Ecology, epidemiology

- Conservation, sources of viruses

The vector of  Mirafiori lettuce big vein virus (MLBVV) is Olpidium virulentus : chytridiomycete fungus, obligate parasite. This fungus is in fact a "non-cruciferous" strain of the species Olpidium virulentus sensu lato which was renamed following recent biological and molecular analyzes carried out in several species of Olpidium . This fungus ensures the conservation and the dissemination of the virus respectively by very resistant resting spores (“resting spores”) and zoospores. The resting spores can persist for many years in the soil and thus perpetuate the virus in the plots.

O. virulentus has a relatively wide host range, whether cultivated or not. Their role in maintaining the viruliferous vector fungus in the soil is not well understood. Symptomatic hosts, such as several species of Lactuca , Cichorium, and Sonchus , may increase the number of resting spores and viruliferous zoospores in the soil. The importance of this inoculum in the epidemic of this virus on lettuce is undoubtedly low if one considers the multitude of viruliferous conservation and dissemination spores formed during a culture of lettuce or chicory. The possibility of a reliable serological diagnosis of MLBVV should allow a study of the hosts of the virus.


- Transmission, dissemination

In the presence of sensitive hosts, lettuce in this case, primary infections occur via viruliferous zoospores which are released by the resting spores. These motile spores infect the epidermal cells of young roots. The fungus then remains in the roots. It forms there many zoosporangia which produce a large number of zoospores, some of which are viruliferous. Subsequently, many secondary infections take place if the soil moisture is appropriate.

O. virulentus is favored by cool soils, usually heavy and poorly drained, which remain saturated with water for several days. The expression of symptoms requires temperatures below 18 ° C.

These favorable conditions largely explain the distribution of the disease in the plots (wetlands) and over time (late autumn, winter). It should be noted that exceptional Big-vein attacks sometimes took place in April, when the temperature conditions were favorable. Finally, damage was sometimes observed in sandy soils in which permanent humidity was abnormally maintained.