Plant viruses are extremely minute infectious particles consisting of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors (including humans) to transmit them from infected to healthy plants. Once viruses penetrate into the plant cells they take over the cells’ nucleic acid and protein synthesis systems and ‘hijack’ them to produce more virus.
Viruses are frequently transmitted through propagated material but, depending on the virus, can also be transmitted via insect or mite vectors, pollen, mechanical transfer via contaminated hands and tools, and nematode vectors in the soil. Some viruses can be transmitted via seed, but generally these are a minority and therefore seed propagation is often a useful way to ensure virus free plant material.
INSV and TSWV are 2 of over 20 viruses in the genus Orthotospoviruses (named after TSWV). INSV and TSWV are very closely related, and INSV was initially thought to be a strain of TSWV when it was first discovered. TSWV infects ornamentals and glasshouse vegetables such as tomatoes and peppers; INSV tends to be confined to ornamentals. Both viruses can sometimes be found in a single plant.
TSWV and INSV are transmitted exclusively by thrips. A number of different thrips species can transmit these viruses with the most important being the western flower thrips (Frankliniella occidentalis). The thrips vectors can only acquire TSWV and INSV in the larval stage when feeding on an infected plant. Adult thrips cannot acquire the virus through feeding, but an adult thrips that has acquired the virus at the larval stage will then go on to transmit the virus to susceptible plants for the rest of its life.
In addition to the symptoms described above, some affected plants remain symptomless for extended periods. These plants are a particular risk as they appear healthy yet act as a source of the virus.
A range of weed species can also be infected by these viruses.