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Mycologia, MASTIGOMYCOTINA, PERONOSPORALES, PERONOSPORACEAE .

Crops attacked: lettuce .

Downy mildew

Identification, Description, Biology, Epidemiology, Treatement, Possible misleading, Images

[R]Description
The disease can occur in nursery crops starting in the cotyledons.
Pale lesions with geometrical margins occur on leaves throughout the crop. They are bounded by the veins and correspond to white sporulation on the lower surface of the leaf.
The severely affected leaves dry out or rot.

[R]Biology
The fungus is preserved as oospores in the dead leaves on the soil and is so passed from one crop to the next.
Oospores cause the primary infection of the seedlings generating foci.
The hosts in the foci produce sporangia on the lower surface of the leaves and cause the secondary contamination of healthy plants in optimum temperature and RH conditions.

[R]Epidemiology
Cool and humid weather favour the spread of the disease. The optimum temperature is 15 °C.
Conidial germination (zoosporangia) requires the presence of free water.

[R]Treatement
Remove the debris from the soil.
Avoid planting lettuce after a crop affected by mildew.
Avoid tight sowing in the seedling nursery area.
Minimize free water retention.
Ventilate properly the crops in glasshouses.

[R]Possible misleading
.

[R] Images

1. Bremia lactucae , Downy mildew (RAYNAL G., INRA)
   Lettuce downy mildew Spots of Bremia lactucae on scarole leaves.
   
2. Bremia lactucae , Downy mildew (VEGH I., INRA)
   Lettuce downy mildew Sporulating spot of downy mildew limited by secondary veins on a lettuce leaf.
   

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