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''Bremia lactucae'' is a plant pathogen. This microorganism causes a disease of lettuce (''Lactuca sativa'') denominated as downy mildew. Some other strains can be found on 36 genera of Asteraceae including ''Senecio'' and ''Sonchus''. Experiments using sporangia from hosts do not infect lettuce and it's concluded that the fungus exists as a quantity of host-specific strains (''formae speciales''). Wild species, such as ''Lactuca serriola'', or varieties of ''Lactuca'' can hold strains that infect lettuce, but these pathogens are not sufficiently common to infect seriously the plant. The damage to caused by ''Bremia'' to lettuce may not in itself be serious, but infected plants are susceptible to secondary infection by the more severe mould, ''Botrytis cinerea''. The plant can suffer systemic infections. Metalaxyl is effective against this microorganism. == Description == : * Coarse intercellular mycelium. : * Haustoria are sac-shaped, many times they are present in each host cell. : * Sporangiophores emerge singly or in small groups through the stomata and branch dichotomously. : * Tip of each branch expands to form a cup-shaped disc bearing short cylindrical sterigmata at the margin and occasionally in the centre, and from these the hyaline sporangia arise. : * Germination of the sporangia is usually by means of a germ tube which forms an appressorium to penetrate epidermal cells or it enters through a stoma. : * Zoospore formation has been reported but not confirmed. : * Sexual reproduction is usually heterothallic, but homothallic strains also exist. : * Oospores are formed in leaf tissue and remain viable for 12 months. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bremia lactucae」の詳細全文を読む スポンサード リンク
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