LESIONS on stems, cotyledons, leaves, sepals, petals, and siliques. On stems, spots numerous, circular or irregular, red-brown, up to 1 mm diam. On stems, spots brown-black, elongate, slightly immersed. On leaves, spots brown-black, gradually enlarging with age. On sepals and petals, spots brown. On siliques, spots numerous, small, frequently connecting with each other; most spots occur along the raphe of the silique.
The spots on all infected plant parts always are covered with an olive coat, usually composed of concentric zones formed by aggregations of conidiophoreswith conidia. Each spot frequently is surrounded by a chlorotic halo.
CONIDIOPHORES (cp) dark olive, 0-7-septate, geniculate, with a prominent scar at each geniculation, 14-48 x 6-13 µm.
CONIDIA (c) usually single when produced on their plant hosts, sometimes in chains of up to four when formed in agar cultures, obclavate, with a conspicuous beak, smooth, greyish-olive, with 11-18 cross septa, and 0-8 longitudinal ones, slightly constricted at the septa, 39-350 x 9-42 µm. The conidia are called "porospores" or "poroconidia", because they arise from a protrusion of protoplasm through a pore in the wall of the conidiophore.
PLANT HOST AND DISTRIBUTION. The plant hosts of A. brassicae are different species of the family Brassicaceae (Brooks 1953; Holliday 1989; Kochman 1973; Smith et al. 1988).
Alternaria brassicae has a worldwide distribution (Kochman 1973; Smith et al. 1988).
NOTES. Alternaria brassicae mainly causes spots on different plant species, but it may also cause damping off of seedlings. Affected siliques usually split before their harvest and loose seeds. The seeds from infected siliques are also infected.
Alternaria brassicae overwinters as mycelium in infected plant debris and as mycelium or conidia in or on seeds (Agrios 1988; Kochman 1973). The fungus carried with the seed then attacks the seedling, usually after emergence, and causes damping off or stem lesions and collar rot. Spores produced on debris, infected cultivated plants, or weeds are mainly spread by wind. Free water is needed for spore germination (Smith et al. 1988). The optimum temperatures for spore germination, mycelial growth, and infection of plants are 17-24oC. The conditions favouring the production of conidia are heavy dews and frequent rains. The germinating conidia penetrate susceptible tissue directly or through wounds and soon produce new conidia that are further spread by wind, splashing rain, or tools.
Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.
Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, new York, Toronto.
Kochman J. 1973. Fitopatologia. PWRiL. Warszawa.
Holliday P. 1989. A dictionary of plant pathology. Cambridge University Press. Cambridge, New York, New Rochelle, Melbourne, Sydney.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.