Aspects of the biology of Brassica/Alternaria host/pathogen systems
The main purpose of this study was to further an understanding of host-pathogen interactions and th e role of phytotoxins in the host relationships of Altemaria brassicae(Berk.) Sacc. and Alternariabrassicicola(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 Altemaria along with their attributes and behaviour patterns in relation to other members of the anamorph-genus.OOn the leaf surfaces of host plants A.brassicae and A.brassicicola showed broadly similar patterns of development but with some features which distinguished them. T he larger spores of A.brassicae typically produced two to three germ -tubes whereas A.brassicicola gave rise to only one. A.brassicae generally produced m ore 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 m ore or less similar on contrasting host leaf surfaces, although A.brassicae showed somewhat reduced germ -tube numbers and stomatal penetrations on leaves with a pronounced waxy bloom. M arked 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.brassicicolaun successful penetration events were associated with cell wall responses which occurred in whole single cells.In comparing A.brassicae and A. brassicicola with other Altemaria 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.Biochemical studies on different Altemaria species indicated that each produced a diverse range of metabolites which may exhibit fungitoxic and phytotoxic activity. In the present study cytotoxicity of the extracts was low. T he studies failed to identify host-specific phytotoxicity, as only crude extracts were used, but the critical role of a host specific toxin, or resistance suppressor, is postulated in the initial establishment of infection.In considering genetical variation within A.brassicae and A.brassicicola, there is little evidence of marked physiological specialisation in either species. However, in testing a small number of isolates against a dicarboximide fungicide, A.brassicae showed slight and A.brassicicola showed marked intraspecific variation in fungicide insensitivity. In testing for sensitivity to fungicides the isolates of A.brassicae used w ere found to exhibit distinctive colony growth characteristics.Multivariate statistical techniques were applied to morphological, biochemical, and pathogenic characters to examine phylogenic relationships between selected species of Altemaria.Studies of the taxonomic relationships of the Altemariaspecies are constrained by the absence of teleomorphs, but certain genera within the Pleosporaceae are suggested to accommodate this group of toxigenic, leaf spotting, facultative parasites / saprophytes.