Grower site 2: ToBRFV outbreak update

Since the UK grower ToBRFV outbreak case studies were originally reported, site 2 has unfortunately experienced a new outbreak of ToBRFV. Read on for the full details.

The secondary outbreak in summary

How the outbreak was identified

Following the outbreak and subsequent clean up in summer 2020, the site was replanted. The new crop was tested by the local Plant Health Inspector in September/October 2020, who took 20 subsamples, following set protocols. The crop was sampled at shoulder height which corresponded to the middle of the plant for one half of the crop, whilst the other younger half corresponded to the heads. From these 20 subsamples, one came back with a low positive result for ToBRFV. Previous surveillance had shown the importance of being able to trace back to sampling locations, so the grower was able to locate the exact area where the positive sample had come from, which was in the younger crop. Following these results, both leaf and root samples were sent to another testing facility outside the UK, from where the results came back negative. 

Monitoring symptoms and spread

Having experienced weak positive results earlier in 2020, the grower remained vigilant. No symptoms were observed in the crop until a couple of weeks after testing when distinct leaf symptoms in the heads of the plants were identified in the specific area corresponding to the low positive result. More samples were taken from the surrounding area, allowing the grower to map disease progress (confirmed by both positive and negative test results). All symptomatic plants were removed and destroyed, and strict hygiene measures were enforced around this area. Following several more rounds of testing to identify the exact spread of the virus, in early December, around 2,500 m² of crop was removed and the areas were disinfected/deep cleaned. No further symptoms were observed elsewhere across the site. 

From February 2021, new symptoms spread throughout the younger block, at first very distinctly associated with specific worker areas. Similarly to the infection last year, by March, the crop struggled to keep fruit on the trusses – it set, but then dropped off the truss as it swelled. Ordinarily, the crop would stay in until July, but the grower chose to remove areas where production had slowed or ceased (~April 2021).  

Interestingly the older planting remained symptom-free, and the last samples taken (in December 2020) remained negative. The two halves of the glasshouse are only separated by a single wall of glass, indicating that the hygiene protocols on-site are working to some extent. Once the heads were removed from the older plants, some symptoms were seen in isolated areas in the last month of production. The area most affected was adjacent to where the infection had started. Heat stress and immediate pressure were deemed obvious links to symptom expression, but the grower could not rule out biocontrol, rodents or birds influencing the spread. There is no indication that the virus has been transmitted through irrigation, as this would likely have a more consistent impact across the crop. It certainly appeared mechanical.

The grower suspects that the site was somehow recontaminated through workers movements from the glasshouse located close to the packhouse, despite the strict biosecurity precautions taken (uniforms were destroyed, a two-week enforced stand down, and the issue of new uniforms/ shoes/equipment on arrival on-site).

Lessons learnt

Biosecurity

Although strict biosecurity procedures were followed when moving staff from a site with an active outbreak to the new crop, the virus seems to have spread via this route. The previous crop finished in April, and the additional workers did not arrive until after planting in August/September. The staff did not work at the packhouse, but some lived in accommodation nearby. Although there were clear risks, further precautions were taken, including the enforced two-week stand down and reissuing of new uniform. More controls were put in place with these workers than with a usual new intake of seasonal staff.

Despite the outbreak in the younger block, growth in the older planting block remained strong and symptom-free until the last month of production, when the plants had already been stopped. These crops were only separated by a pane of glass, demonstrating that the heightened level of biosecurity has helped prevent further spread of the virus.

ToBRFV testing and surveillance

AHDB research findings on the spread and detection of ToBRFV have helped guide Plant Health protocols for virus surveillance in 2021. As the virus can be most readily detected in the plant heads in young crops, this is where the sampling was targeted. In addition, research has shown that growers should not dismiss weak positives for ToBRFV in crops without symptoms. This may be an early indication of symptom development in the crop or virus presence that could be spread around the nursery, potentially affecting other crops.

New and ongoing measures taken to protect against the virus

Biosecurity

• Workers are banned from wearing watches on-site

• Only five people across the whole business are permitted to go between the two blocks - they are aware of the risks and are extremely vigilant about biosecurity measures (handwashing, disinfection etc.); always visit the clean crop before the affected block

• Full disinfection of workers accommodation and staff vehicles is due to be carried out under the supervision of site management at the same time the glasshouse is disinfected

Staff training:

• The staff have seen how this virus can have a devastating impact on the crop and affect the work available

• The induction of new staff includes management of and biosecurity measures for the virus 

• The coronavirus pandemic has helped show how easy viruses can spread and the lengths that you have to go to prevent spread – the comparison of ToBRFV as the ‘plant version of COVID-19’ has helped the message land 

• Ongoing/top-up training of staff about the disease and measures in place to prevent the spread should ideally be implemented