Greenhouses are cleared where the Tomato brown rugose fruit virus (ToBRFV) is found. But that is not the end, because how do you remove the virus from substrate slabs? This is the subject of a lot of research, because too little is known about it.
In case of doubt, ToBRFV cultivation waste is now incinerated in the Netherlands, but steaming and/or treatment with hot water may also 'deactivate' the virus. In any case, wet heating is more effective than dry heating.
It is expected that ToBRFV will be similarly heat tolerant as other tobamo viruses, as is shared by the NVWA (The Netherlands Food and Consumer Product Safety Authority) in a document about inactivation of ToBRFV by heat treatment. Waste processors also assume this (for the time being due to a lack of more knowledge) and if in doubt they opt for incineration.
Knowledge about related viruses
Quite a bit has been published about the related viruses tobacco mosaic virus (TMV) and tomato mosaic virus (ToMV). Researchers have searched the internet looking for information.
Reports/publications on disinfection of seeds through dry heating are not included. That's because it is uncertain whether virus concentrations in/on seeds are comparable to those in/on other materials such as roots in substrate slabs (which may contain high virus concentrations) and whether results with seeds can be extrapolated to other materials.
According to the NVWA, the search yielded two relevant reports and three relevant publications (see the NVWA document, in Dutch). Duration and temperature for inactivation of the virus differ per method (treatment in hot water, with calcium oxide, steaming or dry heating) and also treatment of different substances/materials cannot be summarized just like that.
Difference between dry and damp material
The temperature and time required to eliminate ToBRFV will depend on the virus concentration and the availability of water. In general, dry conditions require higher temperatures and/or longer treatment periods to inactivate pathogens compared to humid conditions, the NVWA concludes in a discussion passage.
The higher effectiveness of wet heating also applies to tobamoviruses, and research shows that dry heating at 100° C for 20 minutes is insufficient for inactivation. No experimental data have been found with TMV, ToMV or ToBRFV in which inactivation by heat was obtained with dry heating (seed treatment publications have not been included for reasons previously mentioned).
Heating is most effective with aqueous suspensions of virus particles. Research results with ToBRFV from Fera (2020) indicate that ToBRFV can be inactivated within 5 minutes in 90° C water. This was an experiment in which contaminated boxes were immersed in a water bath at 90° C and some of the virus particles may have been rinsed off. Therefore, it cannot be excluded that with (very) high virus concentrations not all virus particles are inactivated within 5 minutes.
A research study (by Broadbent et al., 1965) with infected plant sap, showed in one experiment with TMV that no infectious virus was detected after 5 minutes at 85° and 88° C, while in another experiment still infectious virus could be detected at both temperatures after 10 minutes. Zaitlin (2000) even indicates that 90° C - 10 minutes is insufficient for complete inactivation of TMV in plant sap.
Differences in virus concentration and temperature tolerance of virus strains
The most obvious explanation for differences between experiments within the same study and differences between studies are differences in virus concentrations. In addition, differences between studies may also arise due to possible differences in temperature tolerance between virus strains within a variety.
Steaming
Inactivation of ToBRFV in plant parts and substrate with hot moist air (steaming) probably requires higher temperatures or longer treatment times. Research has shown that the minimum required temperature and treatment time depends on the material treated (fresh/dying roots with high virus concentrations versus old roots with lower virus concentrations).
The results of the study by Broadbent et al. (1965) indicate that a temperature of about 90° C is required for at least 20 minutes to inactivate all present TMV particles. Steaming at 100° C for 5 minutes was also effective, but this concerned only one experiment. Runia (2000) advises to steam stone wool, that contains roots infected with a tobamovirus, at 100° C for at least 10 minutes. At these temperatures and treatment times (90° C - 20 min and 100° C - 10 min) ToBRFV is inactivated.
Since there is no experimental data with ToBRFV and conditions in practice may differ from those in the published experiments (for example, differences in virus concentrations and temperature development before the material has been heated to the desired temperature), the advice is to test the effectiveness of steaming.
In practice, spatial differences could occur in temperatures in the to-be-treated material. It is therefore important to measure the temperature in different places and in any case in places where the lowest temperatures are expected. The required temperature - treatment time will demonstrably have to be achieved in all the to-be-steamed material.
Drying and storing contaminated materials, such as stone wool slabs with roots, for several months will decrease virus concentrations and it is expected that the minimum temperature and treatment time required for virus inactivation will decrease, as was demonstrated for TMV in tomato root pieces (which can also be tested experimentally).
Conclusions
After the study, the NVWA draws the following (provisional) conclusions:
- ToBRFV will, it is expected, be inactivated in 90° C water; the duration of treatment for complete inactivation probably depends on the concentration of virus particles. Experimental studies are needed to determine the minimum temperature - treatment time that is required to inactivate ToBRFV.
- Steaming at 90° C for 20 minutes or at 100° C for 10 minutes is expected to be sufficient to inactivate ToBRFV. It is recommended to experimentally test the effectiveness of steaming, taking into account the effect of virus concentration and the temperature trend in the to-be-disinfected material.
- Dry heating is less effective than wet heating. No indication of the required temperature and duration of treatment for inactivation under dry conditions has been found.
Source: NVWA