The article “Multi-genotype cross-protection against Pepino mosaic virus in tomato” describes how young plants are inoculated with V10, a product made up of two mild isolates of the virus. Introducing a mild virus into a plant activates RNA silencing in the plant. RNA silencing is a process in which the plant protects itself against subsequent infections with similar viruses. A certain length of time is required before the plant can protect itself against pepino mosaic virus. After being treated with V10, there is a short period of time in which the plant needs to build up resistance. After this period, it can protect itself against aggressive isolates of the virus.
In summary, therefore, the earlier we inoculate a plant with a mild virus, the earlier the plant is protected. Moreover, young plants are also more susceptible to virus infection, so a mild virus spreads more rapidly through the whole plant at this stage.
Read the article for more information on the use of cross-protection against PepMV in tomato.