Main symptoms

Bremia lactucae can attack lettuce throughout their growing cycle.

The seedlings are particularly susceptible to mildew. Bremia lactucae grows very rapidly on the cotyledons which it covers with its numerous white fruiting bodies (figure 1). It invades the leaf tissues which chlorose ; some young leaves have their blade curling at the edge. It also causes stunted seedlings and ultimately their death .

On older plants, it first develops on the leaves of the crown (Figures 2 to 5). It causes large pale green to yellow spots , delimited by the veins (figures 6 to 9) and therefore more or less angular. These spots eventually necrode and take on a light brown tint (Figure 10). Bremia lactucae fruits fairly profusely, especially on the underside of the leaves before or after the chlorotic spots are visible on the leaf blade. Arbuscular sporangiophores (figures 11 to 13), carrying the sporangia, emerge from the stomata and constitute a white felting more or less dense at the origin of the common name of "sucker" formerly used. Subsequently, spots develop on the innermost leaves and those of the heart. Heavily affected leaves, on which the spots have coalesced, completely necrotize and die .

Systemic infections sometimes occur. Browning of the internal tissues of the stem and the base of the leaves is then visible.

It should be noted that damaged tissues constitute nutrient bases for secondary bacterial ( invaders Pseudomonas spp., Pectobacterium carotovorum subsp. Carotovorum ...) or fungal ( Botrytis cinerea ...) which, when conditions are wet, cause wet and soft rots in the field and sometimes even in storage.