Biology, epidemiology

- Conservation, sources d'inoculum

Bremia lactucae seems to be able to be preserved in different forms depending on the production areas (fig 1):
- through its oospores (resulting from its sexual reproduction; this fungus is heterothallic) (figure 2) and its mycelium which form in necrotic tissues and are subsequently found in the soil , at the same time as plant debris ;
- les lettuce environnantes , Meyers ( prickly lettuce ...) or Cultivation charantais sur le champignon sporule plus ou moins;
- the seeds sometimes harbor this fungus; in fact, they have never been shown to be the source of primary contaminations.


- Penetration and invasion

The oospores (figure 3) seem to be able to germinate and directly contaminate the young seedlings in the nursery. The sporangia , which have a lifespan of 6 days, provide the vast majority of contaminations, this in a few hours. They germinate in the presence of free water and emit a germ tube on the surface of the leaves which directly penetrates the cuticle and epidermal cells. Contaminations via the stomata are possible. The formation of zoospores is said to be quite rare. Infection can take 3 hours. Then the mycelium gradually invades the cells of the mesophyll. Its extension can be inter and intracellular. If the climatic conditions are favorable, yellow spots appear 4 to 7 days after the first contaminations. Bremia lactucae could be systemic and invade salads entirely.


- Sporulation and dissemination

The appearance of sporangiophores (Figures 4 and 5) and therefore sporangia requires the presence of high humidity . It is carried out during the night. The sporocystophores emerge through the stomata. The sporulation can occur between 5 and 24 ° C . Once formed at the end of sterigmas, the numerous sporangia are released thanks to the combined action of an increase in temperature and a decrease in humidity. The spores are mechanically ejected, especially during the morning. They are immediately carried away by the wind and the air currents which carry them on neighboring plants or more distant in other plots; they ensure secondary contamination. Splashes of water also contribute to the spread of the fungus.

The spread disease is sometimes through infected plants. In some countries, farmers have specialized in the production of seedlings which they produce in large quantities and market in different production areas. Contamination of plants sometimes goes unnoticed; they are then sent sick to distant producers, thus contributing to the early development of the disease in the crop and its dissemination.


- Conditions favorable to its development

This obligate parasitic fungus is extremely influenced by climatic conditions. It appreciates prolonged periods of cool, weather humid (with relative humidity close to 100%) and cloudy . Long periods of wetting the leaves in the morning are particularly favorable for infections. Sprinkler irrigation promotes mildew more than other watering methods.

The temperature range suitable for the germination of its sporangia lies between 10 and 15 ° C . Infections can occur in 2 to 3 hours for a temperature range of 2 to 20 ° C. Its sporulation is intense for night temperatures of around 5 to 10 ° C and daytime temperatures ranging between 12 and 20 ° C. Conversely, as soon as the weather becomes milder again, the temperature rises above 20 ° C and the humidity decreases, sporulation decreases sharply. Above 25 ° C, mildew would have an increasingly reduced activity up to 30 ° C.

Lettuce having undergone stress during their growth, such as too low temperatures, low luminosities, temporary lack of water, seem to be more sensitive to this fungus. Bremia lactucae achieves a complete cycle in less than 5 days if the climatic conditions are very favorable.