Biology, epidemiology

• Conservation, source d'inoculum

The methods of conservation from one season to another of Pseudoperonospora cubensis are still poorly understood, especially in the more northern production areas. It produces oospores (from its sexual reproduction) which could sustain it in the soil and on infected plant debris, but these conservation spores were rarely observed in nature and did not seem to play an important role in the life cycle of the stramenopil. . Indeed, they have only been observed very occasionally in the world, reported in China, Japan, India, Russia and Italy, Austria, Bulgaria. Their germination had never been observed. The role attributed to oospores has evolved in recent years *.

The sporangia have a limited implication in the conservation of downy mildew of Cucurbitaceae. For example, they would only last for 14 days in soil containing 30% water. It should be noted that primary contaminations most certainly take place via the sporangia carried by the wind over long distances from already affected areas. This is the case, for example, in central Europe where each year the source of inoculum comes from south-eastern Europe and remains there during the winter on various living Cucurbitaceae.

In hot production areas, this chromist can persist on various cultivated or spontaneous Cucurbitaceae (around sixty species belonging to around twenty genera), and therefore be present all year round. In more temperate zones, Cucurbitaceae grown out of season can also play this role.

* Indeed, the pathogenic power of P. cubensis on Cucurbitaceae seems to have evolved. For example, in Czechoslovakia, new species of Cucurbitaceae have been shown to be sensitive to this stramenopile in recent years. Known mainly on cucumber, it has appeared since the 2010s on melon, watermelon and musk squash ( Cucurbita moschata ). Then later the zucchini, and the species Cucurbita maxima , Cucurbita ficifolia , and Lagenaria siceraria were also affected.

At least 7 pathotypes have been reported from a differential host range consisting of several Cucurbits (Figure 1). Depending on the country, the range of species showing symptoms of late blight varies according to the strains prevalent, and their virulence. Note that pathotype 5, capable of attacking species of the genera Cucumis , Cucurbita and Citrulus , was already described in one of our neighbors: Italy in 2002.

This microorganism is heterothalic , two sexual types complementary (mating type) have been demonstrated: type A1 and type A2 . The second was discovered in Israel. Recent studies carried out in this country have made it possible to demonstrate certain affinities of Type A1 for species of the genus Cucumis , and of type A2 for those of the genus Cucurbita .

The presence in France of downy mildew on zucchini could materialize the introduction and extension of type A2, with the risk that the production of oogonia is now more frequent and has a significant impact on the conservation of this stramenopile and on the early in its epidemics.

The oospores are spherical , hyaline to reddish brown, and measure 40 microns in diameter. They are formed in large quantities on cucumber and melon, more rarely and in low numbers on zucchini, pumpkin, watermelon, and not at all on butternut squash.

• Penetration, invasion

Present on the leaf blade and in the presence of free water, the sporangia release flagellated zoospores or germinate directly by emitting a germ tube. Zoospores, once encysted, also produce a germ tube which allows them to enter the leaves through the stomata. In place, this obligate parasitic chromist colonizes the leaf tissues of the mesophyll and palisade, and its non-partitioned mycelium emits haustoria inside the cells which allow it to draw the elements necessary for its survival.

The incubation period can last from 4 to 12 days depending on the leniency of the climatic conditions.

• Sporulation and dissemination

P. cubensis sporulates especially on the underside of the leaves (figure 2) (more rarely on the upper side), especially if the ambient humidity is very high (close to 100% for at least 6 hours) and if the temperatures are included between 5 and 30 ° C (with an optimum between 15-25 ° C). It produces arbuscular sporangiophores (Figures 3 and 4) exiting through the stomata, at the end of which sporangia form (Figure 5); they will subsequently constitute the secondary inoculum.

It should be noted that the sporangia are very easily disseminated by wind and air currents over fairly large distances, of the order of several hundred meters, as well as by splashing water, runoff following a rain or sprinkler irrigation. A hot and humid wind can allow the generalization of mildew to a production area, or even a region, thanks to successive transplanting from a plot or an outbreak production area.

It should be noted that workers during cultivation operations and the tools they use also contribute to the spread of the disease.

• Conditions favorable to its development

Like many mildews, it particularly appreciates high humidity occurring in periods of fog, dew, rain and sprinkler irrigation. The presence of free water on the leaves is essential for infection which takes place for example in 2 hours if the temperature is between 20 and 25 ° C. It can occur for temperatures between 8 and 27 ° C, the optimum being between 18 and 23 ° C. This chromist tolerates high temperatures well; several days at 37 ° C do not affect its viability, the cooler night temperatures allow it to survive. These conditions would be the most favorable for the development of downy mildew.

Its cycle is relatively short since the first conidiophores appear 3 to 4 days after infection.

It should be noted that the best conditions for easily observing the fructifications of late blight are found fairly early in the morning, at a period when the ambient humidity is high and when the sporangia have not yet been disseminated.