Tomato brown rugose fruit virus: Best practice findings from Europe and Israel

At the end of 2019, Dave Kaye (Plant Pathologist, RSK-ADAS) completed fact-finding visits to the Netherlands, Germany and Israel, to determine how growers, researchers and related industries in those countries are tackling the issue. Find out the key recommendations for managing ToBRFV from his trip below.

What are the seed houses doing?

• Although ToBRFV has not been found to be present within the seed, infections may develop from inoculum sources on the seed coat. Seed transmission is predicted to be rare
• Seed is treated (e.g. with hydrochloric acid, sodium hypochlorite and trisodium phosphate), to eliminate seed transmissible pathogens (including viruses)
• Seed is already tested for tobamoviruses including TMV and ToMV, and confirmed free of all tobamoviruses before dispatch
• Many seed producers are members of accredited bodies such as Good Seed and Plant Practices (GSPP) and follow their guidelines
• Some producers have moved production to lower risk area regions, keeping parental material isolated
• Seed production on some sites is structured to allow areas to be isolated, allowing production to continue if suspicious symptoms are found
• In-depth records, and subsamples of all seed batches, are kept for retesting, if required

Reducing risk in propagation

• Propagators and growers can request seed health certificates from seed producers for each variety grown. Check that the diagnostic tests were performed by an accredited laboratory and sufficient seeds were tested (>3,000 per batch)
• Trial varieties can pose a higher risk and should not be grown unless suitable seed certificates are available, ensuring sufficient seeds (3,000) were tested. Trial seed of unknown origin should not be used
• Visible ToBRFV symptoms are unlikely to develop until after plants have been dispatched from propagation and planted out on grower sites

Host plant resistance

No commercial tomato variety has been demonstrated to be fully resistant to ToBRFV, but some infected varieties in Israel remained asymptomatic. European sweet pepper varieties do not contain the Tm-22 gene, but instead the L4 gene. This provides some protection against ToBRFV. The L4 gene is not present in pepper varieties grown in Mexico and so the impact of ToBRFV has been catastrophic.

Varietal differences in the severity of ToBRFV expression have been seen in Europe and Israel:

• Varieties showing symptoms on leaves alone, but fruits failed to ripen
• Varieties exhibiting symptoms on the fruit, but not on leaves
• Varieties that are asymptomatic, showing no visible symptoms
• Highly susceptible varieties can be killed in as little as six weeks after first visible symptoms develop

For some varieties in Germany, symptom severity may have been confounded by poor plant vigour/health.

Resistance breeding is underway in Israel, with potentially two sources of resistance identified, including a ToBRFV-resistant rootstock to be made available in 2020.

Hygiene

• Almost all sites had limited access, with only essential visits permitted
• Some sites have installed mechanised foot and hand-cleaning machines at all site entrances. All staff and visitors, without exception, are required to pass through these when entering or exiting
• Launder clothes at high temperatures. Clothing capable of being laundered at ~95°C is being researched alongside viricidal detergents
• No tomato products are permitted to be brought onto or consumed on site
• Returning fruit crates/trays is considered one of the greatest risks of spreading ToBRFV between sites. Some sites are now using automated machines to disinfect trays, while many have switched to using single-use trays
• Some German and Dutch growers sterilise their knives overnight in Virkon S or Menno Florades. One Dutch grower uses a two-knife practice, re-sterilising each blade while the other is in use; while a German grower provides a pre-sterilised knife for use down each single row
• Many European and Israeli sites use Virkon S to clean down equipment and machinery, e.g. knives and trolleys. Virkon S is not considered food-safe in the UK and can cause phytotoxic effects on fruit and leave detectable residues
• Some sites choose to spray down trolleys using a knapsack containing Virkon S between rows, while others use a trolley cleaning machine. Other production sites monitor staff electronically through recording software, ensuring that sufficient time is spent disinfecting equipment
• There is increasing interest from growers in the use of enzyme products based on milk (e.g. Nzym Rugo) for handwashing and disinfection of other surfaces

Successful eradication of the virus in Germany shows that it is possible to eliminate ToBRFV from infected sites, as long as ToBRFV biosecurity protocols are enforced and strictly adhered to. Visit our ToBRFV: survival and disinfection page to find out recommended disinfectants for use in the UK.

The clean-down procedure recommended in Germany for response to an on-site ToBRFV infection is shown in Table 1. Note some chemical disinfectants recommended for use in Germany may not be authorised for use in the UK, and products must only be applied in an empty glasshouse, free of plant material. Appropriate personal protective equipment (PPE), should always be worn.

Table 1. Recommended procedure (Germany) for the disinfection and decontamination of ToBRFV in an empty greenhouse

This table was sourced and translated from the Tomato Brown Rugose Fruit Virus (ToBRFV) – ‘Jordan Virus’, Practical information and experiences.

Other relevant strategies for eradication (non-exhaustive) are as follows:

• ToBRFV-infected material must be incinerated in Germany but in the Netherlands, material is composted
• Growers at the infected German sites now replace their coir each year, rather than reusing it year on year. Boxes and drip pegs were also replaced after outbreaks

In Israel and other countries where plants are grown intensively in the ground, or where phytosanitary practices are poor, ToBRFV is difficult to eradicate and will continue to be an issue until resistant varieties are developed.

The aim in Israel is to delay symptoms rather than to avoid development of ToBRFV. Some relevant strategies are summarised below:

• Plant stress has been implicated as a major trigger of symptom expression. E.g. in Israel symptoms are most severe at the height of midsummer (heat stress), while symptoms are greatest in Turkey when crops are exposed to very cold temperatures (~4°C)
• In Israel, dependent on the variety, early plant health and the time of year, growers can continue to produce a degree of marketable fruit despite infection. The increased use of grafted plants allows splitting of plants into two heads, maximising marketable yields in the two crop cycles
• Mixed infections of ToBRFV and mild strain PepMV do not appear to be more severe than plants infected with ToBRFV alone. The impact of mixed infections with ToBRFV and wild strains of PepMV is unknown. Mild strain ‘vaccines’ are in development, and may provide cross-protection to ToBRFV, but these are several years away from being registered for use

See our growers’ checklist for more information on the ‘best practice’ procedures identified from the two study tours.