Bremia lactucae. [Descriptions of Fungi and Bacteria].
A description is provided for Bremia lactucae. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: At least 230 species in the following genera of Compositae: Agoseris, Arctium, Carduus, Carthamus, Centaurea, Cichorium, Cineraria, Cirsium, Crepis, Cynara, Dendroseris, Dimorphotheca, Erechtites, Gaillardia, Gnaphalium, Helichrysum, Hemistepta, Hieracium, Hypochoeris, Jacobaea, Knightia, Lactuca, Lagoseris, Lapsana, Lappa, Leontodon, Mulgedium, Nabalus, Picris, Prenanthes, Rhodanthe, Saussurea, Senecio, Solidago, Sonchus, Taraxacum and Tragopogon. DISEASE: 'Downy mildew' of lettuce, globe artichoke, endive, chicory and many ornamental and wild species of Compositae. Infection can occur on any part of the lettuce plant (Lactuca sativa) except the capitulum (40: 197) and the fungus may colonize the plant systemically even as far as the roots (42: 429). In infected seedlings, the cotyledons stop growing, leading to stunting or death of the plant. Sporulation occurs on both sides of the cotyledon, which becomes chlorotic. As seedlings age, they become less susceptible, systemic infection becomes progressively less and fewer sporangiophores are produced (53, 3262). Newly formed true leaves are less susceptible than cotyledons (53, 3262). On mature leaves, profuse sporulation on the lower surface is sometimes preceded by a slight chlorosis; in widespread infections of wrapper leaves conidiophores are often the first sign of infection. On the upper surface chlorosis becomes severe and the lesion, bounded by the main veins, is frequently angular in appearance. Browning may occur later, probably due to secondary infections. On globe artichoke (Cynara scolymus) infection of the peduncle and basal bracts allows access to secondary infections (43, 3104). GEOGRAPHICAL DISTRIBUTION: World-wide except Antarctica and Arctic (CMI Map 86, ed. 3, 1969). TRANSMISSION: Sporulation is usually profuse and conidia are air-borne. Conidia from cultivated lettuce are viable for at least 60 days at low temperatures and high humidity (40: 197) and, where lettuce is grown throughout the year, conidia are the most important means of transmission of the disease. The fungus is probably also carried over from season to season by means of oospores in soil debris (51, 3663).