Eight years after its discovery, tomato growers are still frustrated with a resilient virus that so far has proven difficult to manage. The rapid spread of tomato brown rugose fruit virus (ToBRFV) across the world has caused devastation in tomato greenhouses from Canada to China.
First observed in tomato plants in Israel in 2014, ToBRFV was formally reported as a new viral disease by researchers in Jordan in 2016. European observations followed shortly afterwards in several countries. By fall of 2018, the virus was reported in North America at a California greenhouse facility. It is likely ToBRFV was present soon afterwards in Canada with the first reports in 2019. The virus has now been officially recorded in more than 20 countries globally, with researchers suspecting its distribution is far larger than records indicate.
The symptoms of ToBRFV range widely from different growing regions, growing systems, and tomato varieties around the world. The virus may be hardly noticeable on foliage while it has also been reported to kill plants outright. It is the impact on the fruit however that causes the most economic damage. Tomato fruit from infected plants are often blotchy, pale, ripen unevenly, malformed, and can be undersized. Up to 100 per cent of fruits from infected plants can be rendered unmarketable.
Greenhouse tomato crops are most affected by ToBRFV. It has not been an issue with field-grown tomatoes in Canada, possibly due to their much shorter growing cycle and reduced human contact.
Categorized as a type of tobamovirus, ToBRFV is related to other commercially important viruses such as the tomato mosaic virus (ToMV). Tomato growers have had a management tool for genetic resistance to these previous tobamoviruses such as ToMV in the form of the Tm-22 gene. Varieties that incorporated this gene have had effective protection from these viruses for more than 50 years. But it fails to protect from ToBRFV.
Adding to the difficulty of managing ToBRFV, researchers have discovered that the virus is exceptionally stable in the environment. While viruses are considered obligate parasites and need a live host to reproduce, trials have found ToBRFV can survive months or even years on plastic, metal, or concrete surfaces and still remain infectious. This stability is a key factor in why managing this virus has proved so difficult.
Transmission of ToBRFV can include seed which is a key first pathway to infection of a new crop. Researchers have evaluated various seed treating methods with some good results, however, seed washes may not be completely perfect. Some commercial greenhouses now invest significantly in their own seed testing of new seed lots prior to propagation to detect any presence of the virus. It may only take one infected plant to contaminate an entire crop so avoiding infection in the first place is the best outcome.
Once ToBRFV is present in a greenhouse, it is primarily spread through mechanical contact by employees from their activities on each plant such as training, pruning and harvesting. This may be upwards of 150 contact events per plant each growing cycle. In an infected greenhouse, researchers have demonstrated ToBRFV can be found everywhere. From the roof, walls, ceiling, floor to employee clothing, door handles, and break rooms. Virus has been recovered from computer keyboards and mice to the washrooms.
Sanitation is critical to avoid further movement of the virus in the greenhouse. Researchers have evaluated numerous disinfectants and there are effective solutions being implemented. Fully eradicating every virus particle from an infected greenhouse remains elusive. Despite some growers’ best efforts to sanitize everything, re-infection has occurred.
One method of dealing with ToBRFV is changing crops. Greenhouses that previously grew tomatoes have been switched to cucumbers, peppers, strawberries, lettuce, eggplant, or cannabis in an effort to avoid severe crop losses. Customers will still be looking for tomatoes however, so avoiding the crop all together is not a permanent fix. With complete eradication of the virus challenging, growers are going to need tools to be able to cope with ToBRFV potentially being in the environment.
More permanent solutions are in development. Private and public breeders have been tirelessly screening domesticated and wild plant material for ToBRFV resistance for several years. Fortunately, resistant lines have been discovered. Major commercial seed companies are already or will be shortly offering ToBRFV-resistant tomatoes in their catalogues. However, the genes conferring resistance to this virus are not well understood and will take further years to investigate. Incorporating ToBRFV resistance into all popular greenhouse tomatoes will take yet further breeding efforts. It is also unclear how the fruit of resistant varieties – which may potentially still host small quantities of the virus without symptoms – will be accepted by phytosanitary screening standards in international trade.
Work is also underway to develop a ToBRFV-specific vaccine. Plant vaccines already exist for pepino mosaic virus in tomato where a mild form of the virus is used to infect young tomato plants. Vaccinated plants then have protection against more severe forms of the virus that plants may be exposed to. A similar strategy is under development for ToBRFV but this is likely some years away from receiving regulatory approval.
There is no single silver bullet available right now to address ToBRFV. The best advice from experts in the field is an integrated approach that layers several reasonably effective measures including clean seed and sanitation with future genetic resistance tools and potentially a virus-specific vaccine. The understanding of ToBRFV continues to evolve rapidly but further losses are likely until these comprehensive tools are available.