In 2004, more than 4 million ha of tomato (Solanum lycopersicum) were cultivated worldwide, producing over 116 million tonnes (FAO, 2005). This crop is the second horticultural species in terms of cultivated surface area and production volume after potato. In the last decades, the viral diseases have become the main limiting factor in tomato crops along the Spanish Mediterranean coast and in many other areas in the world. In this respect, Tomato spotted wilt virus (TSWV) and Tomato yellow leaf curl virus (TYLCV) have been responsible for serious economic losses around the world (Goldbach and Peters, 1994; Picó et al., 1996; Roselló et al., 1996).
Recently, other viral diseases have appeared such as those incited by Parietaria mottle virus (PMoV), which is currently causing serious damage in Greece (Lisa et al., 1998), France (Marchoux et al., 1999), Italy (Roggero et al., 2000), and Spain (Aramburu, 2001); Tomato infectious chlorosis virus (TICV), identified in the United States (Duffus et al., 1996), Italy (Vaira et al., 2002), and Spain (Font et al., 2003); and Tomato chlorosis virus (ToCV), identified in Portugal (Louro et al., 2000), Spain (Navas-Castillo et al., 2000), and Greece (Dovas et al., 2002). Nevertheless, the viral disease that has caused the greatest impact on tomato crops is the one incited by Pepino mosaic virus (PepMV). This virus was first described in Solanum muricatum plants in Peru in 1974 (Jones et al., 1980). However, its presence was not reported again until 1999 when it was found in tomato crops in Holland. A few months later, its presence was reported in England and Germany in 100 ha infected (EPPO, 2000). In 2000, it was reported for the first time in the French region of Brittany and the Spanish provinces of Murcia, Almería, and the Canary Islands (Soler et al., 2000). It was later detected in Italy (Roggero et al., 2001), Canada, and the United States (French et al., 2001), Poland (Pospieszny and Borodynko, 2002), China (Zhang et al., 2003), and Hungary (Forray et al., 2004).
Currently, in Spain, the disease incited by PepMV affects plantations in the provinces of Alicante, Murcia, Almeria, Granada, and the Canary Islands and represents a serious problem for tomato crops along the Spanish Mediterranean coast. The highest incidence of the disease appears in Murcia with a 75% to 90% rate of infected greenhouses. In many cases, losses reach 20% to 40% of production (Soler et al., 2000). In addition, many countries are reticent to buy Spanish tomatoes because PepMV has been occasionally detected in Spanish imported fruits (Zitikaite et al., 2004).
Typical leaf symptoms associated with PepMV infection are yellow mosaics, puffiness, and leaf filimorphism. Dark green grooves appear on stems and sepals. Infected plants show a severe reduction of fruit set. Fruits usually show a mosaic of different shades of red only on ripe fruits, which is incited by an irregular distribution of lycopene on the surface and reduces their commercial value (Soler et al., 2000). The appearance of these symptoms depends on environmental factors, mainly temperature. Symptom severity is reduced when temperature rises above 25 °C and sometimes disappears altogether. In this case, plants may show spotted ripe fruits without symptoms on the leaves or may have systemic infection with high levels of viral accumulation without showing symptoms.
During the 1998–1999 season in Murcia, a sudden wilting of tomato plants was reported and in many cases caused the irreversible collapse of the plants. Studies carried out on collapsed and noncollapsed plants reported a clear association between PepMV and tomato collapse (Soler-Aleixandre et al., 2005b). This syndrome could constitute another symptom associated with infection by this potexvirus. This has contributed to the fact that PepMV is now considered to be one of the most serious limiting factors in the profitability of tomato crops in Spain. In Holland and the United Kingdom, the appearance of severe mosaic and filimorphism symptoms, associated with PepMV, also cause serious losses (Tomassoli, 2001). Fruit spotting causes the most damage in tomato crops in Italy (Roggero et al., 2001).
PepMV can be mechanically transmitted very efficiently. In the field, the disease spreads fast, even more than Potato virus X (PVX), and can be transmitted more efficiently than Tomato mosaic virus (ToMV) (Wright and Mumford, 1999). It is transmitted through the touching of plants with contaminated hands, during the handling of infested tools, and even by clothes brushing against plants. Although it has been detected in tomato seeds (Salomone and Roggero, 2002), there is no evidence of transmission to descendants. No specific transmission vectors have been reported. The transmission role by contact of pollinating insects such as Bombus terrestris and B. canariensis, used in greenhouse crops, has been reported (Lacasa et al., 2003). Because of the lack of severe symptoms in infected plants, growers are reluctant to eliminate the infected ones and, therefore, the disease spreads quickly through greenhouses (Soler et al., 2000).
The highly efficient mechanical transmission of this disease has made the adoption of cultural practices unsuccessful in preventing its spread. The development of varieties that incorporate resistance to the virus would allow effective control of the disease. In this study, results of a screening for resistance to PepMV in a collection of accessions from different Solanum species are presented.
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