Best-practice for Tomato Brown Rugose Fruit Virus: findings from Europe and Israel

ToBRFV has been confirmed in 12 countries (December 2019) although it has been eradicated from the UK, Germany and the USA. Dave Kaye (Plant Pathologist, RSK-ADAS) recently 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. The full report from this trip will be available in the New Year.

Please note, the information contained in this document is not provided as a list of recommendations from AHDB or RSK-ADAS but rather provides an overview of efforts to minimise ToBRFV spread under different production systems and climatic conditions. Before using any product or protocol you must check legality and compliance with current Health and Safety law and legislation.

Ensuring a clean start

What the seed houses are 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 very rare.
  • Seed is treated (e.g. 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 sub-samples of all seed batches, are kept for re-testing if required.

Reducing risk in propagation

  • Propagators & 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 (>3000 per batch).
  • Trial varieties can pose a higher risk and should not be grown unless suitable seed certificates are available, ensuring sufficient seeds (3000) 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.

Keeping ToBRFV out

Host plant resistance

No commercial tomato variety has been demonstrated to be fully resistant to ToBRFV, however some varieties in Israel are asymptomatic (but may still contain infective ToBRFV). European sweet pepper varieties do not contain the Tm-22 gene, but instead the L4 gene. This provides protection against ToBRFV and no commercial outbreaks have been reported in Europe. The L4 gene is not present in pepper varieties grown in Mexico and the impact of ToBRFV there has been catastrophic.

Varietal differences in the severity of ToBRFV expression have been seen in Europe and Israel. Some varieties show symptoms on leaves alone, but fruits failed to ripen; other varieties exhibited symptoms on the fruit, but not on leaves. Some varieties are asymptomatic, showing no visible symptoms, whilst 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 have limited access with only essential visits permitted. No tomato products are permitted to be brought onto or consumed on site.
  • 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.
  • Clothing capable of being laundered at ~ 95°C is being researched alongside virucidal detergents.
  • Returning fruit crates/trays are considered one of the greatest risks of spreading ToBRFV between sites. Some sites now using automated machines to disinfect trays, whilst 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 whilst the other is in use, whilst 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. NzymRugo) for hand-washing and disinfection of other surfaces.

Eradication of ToBRFV

Successful eradication of the virus in Germany and the UK shows that it is possible to eliminate ToBRFV from infected sites, as long as ToBRFV biosecurity protocols are enforced and strictly adhered to.

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.

Step

Product

Concentration

Approach

Comments

Clean up all plant debris and glasshouse structure.

Brooms, etc.

N/A

Remove all organic material from the site.

The cleaner, the better; residual organic material may impact the efficacy of the disinfectants used.

Clear sap residues, and everything which has had contact with plant material.

Venno ‘Hortisept Clean Plus’ (or equivalent)

Up to 2%

Foaming (warm water, 30 °C) for 5 minutes (product must not dry).

Rinse thoroughly with water, allow to dry.

Application with spray lance or spray nozzle, e.g. injector nozzles.

  • or Skumix facility (please refer to Royal Brinkman’s website for more information)
  • or high-pressure cleaner with foam nozzle
  • or Menno dosing foam syringe

OR, in organic production

Fadex H+

(formic acid)

Up to 2%

Foaming (warm water, 30 °C) for 5 minutes (product must not dry).

Rinse thoroughly with water, allow to dry.

Application with spray lance or spray nozzle, e.g. injector nozzles.

  • or Skumix facility
  • or high-pressure cleaner with foam nozzle
  • or Menno dosing foam syringe

Disinfection after thorough cleaning

Menno Florades

 

4%

Reaction time:

  • 4 hrs as a foam, otherwise 8-16 hrs
  • Effective also after drying

 

Foam application offers higher effectiveness.

Attempts by the University of Wageningen in 2019 gave good results against ToBRFV.

Sodium hypochlorite (content: NaOCl, 12.5%)

or stabilised NaOCl

Note: storage reduces effect.

3% dilution of 0.1-0.4% NaOCl in spray mixture.

Apply with spray boom with usual spray nozzle; leave to absorb for 2 hours (wet).

Monitor for corrosion and ensure correct personal protective equipment (PPE) is worn.

