Ecology, epidemiology

- Conservation, sources of viruses

Knowledge about the life cycle of the agent ring necrosis lettuce ( Lettuce ring necrosis agent , LRNA) are pretty basic. The transmission of this disease by soil could be established quickly at first. Subsequently, the involvement of Olpidium virulentus (ex Olpidium brassicae ) as a vector was demonstrated during experimental inoculations via zoospores. The frequency of transmission of LRNA during these experiments was very low as if this virus was lost by the zoospores of the fungus during the cycles and successive transfers from plant to plant. The LRNA vector is therefore Olpidium virulentus (ex Olpidium brassicae ), obligate parasitic chytridiomycete fungus. This fungus ensures the conservation and the dissemination of the virus respectively by very resistant resting spores (“resting spores”) and zoospores. As with MLBVV, the virus is “carried” internally in the resting spores of the fungus. It is therefore likely that LRNA can remain infectious in these storage spores for many years.

It should be remembered that Olpidium virulentus (ex Olpidium brassicae ) has a relatively wide range of hosts, cultivated or not. It is not currently known what influence they may have on the conservation of LRNA in a given plot.


- Transmission, dissemination

As in the case of the Big-vein, one can think that the primary infections occur on lettuce via viruliferous zoospores which are released by the resting spores present in the soil. These zoospores are motile and 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. Thus, secondary infections can occur if the soil moisture is adequate.

Olpidium virulentus (ex Olpidium brassicae ) is favored by cool and usually heavy soils, poorly drained, which remain saturated with water for several days. The expression of symptoms requires temperatures below 18 ° C. These favorable conditions are comparable to those required for the expression of the symptoms of Big-vein; it is therefore not surprising to find the two diseases in the same plots at the same times of the year.

In the Netherlands, it has been shown that drainage water or water collected at very shallow depths could be responsible for contamination and a significant spread of the disease.