LEAF SPOTS on both upper and under sides, circular, subcircular, oblong to irregular, 5-15 x 20-30 mm, at first greenish yellow, later yellow brown, finally brown, vein-limited, sometimes cover the whole leaf surface.
CONIDIOPHORES WITH CONIDIA in loose, rarely in dense, white conglomerations, formed on small or large parts of the under leaf surface.
CONIDIOPHORES (cp) 220-830(-1000) x 6-12 µm, slightly thickened, up to 17 µm, at the base, dichotomously branched.
Branches (b) curved, with globose or plate-like terminal thickenings having 3-5 lateral, rarely top processes (sterigmata), up to 10 µm long, from which conidia develop.
CONIDIA (c) globose, subglobose to slightly prolate, hyaline, with thinner wall at the top and a short pedicel at their base, 15-25 x 12-21 µm.
OOGONIA with a thin, colourless wall.
OOSPORES 26-34 µm diam, with a thin, smooth or slightly warty, yellow brown episporium.
PLANT HOST AND DISTRIBUTION. Bremia lactucae affects plant species belonging to 36 genera of the family Asteraceae (Smith et al. 1988) .
The fungus has a worldwide distribution (Kochman and Majewski 1970; Smith et al. 1988).
NOTES. The genus Bremia differs from the other members of the family Peronosporaceae in the swollen tips of its dichotomously branched conidiophores (Smith et al. 1988).
Bremia lactucae is a very variable species. Hence, in the literature, the fungus exists under many names, varieties, and forms (Kochman and Majewski 1970).
Bremia lactucae is the causal agent of downy mildew of lettuce (Lactuca sativa L.) cultivated in both the field and greenhouses. Both young seedlings and more rarely mature plants may become systemically colonized.
The obligate parasite spreads by means of wind or splash-dispersed conidia. The fungus prefers high humidity (ca. 100%) and low temperatures (5-17oC with an optimum at 10oC). Its conidia germinate by formation of germ tubes that penetrate plant leaves through their epidermal cells or stomata; the conidia do not transform into sporangia with zoospores. Penetration occurs as quickly as within 3 h. Depending on temperature and susceptibility of the plant host or even its tissue, new spores arise in 5-14 days. Sporulation occurs in the nighttime at 4-20oC and requires 6 h darkness and >95% relative humidity (RH). Spore release occurs as RH falls during the day, peaking at 12.00 h.
Bremia lactucae overwinters in the form of oospores in plant debris. Germinating oospores are the primary source of infection.
Kochman J., Majewski T. 1970. Grzyby (Mycota) IV. Glonowce (Phycomycetes), Wroslikowce (Peronosporales). Warszawa, 308 pp.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.