Leveillula taurica (Lév.) G. Arnaud 

Internal powdery mildew

General

• Fungi rather restricted to hot and dry areas, tropical to subtropical, it is nevertheless widely distributed in the world due to fairly wide thermal and water requirements.
• Under the name Leveillula taurica hides in fact 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 shelter.

• Susceptible botanical family(s)

• Production areas affected :

• Organs attacked

Symptoms and signs  

• 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 show brown micro-alterations before becoming totally necrotic (figures 1 to 4 - pepper) (figures 8 to 10 - tomato).
  • Possible confluence of spots leading to complete yellowing of the blade and death of leaflets and leaves, which however do not fall.
  • No visible symptoms on other plant organs.

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• Signs : presence of a more or less discreet down visible on the lesions on the underside of the blade (figures 6 and 7 - pepper) (figures 11 and 12 - tomato) consisting of elongated conidiophores bearing one to a few lanceolate conidia (figures 13 to 16). The fungus can also sporulate on the lamina when the humid climatic conditions allow it.
• Possible confusion : cladosporiose on tomato.

Biology

• Conservation : maintained thanks to its mycelium on living hosts, cultivated plants (peppers, eggplants, artichokes, cucumbers, onions, garlic, leeks, cotton plants, 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 hygrometry above 40%, and direct penetration of the cuticle or via the stomata. Intercellular colonization of leaf mesophyll.
• Sporulation : after about twenty days conidiophores appear at the level of the stomata on which are formed elongated and pointed conidia.
• Favorable conditions : appreciates temperatures around 26°C and hygrometry of 70-80%; infections can take place at temperatures between 10 and 33°C in the presence of more or less high hygrometry. The combination of hot, dry days with cool, humid nights favors the disease, as well as the presence of dew on the foliage.
  
  

Protection

• Use resistant varieties .
• Carry out crop rotations fairly long
• Provide balanced fertilizer to plants.
• Use healthy plants.
• Carefully choose the location of the future plot so that it is located in a fairly airy and sunny place. Avoid proximity to plots already affected.
• Eliminate weeds from the plot and its surroundings, as these can serve as relay plants for the parasitic fungus.
• Carry out a crawl space under shelter as well as a washing, or even a disinfection of the surfaces.
• Avoid too high planting densities in order to promote ventilation and sunshine of the foliage.
• Eliminate the first diseased leaves and plant residues fairly quickly, during cultivation following the various cultivation operations, and at the end of cultivation after uprooting the plants. They will have to be destroyed or buried deep.
• Strip the lower parts of the plants in order to eliminate the first attacked leaves and to favor their aeration and sunshine.
• Ventilate shelters.
• Monitor plant health to detect early symptoms.
• If necessary, spray fungicides taking into account authorized uses.