These viruses have been a significant problem since the late 1980s, when the western flower thrips, their principal vector, arrived in the UK. Each virus has a huge host range and can produce a bewildering array of symptoms.
What are these viruses?
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 usually have been infected when they were raised under protection prior to planting out. There may, however, be localised spread of WFT outdoors during summer.
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:
- Leaf distortion
- Yellow or brown spots or rings on the leaves
- Yellow or brown lines, streaks or zig-zag patterns on the leaves
- Mottles (pale green or yellow blotches) or mosaics (smaller, more regular patterns of yellowing) on the leaves
- Colourless leaf veins (vein clearing) or yellow areas bordering the veins (vein banding)
- Death of parts of the leaf, leaf stalk, or stems
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.
Test kits for INSV and TSWV (and for Cucumber mosaic virus (CMV), which can cause similar symptoms) are available from Forsite Diagnostics.