Effectiveness of Chemo- and Thermotherapeutic Treatments on Pepino mosaic virus in Tomato Seed

In the 10 years since its first report to infect tomato (Solanum lycopersicum L.) in 1999 (19), Pepino mosaic virus (PepMV), a Potexvirus sp., has become an important virus infecting greenhouse tomato plants in Europe (2,5,6,13–16), North America (3,10,12), South America (8,18), and Asia (20). Although commercial tomato seed has long been suspected to be a source of initial virus inoculum, earlier reports on natural seed transmission of PepMV in tomato were in conflict. Salomon and Roggero (17) showed no seed transmission of PepMV in tomato. Cordoba-Selles et al. (1) claimed to have observed a low rate (<2%) of seed transmission. An even lower rate of seed transmission (0.026%) was recently reported in a larger number of grow-out seedlings (approximately 90,000) tested in multiple laboratories in Europe (4). The mechanism of PepMV transmission from tomato seed to seedlings has only recently been elucidated (9). Although no direct seed transmission of the virus in over 10,000 grow-out seedlings was observed, that study showed that localization of PepMV was on the seed coat but not on the embryo, and the ease of mechanical transmission of PepMV from contaminated tomato seed to seedling was responsible for initiating virus infection (9). In general, Potexvirus spp. are not considered to be seed transmitted. Thus, authors in the recent report (4) speculated that the extremely low rate of seed transmission (0.026%) as observed in nearly 90,000 grow-out seedlings was likely due to the contact between the germinating seedlings and the virus-contaminated seed coat. The notion that PepMV is a seedborne but not seed-transmitted virus implies that seedling grow-out tests may not reflect the actual virus status of the seed. Bioassay by inoculating the processed seed extract on indicator plants (Nicotiana benthamiana) is more appropriate in assessing the presence of infectious virus particles remaining on the treated tomato seed. In fact, such a virus assay is also recommended by the International Seed Health Initiative for Vegetables (ISHI_Veg). In seed treatment for PepMV, several chemoand thermotherapeutic techniques were also evaluated by Cordoba-Selles et al. (1). However, authors in that report relied mainly on serological tests (enzyme-linked immunosorbent assay [ELISA]) and seedling grow-out assays to determine PepMV infectivity. In general, ELISA can detect a certain level of PepMV on tomato seed. However, the technique could not differentiate viable from nonviable virus particles on the treated seed. The objective of this study was to evaluate the efficacy of several commonly used chemoand thermotherapeutic methods on PepMV in seed samples from a heavily PepMVcontaminated commercial tomato seed lot. Infectivity of PepMV remaining on the treated seed was evaluated in bioassay through mechanical inoculation on N. benthamiana plants, using symptom expression on the inoculated plants with confirmation test by ELISA.