If you could name one virus that has dominated headlines in the horticulture industry, apart from the obvious choice of course, chances are ToBRFV will come to mind. The tomato virus has been plaguing growers around the world, and slowly but surely the industry is coming to grips with it. Breeders like Bayer and Enza Zaden are reporting breakthroughs, and hygiene protocols are more important than ever.
During this year's British Tomato Conference, ToBRFV was a major topic as well. Adrian Fox (Fera Science Ltd), Bart van de Vossenberg (NVWA), and Dave Kaye (ADAS RSK) gave presentations during the event to share the latest insights on the virus with British growers.
Steering group
In his introduction, Philip Pearson (Chairman of the TGA Technical committee), first pointed to the AHDB knowledge library, which has a lot of information on ToBRFV. Then there's also the UK ToBRFV steering group, formed to help manage the virus from a UK perspective and consisting of commercial growers, AHDB staff, independent researchers and representatives from Defra Plant Health and Fera. All which is to say, the British are well-prepared to deal with the tomato virus.
Dealing with ToBRFV
One of the people at Fera, Adrian Fox, started his presentation with a quick refresher: ToBRFV is a tobamovirus, but unlike those, it can overcome TMV resistance genes, and it can be cured. Something that further complicates matters is that the virus can be transferred even through clothing, and it spreads rapidly within the crop. As Adrian explains, it's possible to have 100% crop infection within one season.
Due to the variety of ways the virus can spread, a holistic approach is needed to contain the virus. EU measures, which came into force August 15 through Regulation 2020/1191, are aimed at achieving exactly this. Needless to say, this is one of the areas where Britain is looking to align its regulations following the country's exit from the EU.
While we already know a thing or two about how the virus spreads, there's still much room for research. For instance, the AHDB is looking into the survival rate of the virus on skin and gloves, and on glasshouse surfaces, as part of its project PE 033.
From the interim results, the advice has been given to use disposable gloves and changes these often, instead of just handwashing - even washing for 60 seconds doesn't always offer an iron-clad guarantee of removing the virus, and asking employees to wash hands for that long is, well, unrealistic.
The virus has also shown remarkable resilience when it comes to surviving on all sorts of glasshouse surfaces. Soaking in hot water, in combination with Virkon disinfectant, does appear to work, but soaking for five minutes in just water at 70 degree Celsius doesn't do the trick - showing just how tough this virus really is.
Real-time tracking of ToBRFV outbreaks
The next speaker, Bart van de Vossenberg, is a molecular biologist with the Netherlands food and consumer product safety authority (NVWA). He shared some information about the ToBRFV situation there, and how the Dutch are dealing with it.
On October 7, 2019, ToBRFV was suspected after a sample was taken in a regular annual survey, which was confirmed on October 17. In a follow-up survey in November 2019, 68 companies were visited and samples were taken, which were tested with real-time RT-PCR. By the end of November, a presence was found at 17 companies, and the current situation (on September 24, the day of the British Tomato Conference) is that there are 20 companies in the Netherlands with a confirmed presence of the virus.
Using Illumina sequencing, Dutch researchers have been able to gain access to the entire genetic info of the virus. "We can use that in tracking and tracing outbreaks, finding out where it comes from," Bart explains. "We can use this information because each time a virus replicates, small changes can occur. Virus genomes with a common ancestor are more alike, they group in a cluster and have a shared (original) origin." Extrapolating from this, it could very well be that some clusters are linked to associated information, with some strains of the virus possibly originating from the same seed batch.
In order to share this information with growers, but also to communicate and disseminate that information more broadly, the NVWA created two web tools - one for internal use and one for public use. This Nextstrain tool, which is available here, also happens to be used for tracking COVID-19 outbreaks. "We're applying the same strategy and hypothesis, but then for ToBRFV."
Concluding, Bart notes that in the Netherlands, ToBRFV genomes were found from three separate clusters, which are believed to represent three different original sources. The data also show that the virus was likely present in the country before 2019, possibly even as early as 2017. The research team have compiled their findings in a paper, which was accepted for publication.
Learning from UK grower ToBRFV experiences
The last speaker, plant pathologist Dave Kaye (ADAS RSK), has certainly been around, having visited the Netherlands, Germany and Israel among others to find out more about the devastating tomato virus. He's carried out an AHDB commissioned desk-based study on outbreak and management in the UK, in which three business were researched.
At site 1, the first outbreak, there were several houses on on site. There was limited information available on the virus when the grower spotted symptoms on site in young plants. Site 2 was in the process of building an expansion to the established parent company - they had to introduce young plants before construction was fully completed due to the planning having slipped. At first, the grower believed the symptoms to be related to the use of LED grow lights. At site 3, the outbreak too place in an old 1.2 ha house, and it came to light when a pipe burst, which triggered very severe ToBRFV symptoms.
So what can we learn from all this? In terms of origins, at site 1 the virus could have come in from propagation, but that's not that clear-cut - it could also have been visitors or contractors. Similarly, in site 2, it could have been the construction contractors. The point is, ToBRFV introductions continue to occur, which is a weak point shared by all businesses. One of the recommendations Dave gives is to have young plants tested before arrival.
Another thing for growers to keep in mind is that plant stress is repeatedly named as a trigger for more severe symptoms. One of those triggers could be simply switching on the lights, as was the case at site 2. Finally, Dave stresses the importance to remain vigilant. "Don't take your foot off the pedal now! Keep your eyes peeled and keep looking."
For more on this presentation, or if you just want to relive the British Tomato Conference in its entirety, you can watch it all on YouTube. Be sure not to miss the very entertaining presentation by Professor David Hughes, aka Dr. Food, which starts at about 5 hours in.