Leveillula taurica (Lév.) G. Arnaud

Internal powdery mildew

General

• A fungus rather subservient to hot and dry areas, tropical to subtropical, it is still widely distributed throughout the world due to fairly wide thermal and water requirements (Korée,).
• Under the name Leveillula taurica is in fact hiding a complex of species attacking a very large number of cultivated plants or weeds: more than 1000 species belonging to 74 botanical families. On vegetables and in tropical areas, it mainly affects Solanaceae, very rarely cucumber.
• Observed in the open field as well as under shelters.
• Organs attacked : leaves.
• Symptoms :
  • Pale green spots appearing on rather low leaves. Rounded to angular when their contours are delimited by the veins, they gradually turn yellow.
  • Chlorosis of the affected tissues which present brown micro-alterations before completely necrosis (figures 1 to 4 - chilli) (figures 8 to 10 - tomato).
  • Possible confluence of spots causing complete yellowing of the blade and death of leaflets and leaves, which however do not fall.
  • No symptoms visible on other plant organs.
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• Signs  : presence of a more or less discrete down visible on the lesions on the underside of the blade (figures 6 and 7 - chili pepper) (figures 11 and 12 - tomato) consisting of elongated conidiophores bearing one or a few lanceolate conidia (figures 13 to 16). The fungus can also sporulate on the leaf blade when humid climatic conditions permit.
• Possible confusion :

Biology

• Conservation : is maintained thanks to its mycelium on living hosts, cultivated plants (pepper, eggplant, artichoke, cucumber, onion, garlic, leek, cotton, etc.) and weeds ( Sonchus asper, Physalis spp., Chenopodium ambrosioides, Oxalis cernua, Urtica urens ). Its cleistothecia resulting from sexual reproduction are rare and therefore have little influence on the parasitic cycle of the fungus.
• Infection : germination of conidia at relative humidity greater than 40%, and direct penetration of the cuticle or through the stomata. Intercellular colonization of the leaf mesophyll.
• Sporulation : after about twenty days conidiophores appear at the level of the stomata on which elongated and pointed conidia are formed.
• Favorable conditions : appreciates temperatures around 26 ° C and humidity of 70-80%; infections can occur at temperatures between 10 and 33 ° C in the presence of more or less high humidity. The combination of hot, dry days with cool, humid nights promotes disease, as does the presence of dew on the foliage.

Protection

• Sources of resistance have been identified but are not yet available in eggplant varieties.
• Carry out fairly long crop rotations.
• Provide balanced manure to the plants.
• Use healthy plants.
• Carefully choose the location of the future plot so that it is located in a fairly ventilated and sunny place. Avoid proximity to plots already affected.
• Eliminate the weeds from the plot and its surroundings, which can serve as relay plants for the parasitic fungus.
• Create a crawl space under shelters as well as washing or even disinfection of surfaces.
• Avoid too high planting densities in order to promote aeration and sunlight of the foliage.
• Do not plant near already affected tomato or pepper plots.
• Eliminate the first diseased leaves and plant residues fairly quickly during cultivation following the various cultivation operations, and at the end of cultivation after the plants have been uprooted. They will have to be destroyed or buried deep.
• Remove leaves from the lower parts of plants in order to eliminate the first attacked leaves and to promote their ventilation and sunshine.
• Ventilate shelters.
• Monitor the health of the plants in order to detect the very first symptoms.
• If necessary, spray fungicides taking into account the authorized uses.