Pseudoperonospora cubensis

Downy mildew of Cucurbitaceae 

General

• oomycetes , especially in tropical and subtropical areas. Several races are currently present in the field, affecting more or less wide ranges of Cucurbitaceae species.
• Very specific organism of Cucurbitaceae, affecting for example: cucumber, melon, various Cucurbita spp . , watermelon, but also on other Cucurbitaceae cultivated or not, at least forty species. Depending on the country and the strains, the range of species affected varies.
• Observed in the open field as well as under shelter.

Biology

• Conservation : still poorly known from one season to another in certain production areas: low persistence of sporangia, uncertain role of oogonia. In hot production areas, this chromist can persist on various cultivated or spontaneous Cucurbitaceae (about sixty species belonging to about twenty genera), and therefore be present all year round.
• Infection : sporangia germinate or release zoospores in the presence of water. penetration then takes place through the sporangia. Its mycelium penetrates the leaves via the stomata and invades the mesophyllous and palisade tissues.
• Sporulation : a few hours, especially in the morning (figure 1), and production of sporangiophores (figures 2 to 4) and a very large number of sporangia, mainly on the underside of the leaves.
• Dissemination : the sporangia (figures 5 and 6) are dispersed by the wind, air currents in shelters, splashing water, tools, workers' clothing.
• Favorable conditions : the higher the humidity, the greater the sporulation. Particularly fond of temperatures of around 18  to 23°C and humid environments. Relatively short cycle, sporulation visible 3 to 4 days after infection.

Protection

• The different Cucurbitaceae cultivated do not show the same sensitivity to downy mildew. Sources of resistance can be exploited and worked on in certain species.
• Use healthy plants .
• Ensure good drainage of cultivated plots.
• Do not plant near Cucurbitaceae crops already affected.
• Avoid too high planting densities in order to promote aeration of the foliage, and excessive manuring .
• Strip the lower parts of the plants in order to eliminate the first affected leaves and improve the ventilation of the plant cover.
• Avoid irrigation , prefer drip irrigation. If they are essential, carry them out in the morning so that the vegetation drains quickly during the day.
• Under cover, ventilate as much as possible.
• Do not allow workers to work while vegetation is wet.
• Eliminate plant residues , during cultivation following the various cultivation operations, and at the end of cultivation after uprooting the plants. They must be destroyed quickly.
• If necessary, spray fungicides taking into account authorized uses ( e-phy ). Strains resistant to several fungicides have been reported in the literature. Therefore, alternate active ingredients with different modes of action.