- classification : Pospiviroid, Pospiviroidae
This viroid is considered to be closely related, if not identical, to the Tomato bunchy top viroid .


Symptoms

The plants are stunted and their leaflets undergo pronounced epinasty, accompanied by necrosis of the nerves.


Transmission mode

PSTVd ​​is transmitted by vegetative propagation, plant-to-plant leaf contact and agricultural implements. Pollen and seeds are likely to harbor and transmit it. Experimental data show that it could also be transmitted by aphids, notably Myzus persicae , presumably with low efficiency, after heteroencapsidation in particles of the potato leaf roll virus (PLRV) in case of mixed infection.


Other characteristics

Described in 1922 by Martin, spindle-shaped potato tuber disease was first identified in the state of New Jersey. It then spread widely to the potato-growing regions of the northern and northeastern United States, then to Canada and Eastern Europe. It has probably been introduced occasionally in some countries of Western Europe but seems to have been eradicated. This virus was detected on tomatoes, in particular in South Africa and New Zealand in 2001 (with losses of up to 60%), and in 2002 in the Netherlands.

In general, PSTVd ​​appears to be predominantly subservient to the Solanaceae family, although it is capable of infecting species in other botanical families and has recently been found in very different hosts such as avocado. Several recent studies seem to highlight the frequent infection by PSTVd ​​of an ornamental species Solanum jasminoides , whose role as a possible reservoir remains to be clarified.

PSTVd ​​has occasionally been observed in tomatoes, in a greenhouse crop located in the south-east of the United Kingdom and in tomatoes originating in the Netherlands and New Zealand. It should be noted that the tomato is today used worldwide as an experimental host for PSTVd.


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 in eliminating infected plants, and in 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.