Four different strains of the virus are known: the Chilean strain (CH2), the European strain (EU), the Peruvian strain (LP) and the American strain (US1). Within these strains, many different individuals, called isolates, can be observed. In Europe, the Chilean strain (CH2) is dominant and present in 90% of the infections. If more strains are found in case of infection, the infection is usually more aggressive than when only one single strain is present. Within the strains, however, there is also variability among different isolates belonging to the same strain. Although different isolates within one and the same strain are genetically very similar, they can show very different levels of aggressiveness in the plant and, as a consequence, in the severity of the symptoms and the damage they cause. For example, there are very aggressive Chilean isolates, but also very mild ones that cause less symptoms.
The impact of a PepMV infection for the tomato grower is variable. Based on a survey among Belgian growers and on several practical trials with various isolates of PepMV, quality losses were estimated to be between 4 and 15%, with additional production losses between 4 and 12%. Several factors contribute to this variability. The genetic identity of the virus – which isolate of which strain – that causes the infection is one them. The moment in the crop cycle when the infection takes place is another factor that might have an important impact on the damage that is caused. For instance, an infection early in the season, when the plant does not yet bear fruit, will typically cause less losses than when the infection takes place when the plants are heavily laden. Typical of the damage caused by PepMV is, that it is very intense for a few weeks and then disappears to eventually return later in the season. This means that damage can be more severe during a certain period of the year, with 50-60% non-marketable fruits.
Moreover, some tomato varieties are more susceptible to quality losses due to PepMV than others and climate also has an impact. In general, when a plant has more stress for whatever reason, losses will be much greater.
In the early 2000s, PepMV became such a huge problem in Belgium and the Netherlands, that there was an urgent demand for support and control measures from the tomato sector. As a first step, fundamental research was carried out to identify the problem and a very strict hygiene protocol was developed to avoid PepMV infections. These strict hygiene measures turned out to be insufficient to ensure the grower a PepMV-free crop. Soon, research was oriented towards concrete, biological management tools and step by step PMV®-01 vaccination strategy was developed to immunize plants in order to prevent infection.
