Phytophthora infestans

Solanaceae downy mildew  

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General

• Oomycete rife with severity in many production areas of the world, particularly formidable and destructive in regions with a humid climate (rain, sprinkler irrigation, fog, dew, etc.).

• Occurring at altitudes above 800 m in tropical zones, and in the plains during humid and less hot periods.

• Several strains are known belonging to two complementary compatibility groups (A1 and A2) allowing its sexual reproduction.

• Observed in the open field as well as under shelter.

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• Microorganism with a rather narrow host range and limited to Solanaceae.

• Susceptible botanical family(s)

Biology

• Preservation : probably in several forms:
  • oospores (Figure 1) from sexual reproduction forming in altered tissues;
  • as mycelium inside diseased potato tubers;
  • on various other alternative hosts, surrounding cultivated or wild plants (eggplant, red pepper, potato, pepino, black nightshade, Solanum incanum, S. indicum …, Datura stramonium, D. metel , several species of Hypomea, Lycium hamilifolium , Nicotiana glauca, Petunia sp., and Physalis angulata, etc.).
• Infection : Penetration into the limbus via the stomata, sometimes directly through the cuticle and epidermal cells. Infection in 3 to 4 hours. Invasion of leaf tissues by its non-partitioned mycelium. The first spots appear between 4 and 7 days after the first contaminations.
• Sporulation : through the stomata, sometimes directly through the epidermis (figure 2). Production of numerous sporangiophores (figures 3 to 6) and citriform sporangia (figures 7 and 8), several thousand per spot.
• Dissemination : by wind and rain, sometimes over long distances (several hundred meters). It is sometimes carried out through contaminated plants, or even seeds in rare situations.
• Favorable conditions : extremely influenced by climatic conditions, developing more or less well at temperatures between 3 and more than 25°C. High relative humidity, above 90%, is essential for its development. Cold nights and moderately warm days, with high humidity, favor its extension. Rainy periods, sprinkler irrigation, dew, are also very conducive to mildew epidemics.

Protection

• If possible, use varieties expressing resistance partial This plant material should be used in conjunction with other methods of protection, in particular with complementary chemical control. Under no circumstances should mixed crops of resistant varieties and susceptible varieties be grown and the proximity of susceptible varieties should be avoided.
• of Crop rotations at least 3 years are recommended.
• If possible, orient the planting mounds and/or the rows in the direction of the prevailing winds in order to favor the ventilation of the vegetation.
• Eliminate sensitive weeds from the plots and their environment.
• Use healthy plants , check their sanitary quality on arrival.
• Ensure good drainage of cultivated plots.
• Consider inter-row production of non-host crop species.
• Protect the plants from the weather by covering them with plastic.
• Do not plant near already affected tomato crops.
• Avoid too high planting densities in order to promote aeration of the foliage, and excessive manuring .
• 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.
• Strip the lower parts of the plants in order to eliminate the first affected leaves and improve the ventilation of the plant cover.
• Remove attacked leaves at regular intervals.
• 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 should be destroyed quickly or buried deep in the ground to promote their rapid decomposition.
• 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.

* Chemical families particularly affected by resistance phenomena: Anilides, Strobilurins.
Note that for multisite fungicides to be relatively effective, they must be applied preventively and every week, especially in the field. Despite their limited effectiveness over time, they still have the advantage of being quite versatile and not being affected by resistance phenomena.
This is not the case for certain single-site fungicides (such as those of the anilide family) which are rarely used alone, often associated with each other and/or with multi-site fungicides in order to limit the risks of the appearance of resistance. It should be remembered that curative treatments, with their relative effectiveness, further favor the appearance of strains resistant to fungicides. In addition, we expressly invite you to alternate fungicides with different modes of action. With certain products, it will not be necessary to carry out more than 2 to 3 applications per campaign, and not to intervene on declared attacks.