Bremia lactucae

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Bremia lactucae - symptoms on lettuce (click on image to enlarge it)
Author(s): Gerald Holmes, Valent USA Corporation
Source: IPM Images

Bremia lactucae Regel 1843

The oomycete causes downy mildew on lettuce which can be an important disease in temperate regions. The symptoms consist of angular, greenish lesions (limited by large veins) which become yellow or necrotic.

B. lactucae also infects various other plants, including crops like artichoke and ornamentals. Management involves mainly the use of fungicides as well as resistant cultivars. However, resistance has often been short-lived.

Vernacular names
• Deutsch: falscher Mehltau des Salats
• English: lettuce downy mildew
• Español: mildiu de la lechuga
• Français: mildiou de la laitue

All growth stages of lettuce are susceptible. B. lactucae enters the plant through an epidermal cell and the mycelium grows between the cells of the mesophyll, obtaining nutrients through haustoria. Asexual conidia are formed mainly on the lower leaf surface and disperse through the wind. In addition, sexual oospores are produced that overwinter in the soil and are apparently the primary inoculum in the next season.

For details see the respective page in BugwoodWiki.