LESIONS on leaves of potatoes and tomatoes, as well as on their tubers and fruits, respectively. Leaf spots concentrically zoned, dark brown, 3-5 mm diam. The number of spots increases with age of plants. Spots on tubers and fruits at first single, later confluent, dark brown to black, slightly depressed. The affected tissues dry, become coriaceous, and cover with a coat composed of conidiophores with conidia.
CONIDIOPHORES short, septate, dark coloured, similar to those of A. brassicae.
CONIDIA single, with a long filiform beak ended with a small pore, smooth, dark olive or olive-brown, muriform, with 3-14 and 0-8 cross and longitudinal septa, respectively, 73.5-231 x 13.5-28.5 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. Alternaria solani is hosted by different species of the family Solanaceae (Agrios 1988; Brooks 1953; Holliday 1989; Kochman 1973).
Alternaria solani occurs in the whole world (Agrios 1988; Brooks 1953; Holliday 1989; Kochman 1973).
NOTES. Alternaria solani causes early blight of potato and tomato, as well as potato tuber rot and tomato fruit rot (Agrios 1988; Brooks 1953; Kochman 1973).
Alternaria solani overwinters in the forms of mycelium and conidia in plant debris, potato tubers, and on the surface of tomato seeds (Kochman 1973). The mycelium in infected tomato seeds may cause a black foot rot of the stems and "damp-off" the seedlings.
During vegetation, A. solani spreads by conidia transported by wind and insects. The conidia quickly germinate and infect plants. The optimal temperature for their germination ranges from 28 to 30oC. The conidial germ tubes penetrate leaves directly through the cuticule, whereas potato tubers and tomato fruits are infected through cracks and wounds. The sporulation of A. solani is favoured by heavy dews and frequent rains.
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.