- classification : Pospiviroid, Pospiviroidae
- synonym: Indian bunchy top viroid (I-TBTVd, Indian bunchy top tomato viroid )


Symptoms

In South Africa the plants show strong stunting and leaf epinasty. In India, in addition to the previous symptoms, leaf distortions and necrosis of the nerves have been observed.


Transmission mode

This viroid has only rarely been observed on tomato; its mode (s) of transmission have not been the subject of specific studies. Like the other viroids, it is probably transmitted mechanically from plant to plant during cultural operations.


Other characteristics

Tomato bunchy top disease was first described in South Africa in the early 1930s, although the causative agent could not be characterized at the time. In the early 1980s, the disease was observed in the state of Maharashtra in India. Characterization of the viroid involved revealed that in this case it was a strain of CEVd. It has since been spotted several times on tomato in the Netherlands. CEVd can naturally infect citrus fruits and grapes and has recently been detected in ornamental plants, such as verbena, which could potentially serve as a reservoir for transmission to 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 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.