Survey on symptom expression and damage caused by Pepino mosaic virus in commercial tomato production in Belgium.

Pepino Mosaic Virus (PepMV) was first reported on greenhouse tomato (Solanum lycopersicum) in the Netherlands and the UK in 1999. Since then PepMV is worldwide established in the tomato industry and has become a significant problem. An extensive survey among Belgian tomato growers was conducted, assessing their experiences with PepMV in the 2005 and 2006 growing season, and their strategies to cope with this relatively new viral threat. Symptom expression and the economical damage caused by PepMV were investigated. An increase in PepMV infected tomato greenhouses was seen during the 2004 and 2005 growing seasons. The infection risk appeared to be significantly higher in dense tomato cultivation areas. It was shown that a PepMV infection early in the growing season does not guarantee fewer symptoms as compared to a late infection. The number of tomato growers that deliberately inoculated their crops early in the growing season as an immunization strategy decreased significantly after 2005, as results were not satisfying. INTRODUCTION Pepino mosaic virus (PepMV), a Potexvirus originally isolated from pepino (Solanum muricatum) in Peru in 1980, was first reported on greenhouse tomato (Solanum lycopersicum) in the Netherlands in 1999 (Jones et al., 1980; van der Vlugt et al., 2000). Since then, this viral disease has been established in the tomato industry worldwide. At present, five PepMV genotypes (EU, LP, CH2, US1 and US2) have been described, four of which (EU, LP, US2 and CH2) have been reported to occur in Europe (Mumford and Metcalfe, 2001; Aguilar et al., 2002; Cotillon et al., 2002; Verhoeven et al., 2003; Lopez et al., 2005; Pagan et al., 2006; Maroon-Lango et al., 2005; Ling, 2007; Hanssen et al., 2007). A survey conducted in commercial tomato production facilities in Belgium revealed that the PepMV population in Belgium in 2006 was dominated by the CH2 genotype, as this genotype occurred in almost 90% of the infected tomato crops. In addition to the CH2 genotype also the EU genotype was present, with both genotypes often occurring in mixed infections (Hanssen et al., 2007). A wide range of symptoms has been associated with PepMV infection in tomatoes, including leaf mosaic, leaf distortions, yellow rectangular leaf spots, nettle heads, stunting, fruit deformation, browning of fruit petals and fruit discoloration. The latter is generally considered as the most damaging PepMV symptom as it reduces the commercial value of the fruit (Soler et al., 2000; Mumford and Metcalfe, 2001; Spence et al., 2006). To reduce economic losses caused by PepMV infection, some tomato growers in the Netherlands and Belgium deliberately inoculate their plants with PepMV at the start of the growing season, as it has been suggested that early PepMV infections are less damaging than infections that occur later in the growing season. In addition, it is believed that inoculation of tomato plants with a weakly aggressive isolate of PepMV offers protection against subsequent infection with a more aggressive isolate (immunization). This effect is based on cross-protection, a principle that was successfully used in the seventies to protect tomato plants against Tobacco mosaic virus (TMV) (Rast, 1972). However, in the case of PepMV the efficiency of this strategy has not yet been proven. In contrast, it was recently shown that a tomato crop infected with the EU genotype was subsequently invaded by the CH2 genotype, or vice-versa (Hanssen et al., 2007). Proc. II Intl. Symposium on Tomato Diseases Eds.: H. Saygili et al. Acta Hort. 808, ISHS 2009 186 Moreover, this secondary infection coincided with an increase in PepMV symptom expression (Hanssen et al., 2007). An elaborate survey was conducted in the Belgian tomato production industry to assess the PepMV damage as perceived by the growers and to study the results obtained by applying the immunization strategy in commercial tomato greenhouses. MATERIALS AND METHODS Study Site The tomato industry in Belgium is located in the Northern, Flemish district of the country. As tomatoes in Belgium are exclusively produced for the fresh market, fruit quality and appearance is of major importance. Since quality standards during production are very high, the majority of the tomatoes have to be sold in the highest or first quality class in order to remain profitable. The major tomato growing areas in Flanders are covered in this study. Survey 2005 A questionnaire was designed to conduct a survey on the experience of the Belgian tomato growers with PepMV in 2005, aiming to reveal grower’s perception on damage caused by and problems associated with PepMV in the Belgian tomato production during the 2005 growing season. Special attention was given to the results obtained by the immunization strategy that was applied by part of the interviewed growers. PepMV infected tomato production sites were divided into two groups: the production sites that were deliberately inoculated with PepMV at the start of the growth season, further referred to as the ‘inoculated sites’, and the ones that suffered from a natural infection further referred to as the ‘natural infections’. An Access database was designed to manage and process the acquired dataset. Survey 2006 A second questionnaire was designed to assess growers’ experiences during the 2006 growing season, composed of multiple choice questions to facilitate the written completion of the questionnaire by the growers. The tomato production sites infected with PepMV during the 2006 growing season were divided into three groups: a group of production sites that were deliberately inoculated at the start of the growing season, further referred to as the ‘inoculated sites’, the ones that suffered from a natural infection in the beginning of the growing season (January to March), further referred to as the ‘early natural infections’ and finally the ones that became infected later in the growing season (from April onwards), further referred to as ‘late natural infections’. In this classification a production site was considered infected as soon as the grower recognized typical PepMV symptoms, as the exact time of infection was not known. An Access database was designed to manage and process the acquired dataset.