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Morphology and cytology of typhula trifolii rostr

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NobleMJM_1936redux.pdf (4.614Mb)
Date
1936
Author
Noble, Mary Jessie McDonald
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Abstract
 
 
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.
 
URI
http://hdl.handle.net/1842/33404
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  • Biological Sciences thesis and dissertation collection

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