I have been told by farmers and researchers in Netherlands that about 90% of the tomatoes in the Netherlands contains Pepino Mosaic Virus – in a good way. Pepino mosaic virus was once a serious problem in the Netherlands because it’s ease of transmission within green houses, and its affects on the yields, and particularly quality, of tomatoes. The affected tomato plants will have ‘blotchy ripening, gold marbling, gold spot’ (Spence et al, 2006).
Rumour has that the practice of using weaker strains of PepMV to infect plants in green houses has been in practice for last 20 years, and until ten years ago it is been approved by the government and EU (EU No. 1107/2009). Nowadays there are companies like Valto in the Netherlands and DMC in Belgium that produces and sells the controlled weaker strains of PepMV for farmers. Valto’s product is named V10, while DMC’s product is named PMV-01. Both address their products as ‘biological solution’ without mentioning the fact that they are also viruses. But in fact, they are viruses that’s been ‘tamed’ by human and use in the beneficial ways.
What is special about plant viruses is that plants do not have immune systems like us. Plants can only go through necrosis and RNA silencing when encounter with viruses. In other words, once infected, the viruses will live with the plant forever. They will evolve on the plants at the same time. Therefore within green houses it is crucial for the growers to check the viruses regularly to make sure no virulent strains have evolved. If virulent strains are present, the plants will be destroyed along with the viruses and re-introduce the weaker strains again. The seeds are also produced with the quality control that it should not contain any viruses by companies like Rijk Zwaan which at the same time also forbids the tomato growers to produce their own seeds. Or in other words, the seeds produced by the grower themselves directly from the fruits won’t have the same genetic composition due to the nature of sexual reproduction.
In a way, the ‘vaccination’ or ‘cross-protection’ for tomato plants in professional tomato growers can be seen as tamed virus zoo in a protected environment without competitors. The whole production chain is designed in a way that the ‘wolves’ of PepMV is eliminated where we grow the ‘dogs’ – the weak strains of PepMV like V10 and PMV-01.