were about three inches high they were transplanted
into open soil, the diseased seedlings at this stage showing
typical lesions on the stalks and leaves. The final result of this
test showed that from the 50 untreated seeds only 14 healthy plants
were obtained, from the 50 seeds treated with "A" 36 healthy plants
were obtained and from the 50 seeds treated with "B", 30 were obtained
Photographs illustrating this experiment are given in Plates 5 & 6.
On the assumption that the only source of infection was the seed then
it may be concluded from this experiment that such diseases as that
caused by Ascochyta Pisi may be controlled by seed dressings.
Experiments on artificial infection of peas and beans have also
been carried out in order to discover all the possible methods of
transmission of the fungus from the parent plant to the seed and also
to build up a stock of seed known to be heavily infected with certain
diseases for use in testing sterilisers.
In one of these experiments, young flowers of culinary peas were
infected with Ascochyta Pisi by placing spores on the stigmas. The
pods developed from the infected flowers bore the typical lesions of
the disease ( pi, 3. fig. 3) . Other flowers infected in this way
were fixed, embedded in paraffin, and microtomed.
The sections were then stained and examined microscopically for the
presence of the fungus. So far, it has not been possible to trace
the entire path of the fungus in this way but the experiments are
being continued.
In another experiment on artificial infection healthy seeds were
soaked in a suspension of fungal spores and sown immediately in soil.
The seedlings which subsequently appeared bore quite typical lesions
is is shown in Pl.3. fig. 1. Infection of more mature plants was
thought about by watering them with a suspension of spores and also
injecting spores into the plants by means of a hypodermic syringe.
Experiments are also being carried out with a view to determining
he possibilities of insects carrying infection to the seed. Seeds
which have been attacked by the larvae of insects are also often
infected by a fungus. In Pl. 7, fig. 2 a pea seed is illustrated
which had been attacked by the larva of some insect, probably the pea
ridge. It was sterilised superficially and planted on oat agar. At
the pycnidia of Ascohyta Pisi are shown, this fungus having grown
out from the seed. Such seeds are being sectioned in order to
determine the correlation, if any, between the insect attack and the
fungal infection.
An important point which has emerged from experiments with peas
naturally and artificially infected, is that while many infected seeds
bear an obvious lesion (P1.1 fig.4) many show no external symptoms of
the presence of such fungi as Ascochyta. Such lesions as those
illustrated in P1.1 fig. 4, are caused by the presence of deep cankers
m the pod immediately above the seeds which become infected by direct
contact. In seeds which are infected without showing external
symptoms, however, the fungus is found in the tissues of the hilum and
it appears reasonable to suppose that the fungus has grown down the
funicle from the pod and so into the seed. This supposition has been
partially verified by isolating the fungus from the funicle of such
seels. The occurrence of seeds which are-infected while showing no
external symptom is of economic importance to seed growers as well as to seed testers, since both depend almost entirely on the presence
of external symptoms for the detection of diseases.
A considerable number of papers have been written on seed-borne
diseases but they are scattered through various scientific publications
and are mostly written in very technical language. Information
gained from this literature, augmented and confirmed by personal
observation, is to provide the basis for an account of some of the
common seed-borne diseases illustrated by photographs such as are
given in plates 1-12. The effect of certain methods of seed
growing on the occurrence of diseases would also be considered as it
appears that information on this point, written in non-technical
language, would be of interest to seed growers and seed testers.
