Impatiens necrotic spot virus and tomato spotted wilt virus

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.

Symptoms

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 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.

Control

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.

Biology

• Plant viruses share many of the characteristics of those that infect animals, though they do not cross infect (plant viruses only infect plants)
• Viruses are among the smallest of living organisms and consist simply of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors 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 particles
• INSV and TSWV are very closely related; in fact, INSV was thought to be a strain of TSWV when it was first discovered. TSWV can be found on both 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
• The thrips vector picks up the virus when feeding on an infected plant as a larva, and can then transmit it 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 affected