  • Research by the University of Wageningen in 2019 at 1% concentration gave no kill after 8 hours.
  • NaOCl gave good control against Tobamo viruses in Israel.

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 webpage.

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 re-using it year-on-year. Boxes and drip pegs were also replaced after outbreaks.

Minimise the impact of ToBRFV

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), whilst 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.

Growers' check list

The following lists the ‘best practice’ procedures identified from the two study tours.

Implementating these practices may not be practically, or economically viable for all sites, but demonstrates what can be done to mitigate against ToBRFV infections developing, and how to better address infections which do occur on sites.

Maintaining a virus-free site

  • Review and update site hygiene protocols to include ToBRFV management practices, and train staff on ToBRFV symptoms and what to do if an outbreak occurs. Use the resources on ahdb.org.uk/knowledge-library/tomato-brown-rugose-fruit-virus  and discuss this further with a local PHSI officer.
  • Stop all site visits unless essential. Where visits are required, request they take place first thing in the morning.
  • Ensure visitors read and understand site hygiene protocols, including wearing protective clothing at all times, ideally changing into this before leaving their vehicles. Where sites are visited by individuals frequently, shoes can be purchased to remain on-site and be routinely disinfected.
  • When using contractors, ensure their equipment is clean and sterilised before allowing entry on-site.
  • No tomato products should be brought onto, or consumed on-site – by staff or visitors.
  • Consider supplying fresh, laundered clothing for staff to wear during shifts which is replaced each day. Staff should wear gloves at all times, replacing them often and frequently sterilise their hands with an appropriate disinfectant.
  • Limit staff movement between production areas on each site, do not move equipment or machinery between areas. Identify and designate high and low risk areas, and control movement between these areas.
  • Consider installing foot and hand cleaning machines at the entrances to each site. These must be maintained, ensuring that the active ingredients used are at recommended rates and have activity against ToBRFV. Alcohol hand gel has no activity against ToBRFV.
  • During grafting change blades after each new tray, or more frequently if necessary.
  • Use the German one knife per row, or Dutch two knife method to ensure disinfected knives are used at all times.
  • Clean and disinfect all equipment with an effective, crop-safe disinfectant between each row, or use a trolley cleaning machine.
  • Ensure crates are cleaned using a crop-safe disinfectant, or alternative method e.g. low pressure steam treatment, before they are used.
  • Ensure trucks are disinfected with a suitable disinfectant e.g. Virkon S, or Menno Florades, before arrival on-site and again before use.
  • Where water is recirculated, the sterilisation process should be checked to ensure it is sufficient to eliminate infectious ToBRFV.
  • Ensure all waste-water enters the sewer system. Do not allow water to enter natural watercourses which may establish ToBRFV in alternative host weed species nearby.
  • Ensure seed health certificates are sourced for each variety grown, especially trial varieties. Check the diagnostic tests were performed by an accredited laboratory, and that a sufficient number of seeds was tested (3000).
  • Avoid growing varieties which are extremely sensitive to ToBRFV (currently unknown).
  • Monitor crops frequently for disease symptoms, be aware of typical ToBRFV patterns of infection (symptoms along rows), and if a plant looks suspicious have it tested.
  • Avoid excessively stressing plants and take care to avoid unnecessary wounding which may act as an entry point for infection to occur.
  • Monitor weeds species on site and treat as necessary.
  • Clean and disinfect all site architecture using products known to eliminate ToBRFV. Ensure all products are used at their recommended rates, pressures and temperatures and applied to maximise their effectiveness e.g. foam.

 

Suspected / confirmed ToBRFV infection

  • If an outbreak is suspected quarantine the affected house and immediately notify Defra, or appropriate plant health authority, and comply with their recommendations.
  • Isolate any suspicious plants and test for ToBRFV. When outbreaks are confirmed carefully double bag infected plants and remove them from the glasshouse following PHSI recommendations. Remove additional plants along the row and plants from the rows opposite the infected plant (exact number to be removed to be taken on a case by case basis).
  • When suspicious plants are quarantined, shut down bee hives in these areas, until a result is returned. If a positive result is returned these hives should be destroyed.
  • Implement a clean-down policy based on PHSI advice and available information on disinfectant efficacy.