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Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV) are closely-related viruses. They are transmitted by thrips species, particularly the western flower thrips (Frankliniella occidentalis). This species first arrived in the UK in the late 1980s and, in addition to causing direct feeding damage, created huge problems by infecting plants with INSV and TSWV.

Western flower thrips (WFT) does not overwinter well outdoors in the UK, so the problems it causes are found mainly on plants grown in glasshouses, polytunnels, conservatories, etc. Where the viruses are found on plants such as dahlias and impatiens growing outdoors, these will often have been infected when they were raised under protection prior to planting out. There may, however, be localised spread of WFT outdoors during hot, dry summers.

Plants on which one or both of the viruses can be found include Begonia, chrysanthemum, Cineraria, Cyclamen, Dahlia, Gloxinia, Hoya, Impatiens, Pelargonium and tomato.

Each of the viruses has a huge host range (TSWV, for example, is known to affect over 900 plant species). The number of symptom types produced is bewildering, and can vary not only according to the host plant affected, but also with environmental conditions (especially temperature).

The following symptoms are among those caused most commonly by INSV or TSWV:

• Stunting
• Leaf distortion
• Yellow (chlorotic) or brown (necrotic) spots or ringspots on the leaves
• Yellow or brown lines, streaks or zig-zag patterns on the leaves
• Mottles (blotches) or mosaics (smaller, more regular patterns like those of a tile mosaic) on the leaves - these can be various colours
• Colourless leaf veins (vein clearing) or yellow areas bordering the veins (vein banding)
• Death of parts of the leaf, leaf stalk, or stems
• Irregular patterns of colouration in flowers or fruit

Some of the most severe symptoms can look more like those of a chemical scorch than a virus infection, and plants can even collapse and die.

The gallery below shows a range of symptoms on different host plants.

Non-chemical control

• Dispose of affected plants as soon as symptoms are seen. It may be prudent to dispose of an entire batch of, for example, bedding or pot plants, even if some do not show symptoms – they could still be infected
• Control weeds in and around glasshouses and polytunnels
• Seed-raised plants will be free from virus initially
• Control measures (chemical, biological or physical) used against the thrips vectors will reduce the risk of virus transmission

Chemical control

There are no chemicals available for the treatment of plant viruses.

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 two of over twenty 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.