Biology, epidemiology

• Conservation, sources d'inoculum
  

As previously suggested, Leveillula taurica is most prevalent in the summer. This obligate parasitic fungus is able to maintain itself in its mycelial form on living hosts: cultivated plants (pepper, eggplant, artichoke, cucumber, several Allium spp. [Onion, garlic, leek], cotton plant ) and weeds ( Sonchus asper) , Physalis spp., Chenopodium ambrosioides, Oxalis cernua, Urtica urens ) which contribute to its conservation and multiplication. It should be noted that over 1000 different hosts have been listed. This information should be put into perspective if, as we suggested previously, L. taurica is in fact a complex of species. For the same reason, some of the potential hosts described have only limited indicative value.

This fungus has a perfect shape materialized by cleistothecia measuring 140 to 250 µm in diameter. They are very rarely seen in nature; their role in the parasitic cycle of the fungus therefore appears rather reduced.

• Penetration and invasion

The conidia of the fungus polluting the limbus germinate relatively well at relative humidity above 40% . Subsequently, the mycelial hyphae , after entering the limbus directly through the cuticle or via the stomata , invade the interior of the leaf tissues by progressing between the cells of the mesophyll. The fungus thus protected is preserved from desiccation, ultraviolet rays and leaching. It is also less exposed to fungicide treatments.

• Sporulation and dissemination

After about 20 days , long conidiophores emerge from the stomata on the underside of the leaf blade and produce conidia terminal spearhead (Figures 1 and 2). These spores will ensure the dissemination of L. taurica , often via wind and drafts in the shelters. Workers can also disperse them during cultivation operations. In some countries, infected plants are believed to contribute to the introduction and spread of this disease.

• Conditions favorable to its development

Of temperatures adjacent to 26 ° C and 70-80% humidity levels are favorable for its extension, even though infections may still occur at temperatures between 10 and 33 ° C in the presence of more or less high humidities . The combination of hot, dry days with cool, humid nights promotes disease. The presence of dew on the foliage plays an aggravating role.