New pathogen of the genus Tobamovirus. Tomato brown rugose fruit virus (ToBRFV)

Tomato brown rugose fruit virus (ToBRFV) or Jordan virus is a new representative of the genus Tobamovirus. Due to the stability of the particles and effective mechanical transmission, ToBRFV and the entire genus are characterised as highly aggressive viruses. Once the virus appears in the greenhouse, the opportunities of taking control measures are extremely limited and are mainly based on the destruction of infected plants and strict hygiene measures [2]. For this reason, ToBRFV poses a real threat to tomato and pepper producers. In Florida (USA) alone, the virus has caused damage in the amount of 262 million US dollars [1].

The virus was first discovered in Israel in 2014. A year later, the disease was detected in Jordan, then in Mexico, China and the USA. In 2019, the EU Commission took extraordinary measures to prevent the virus from entering and spreading in the EU. Nevertheless, it has also spread in Europe. In Russia, this virus has been declared a quarantine disease. On 1 July 2023, testing of all imported peppers and tomatoes for the presence of this pathogen began [2].

The symptoms vary depending on the crop. In tomatoes, the manifestation of symptoms is also linked to the characteristics of the variety. In some varieties, the symptoms manifest themselves on the fruit, in others on the leaves. Environmental conditions such as lighting, temperature and nutrient availability can influence the appearance of symptoms. Peppers are mainly characterised by leaf deformation, yellowing and mosaic. The fruits deform and show yellow or brown areas or green stripes. The symptoms on a tomato are: Chlorosis, mosaic pattern, leaf spots, narrowing (needling) of the leaves, blistering on the leaf surface, gradual wilting of the leaves with subsequent yellowing and death of the entire plant. Brown necrotic lesions appear on the stems, cups and stalks of infected tomato plants. The fruits may have a latent infection and, without showing symptoms, serve as a source of contagion by infecting other fruits. As a rule, tomato fruits infected with ToBRFV show chlorotic spots, marbling, dark spots on green fruits and deformation. Uneven ripening of the fruit is often observed [3].

There are various routes of infection: through infected seed material, through fruit (in any form), through transport containers, tools, equipment, packaging material and vehicles containing the sap of infected ToBRFV plants, and through a substrate [3].

The main cause of the ToBRFV epidemic is that this virus overcomes all known tobamovirus resistance genes in tomato, including Tm-1, Tm-2 and the persistent Tm-22. A computer analysis of the ToBRFV genome has revealed that this virus contains a variability of 9-15% (comparison of RNA or protein) compared to other representatives of the Tobrfv genus. It has been found that 21 amino acids are probably involved in overcoming the tomato resistance genes. It is thought that the reason for the persistence of Tm-22 against TMV and ToMV is that it targets the element movement proteins (MP) that are required for the spread of both viruses. One such element is the amino acid cysteine 68 (C68). The presence of C68 in TB tobamovirus activates the immune response of Tm-22, but in ToBRFV, C68 is replaced by histidine, preserving viral movement. This finding could explain the stability of the manifestation of Tm-22 against viruses other than ToBRFV (1).

Studies have shown that the various capsicum L alleles are effective against ToBRFV, but probably do not provide complete immunity against the virus [1].

The most obvious strategy for developing tobamovirus resistance is to screen the genotypes of tomato and related wild species for ToBRFV resistance. Thus, 29 lines have been found to be tolerant to ToBRFV. Genetic analysis of the tolerance trait has revealed that it is controlled by a single recessive gene. Mapping of the tolerance locus for ToBRFV has revealed a specific position on chromosome 11 of the tomato. It is noticeable that only one resistant genotype has been identified in the analysis. The mapping of the resistance trait has shown the involvement of two different quantitative trait locus (QTLs): One was on chromosome 11 the same tolerance locus, the other was on chromosome 2 in region Tm-1. Taken together, these loci were the first evidence of ToBRFV resistance in natural tomato genotypes. The proximity of the second QTL to Tm-1 suggests the existence of new Tm-1 alleles that may be capable of conferring ToBRFV (1) resistance.

Currently, the multinational companies of ‘Singenta’, ‘Enza Zaden’ and ‘Rijk Zwaan’ are increasingly involved in the development of sustainable hybrids and have made some progress.