Pathogen Profiles

The BSPP - Publications - Molecular Plant Pathology - Pathogen Profiles - Alternaria spp.: from general saprophyte to specific parasite

Molecular Plant Pathology - Pathogen Profiles

Alternaria spp.: from general saprophyte to specific parasite

Bart P. H. J. Thomma

Centre of Microbial and Plant Genetics (CMPG), Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee-Leuven, Belgium

Summary: Alternaria species are mainly saprophytic fungi. However, some species have acquired pathogenic capacities collectively causing disease over a broad host range. This review summarizes the knowledge on pathogenic strategies employed by the fungus to plunder the host. Furthermore, strategies employed by potential host plants in order to ward off an attack are discussed.
Taxonomy: Alternaria spp. kingdom Fungi, subkingdom Eumycotera, phylum Fungi Imperfecti (a non-phylogenetic or artificial phylum of fungi without known sexual stages whose members may or may not be related; taxonomy does not reflect relationships), form class Hypomycetes, Form order Moniliales, form family Dematiaceae, genus Alternaria. Some species of Alternaria are the asexual anamorph of the ascomycete Pleospora while others are speculated to be anamorphs of Leptosphaeria.
Host Range: Most Alternaria species are common saprophytes that derive energy as a result of cellulytic activity and are found in a variety of habitats as ubiquitous agents of decay. Some species are plant pathogens that cause a range of economically important diseases like stem cancer, leaf blight or leaf spot on a large variety of crops. Latent infections can occur and result in post-harvest diseases or damping-off in case of infected seed.
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Symptoms caused by, and appearance of Alternaria spp. (A) Stand of A. alternata conidiophores with chains of conidia (picture kindly provided by G. Barron). (B) Germinating conidia of A. alternata f.sp. citri, the causal agent of brown spot, infecting a citrus leaf (SEM picture kindly provided by A. Bhatia and P. Timmer). (C) Black spot on potato caused by A. solani (picture kindly provided by Carlos A. Lopes). (D) Typical ‘target spot’ symptom of Alternaria: a series of concentric rings at the site of attack. (E) A. brassicicola on a susceptible Arabidopsis leaf. (F) developing chains of A. brassicicola conidia on the surface of an inoculated Arabidopsis leaf. (G) A. brassicicola conidia with longitudinal as well as transverse septa (phaeodictyospores).

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