ToBRFV Spreading in the greenhouse

The spread of Tomato Brown Rugose Fruit Virus (ToBRFV) through a greenhouse can be slowed down so overall damage can be less severe. Financial impact of the disease have to be taken into account as producers will need to find the right balance between cost of prevention and yield loss.
It is unknown where, when and at how many locations in the greenhouse the virus starts. Early detection is important as described before. Hygiene is also key when it is not known if the virus is already inside the greenhouse.

Basic spreading theory
Assuming that the infection starts with 1 plant. After around 10-14 days there is so much virus in the first plant that transmission takes places to the neighboring plants, it is assumed that 5-10 plants will be infected. This second group of 10 infected plants will be spreading the virus also after 10 days, to the next 50-100 plants. Taking into consideration of the speed of the virus spread; approximately up to 3 -4 rows of plants will be infected in  2-4 weeks.

This is also described in this article: Spread of Tomato Brown Rugose Fruit Virus in Sicily and Evaluation of the Spatiotemporal Dispersion in Experimental Conditions.  Stefano Panno, et al, 12 June 2020 (https://doi.org/10.3390/agronomy10060834)

See picture below coming from this article.

In this research 2 plants were infected indicated by the first 2 red dots in the first month. One month later the virus was detected in an additional 3 and 4 plants. At 3 months the virus is spread mainly downstream with labor activities reaching about 10 plants and also a little upstream and even into the adjacent rows. In month 4 the spread of the virus has been enormous. This can be due to continued crop work, weather changes or other factors. Bumble bees are also mentioned as the reason for the diagonal spreading of the virus.

It is important to realize that when plant 1 and certainly when 10 plants are infected, the virus will most likely move further away than the neighboring plants and rows. As can be seen in the figure, at the 4th month where there is a single red dot at the end of one of the middle rows with many ‘clean’ plants around. This shows that people,  the trolleys, and possibly bumble bees can spread the virus to clean plants further away, up to 5 to 10 rows, to the opposite site of the path and maybe even much further. It is possible that after roughly 4 weeks the virus is present in multiple spots around the initial spot which then lead to spread to more plants in a similar way.  

Spreading in a greenhouse
This is also seen in the grower greenhouses. Infected areas, represented by red spots, could be followed by noninfected areas, indicated by green spots, see table. The experience is that red spots do increase with a speed of 1-2 rows a week.

Between those red spots, green spots are present which stay sometimes surprisingly a long time ‘green’, that is, not so much infected. Of course infection continues and green spots generally are “absorbed” by the red spots over time.

Our experience tell us that the spread of the virus from the initial spot is limited. The studies have shown that the virus is picked up by trolleys and personnels and is moved mainly to the next rows. Despite high virus titers often measured in plants and on trolleys, the virus is not moved to all following rows, or at least not moved in a way that results in infection developing.

The table below shows example of the differences of ToBRFV concentration between rows in the same block in a greenhouse. ToBRFV measured with qPCR and expressed in Ct values.  Rows not presented were not tested.