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Title: Aspects of the biology of Brassica/Alternaria host/pathogen systems
Author: McRoberts, Neil
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1992
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The main purpose of this study was to further an understanding of host-pathogen interactions and the role of phytotoxins in the host relationships of Alternaria brassicae (Berk.) Sacc. and Alternaria brassicicola (schw.) Wilts., using microscopical, biochemical and statistical approaches. A further aim was to assess the taxonomic positions of A.brassicae and A.brassicicola within the Alternaria, along with their attributes and behaviour patterns in relation to other members of the anamorph-genus. On the leaf surfaces of host plants A.brassicae and A.brassicicola showed broadly similar patterns of development but with some features which distinguished them. The larger spores of A.brassicae typically produced two to three germ-tubes whereas A.brassicicola gave rise to only one. A.brassicae generally produced more extensive extra-matrical growth with hyphal branching and appressoria in intercalary as well as terminal positions; hyphae of A.brassicicola produced very few branches and usually terminated in appressoria. For both fungi appressoria were formed most frequently near to or over anticlinal walls of epidermal cells. Smaller numbers of appressoria were formed over periclinal walls and, in the case of A.brassicae, over stomata. The extra-matrical development of both species was more or less similar on contrasting host leaf surfaces, although A.brassicae showed somewhat reduced germ-tube numbers and stomatal penetration on leaves with a pronounced waxy bloom. Marked differences in behaviour on different hosts or between hosts and non-hosts were evident only after penetration. In the case of A.brassicae unsuccessful penetrations were associated with only localised deposition of callose in the cell wall of the challenged cell, while with A.brassicicola unsuccessful penetration events were associated with cell wall responses which occurred in whole single cells. In comparing A.brassicae and A.brassicicola with other Alternaria species varying in their host range and degree of parasitism, all exhibited essentially similar patterns of extra-matrical behaviour on leaves, culminating in the formation of appressoria. Distinctive host relationships were evident only in the post-penetration phase. From successful penetrations A.brassicae produced a short sub-cuticular phase whereas A.brassicicola appeared to penetrate to intracellular position in the epidermis. Subsequent colonisation involved intercellular hyphal growth and extensive callose formation in host cell walls in response to both species.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available