- classification : Pospiviroid, Pospiviroidae


Symptoms

The plants show a stunted appearance, chlorotic leaves and a reduced size, with necrosis of the nerves and petioles (Figures 1 to 3).


Transmission mode

We do not currently have information on the possible transmission of this viroid by tomato seeds.


Other characteristics

This viroid has been observed in Canada in greenhouse tomato plants, and more recently in the Netherlands in plants from the United States. It therefore appears to be predominantly North American. Note that this viroid has been spotted at least once in Brittany.

Experimental transmissions to other hosts have made it possible to demonstrate the sensitivity of a certain number of other Solanaceae (potato, Nicandra physaloides ...) but it has not, until now, been found in natural infection than in tomatoes.

Possible protection methods

The protection methods put in place to control epidemics linked to viroids affecting tomatoes vary according to their nature and therefore their biological properties. They can be subdivided into 3 categories:

- general measures , applicable to all viroid diseases, which consist of eliminating infected plants, and implementing prophylactic measures intended to eradicate the epidemic, to prevent its spread in the plot and / or its spread to new cultures. We recommend that you consult the protection methods described for Tomato Mosaic Virus (ToMV), where many of these measures are described. It should be noted that for several viroids (PSTVd, CEVd, TASVd, CSVd ), recent work has made it possible to highlight the existence of essentially asymptomatic infections in ornamental species, which could possibly play a role of reservoir. for transmission to tomato crops. It is therefore necessary to be vigilant as to the possible presence of such neighboring crops;

- measures specific to viroids potentially transmissible by seeds , as is the case in particular with TASVd, and undoubtedly with CLVd. Particular attention should be paid to the quality of seeds in production areas where these viroids have been reported, especially if these are located in tropical environments. Depending on the evolution of the incidence of the disease, it will certainly be necessary to develop and apply a detection method to control the quality of the seeds;

- the measurements used for viroids propagated by vegetative means , mainly PSTVd. For this viroid, the epidemic cycle can only be prevented by the use of free seeds, which generally requires the establishment of a certification system for seed potatoes, and can only be broken by procedures. heavy eradication. Tomato production is currently only indirectly affected. However, care should be taken not to grow potatoes near tomato plots, or even, if possible, not to ensure these two crops on the same farm.

In general, efforts by breeders to identify sources of resistance to viroids have so far been very limited. On the other hand, intensive screening (at least for PSTVd) of genetic resources has not made it possible to demonstrate such resistance.

On the other hand, thanks to recent advances in plant biotechnology, transgenic plants exhibiting lower sensitivities have been obtained, or even, it seems, resistance to PSTVd, the model viroid on which these studies were carried out.

The two approaches which seem the most promising appear to be the construction of plants expressing viroid sequences in antisense orientation, or plants expressing a yeast RNAse specific for double-stranded RNAs, Pac1 nuclease. These plants are still only at the experimental stage, but the first data obtained demonstrate the feasibility of these new approaches to fight against viroids. However, it remains to take into account the acceptability of such resistant transgenic plants by the consumer.