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dc.contributor.advisorLucas, R. L.en
dc.contributor.authorWhiteman, Caroline Maryen
dc.coverage.spatial117 pen
dc.date.accessioned2021-04-08T08:59:34Z
dc.date.available2021-04-08T08:59:34Z
dc.date.issued1967
dc.identifier.urihttps://hdl.handle.net/10023/21948
dc.description.abstractThe appearance of serious outbreaks of take-all in wheat and barley crops can often be correlated with the presence of infected perennial grasses, such as Agropyron repens, Agrostis spp., and Holcus lanatus, growing amongst the crop. Holcus mollis is a weed prevalent amongst cereals growing on light, acid soils. Although this grass has been mentioned in the literature as a possible carrier of the disease, little experimental work appears to have been done to determine the susceptibility of the grass to the take-all fungus, Ophiobolus graminis Sacc.. An investigation was undertaken in this connection. Vegetative segments of H. mollis were found to be only slightly less susceptible than vegetative segments of Agropyron repens, which are heavily attacked by the fungus, but more susceptible than vegetative segments of Agrostis stolonifera. A number of methods were used for estimating the amount of disease on these grasses. The symptoms of the disease, a blackening of the stele of the roots and ectotrophic runner hyphae, are less easy to discriminate on grass than on wheat roots, so an attempt was made to find a method particularly suitable for use with grasses. A method used extensively throughout this work was one of direct observation. Grass plants, seedlings or vegetative fragments, were viewed under water against a white background to determine the presence of stelar lesions. This was the most suitable method for differentiating between a discoloured, diseased root and. one discoloured by some other cause. The presence of runner hyphae, on what appeared to be healthy roots, could be seen when the roots were viewed under a binocular dissecting microscope under the same conditions as for stelar lesions. The percentage infection was calculated from the total number of roots and the total number infected with one or more stelar lesions. The runner hyphae were not used in this calculation but their number was noted. When present on Holcus spp. and A stolonifera seedlings but without any stelar infection they were found to cause stelar lesions on subsequent wheat seedlings. These hyphae must, therefore, be considered a potential hazard, to cereal crops. There were seldom any above ground signs of attack by the fungus on the grasses; no withering of the leaves nor plate mycelium, which are generally associated with severe attacks of the fungus on wheat seedlings. However, A. repens did show some browning and withering of the leaves when the roots were heavily invaded. Mature vegetative fragments of A. repens and A. stolonifera were found to be more susceptible to the disease than those nearer the terminal growing point of a rhizome or stolon. Seedlings of A. repens and Holcus spp. were found to vary in their susceptibility to the fungus, and this did not appear to be affected by the age of the seedlings, over the duration of time of the experiments. Six mycelial isolates of Ophiobolus graminis were obtained from different parts of the country. Five of the isolates were found to be highly pathogenic on wheat, but the sixth was only moderately so. This sixth isolate (3s) survived as chaffed inoculum in unsterile soil in a heated greenhouse for only four months, whereas the other five were still present, although at a low level, after eight months in the same soil under identical conditions. This isolate, was found on two occasions to cause more severe damage to A. repens vegetative segments than to wheat seedlings, a result which was contrary to those obtained with the other isolates. Throughout this work an effort was made to use grass material from the same source in order to avoid variation wher^-ever possible. Hblcus mollis exists as a number of different cytotypes, tetraploid (4n = 28), pentaploid (5n = 35), hexaploid (6n = 42) and heptaploid (7n = 49). Triploid (3n = 21) and aneuploid plants are also frequently found. Rhizome material was collected from different habitats and localities throughout the British Isles. In order to determine the cytotype, chromosome counts of the rhizome material had to be made. Suitable lengths of rhizome were placed in shallow dishes of water, and when adventitious roots, which grew out from the nodes, were of sufficient length, the root tips were stained with aceto-orcein, squashed and the number of chromosomes counted. Seven clones of vegetative H. mollis were grown, three tetraploid, three pentaploid and. one aneuploid clone. A comparison made of the vegetative characters of these clones shaved that they varied considerably. These differences could not have been entirely due to the different environmental conditions of the habitats from which they were collected, as they did not alter after a year in identical conditions in a heated greenhouse. The clones were found to vary in their susceptibility to the six isolates of O, graminis. Vegetative fragments of the clones, all as similar as possible, were inoculated with each of the isolates. The results of the experiment were interesting in several respects. Three of the clones were found statistically to be significantly less susceptible to the isolates than the other three. (The aneuploid material was not included in the statistical analysis of the results owing to the limited, amount available for inoculation). This is interesting in view of the fact that no one variety of wheat has been found to be more resistant to take-all than any other. The more resistant clones were not all of one cytotype, but two were pentaploid and one tetraploid. It appeared that the aneuploid was highly susceptible to the fungus, but few definite conclusions can be drawn from the results because of the small amount of material. There was considerable variation of the results of some experiments, which could have been caused by differences in the O. graminis isolates as well as in the grass material. These isolates were mycelial and therefore not as pure as single ascospore isolates, the mycelium having possibly grown from a number of ascospores. This may have accounted for some of the variation in the cultural characteristics of the isolates,, and their varying pathogenicity in different experiments, as it has been found that mycelia grown from different ascospores from the same ascus, vary in their pathogenicity on wheat seedlings.en
dc.language.isoenen
dc.publisherUniversity of St Andrewsen
dc.subject.lccQK918.W5
dc.subject.lcshGaeumannomyces graminis.en
dc.subject.lcshMycorrhizasen
dc.subject.lcshMycorrhizas in agricultureen
dc.titleInfection by Ophiobolus graminis of some perennial weed grassesen
dc.typeThesisen
dc.type.qualificationlevelDoctoralen
dc.type.qualificationnameMSc Master of Scienceen
dc.publisher.institutionThe University of St Andrewsen


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