CropsBiology, epidemiology
- Conservation, sources d'inoculum
In France, Plasmopara viticola can only be conserved thanks to its oospores (figure 1), or winter eggs, which form at the end of the season in the mildew tissues: young twigs, berries, but more particularly the leaves affected by mosaic mildew. ”(Figure 2). These latter, present on the ground or accumulated in certain places of the plot or in its environment, therefore indirectly ensure the survival of this chromist during the winter. In the spring, when the temperatures become milder, the eggs germinate in 2 days, or even more than a week depending on the temperature. A germinal filament appears at the end of which a piriform macroconidia forms (Figures 3 and 4). Subsequently, this structure gives rise to several dozen zoospores with 2 flagella allowing them to move in water. These ensure the primary contaminations in the spring and remain mobile for about thirty minutes to achieve it. Let us add that the sporangia can sometimes germinate and directly emit a germ tube.
Let us add that these winter eggs are able to keep for several years in wine-growing soils. Primary contaminations can take place throughout the vegetative period of the vine.
- Penetration and invasion
During spring rainy periods, the zoospores are dispersed during splashing and pollute the aerial organs of the vines closest to the ground (figure 5). Once in place on the leaves and young twigs, and in the presence of water, they lose their flagella, and emit a promycelium which penetrates in particular the blade through the stomata (numerous on the underside of the leaves), and then gains the more internal tissues. Subsequently, many mycelial hyphae develop between the cells and form suckers (or haustoria) which allow it to feed at the expense of the vine (invasion phase). The first symptoms appear about ten days after contamination, which corresponds to the incubation period.
Secondary contamination is also provided by zoospores, but this time from sporangia. These sporangia, resulting from the asexual reproduction of P. viticola , are formed on sporangiophores having the shape of arbuscules as we will study later. They harbor a smaller number of zoospores which they release at maturity in the presence of water. The same parasitic process as before takes place thereafter, but the duration of the incubation period may be shorter, and last only 4 to 5 days under particularly favorable conditions. Remember that the oospores will develop in the tissues of diseased leaves at the end of the season and ensure the preservation of the fungus during the winter.
- Sporulation and dissemination
On the majority of affected organs, P. viticola form arbuscules emerging through the stomata: the sporangiophores, at the end of which sporangia are produced (figure 6). These structures, when they are numerous, constitute the white felting often associated with the symptoms of late blight on the organs of the vine (Figures 7 to 9). These sporangia are mainly disseminated by the wind when the humidity of the air drops. They may be due to splashing occurring during heavy rains.
- Conditions favorable to its development
This stramenopile particularly appreciates humid climatic conditions, rainy periods, high humidity which will allow it to perform several cycles quickly. Downy mildew is indeed a polycyclic disease. These conditions promote sporangia germination and sporulation.
Temperature also influences its development. The oospores tolerate temperatures up to -20 ° C, which allows them to survive during the winter without problem. On the other hand, too low temperatures in spring seem to hamper its development. The thermal optimum of P. viticola is of the order of 25 ° C, and its range of activity is between 11 and 30 ° C.
P. viticola affects in particular young, waterlogged organs in the growth phase, in particular present on overly vigorous vines. Its epidemic development is essentially dependent on the rains. Currently all cultivated varieties of vines are more or less sensitive to mildew.
- Synoptic of the development of P. viticola (figure 10)