Pathogenic Variability
The most dynamic and significant aspect in host–pathogen interactions is that characteristics of individuals within a species are not ‘fixed’ in their morphology, physiology, biochemistry and pathogenicity. During reproduction, all individuals are expecte
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Pathogenic Variability
6.1
Introduction
The most dynamic and significant aspect in host– pathogen interactions is that characteristics of individuals within a species are not ‘fixed’ in their morphology, physiology, biochemistry and pathogenicity. During reproduction, all individuals are expected to be different from each other and from their parents in a number of characteristics, although they retain most similarities with them and belong to the same species (Agrios 2005). When individuals are produced asexually, the frequency and degree of variability among the progeny are reduced greatly, but even then, certain individuals among the progeny will show different characteristics. Three categories of such populations are of direct interest to the Brassicalogists of the world: 1. Populations that differ in their ability to attack particular varieties of Brassica hosts 2. Populations differing in their physiological adaptations to specific environmental conditions 3. Populations differing in their ability to tolerate the effect of toxicants In Alternaria–Brassica host pathosystem, the following categories of variability exist: although the genus Alternaria is known as an imperfect fungus, it shows genetic variability within a species, and this variability might be due to the existence of mutation, somatic hybridization,
heterokaryoses, uniform host selection, extensive dispersal and/or a cryptic sexual stage.
6.2
Historical Developments
Initially, variations in cultural characteristics and pathogenesis of different isolates of three Alternaria species infecting Brassicaceae hosts were observed during 1952–1953 by Stoll (1952) in A. brassicicola (vegetables), by Van Schreven (1953) in A. brassicae (Brassica) and by Atkinson (1953) in A. raphani (radish host pathosystem). Therefore, it can be considered as the beginning of research development on pathogenic variability in Alternaria–crucifers host pathosystem. In A. alternata strains showing differences in their physiological and pathological characteristics have been isolated from Crambe (C. abyssinica). Strain A occurs on leaves, stem and siliquae, whereas strains B and C are mainly found on siliquae and leaves, respectively. In pathogenic ability, strain B has been reported to be most virulent, strain A as moderately virulent and strain C as least virulent (Czyzewska 1969, 1971). These strains have different temperature optima for sporulation. Strain A sporulates abundantly at 17–35 °C, whereas B requires 20–30 °C and strain C sporulates best at 12 °C (Czyzewska 1970). Alternaria brassicae is generally most virulent on all brassicaceous hosts. Preliminary reports on variability in this species were made
© Springer Science+Business Media Singapore 2016 G.S. Saharan et al., Alternaria Diseases of Crucifers: Biology, Ecology and Disease Management, DOI 10.1007/978-981-10-0021-8_6
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from Holland (Van Schreven 1953) and the UK (Mridha 1983). Isolates of A. brassicae from rapeseed (colza) showed differences in cultural growth on cherry agar and differed
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