TSWV was first recorded on sweet peas in 1936. Although it is capable of infecting a huge range of plant species, it was rare in Europe until recently due to the lack of an efficient vector. Only with the introduction of Western Flower Thrips (WFT) from North America has it been able to establish itself effectively as a widespread problem of increasing economic importance.
It is impossible to list every known host plant, but the following lists indicate a few of the possible sources of infection in the UK. Other countries worldwide are similarly affected.
Farm Crops: Beetroot, Broad Bean, Celery, French Bean, Lettuce, Onion, Spinach, Squash, Potato, Pumpkin.
Glasshouse Crops: Aubergine, Cucumber, Melon, Pepper, Tomato. Chrysanthemum, Cyclamen, Gypsophila, Lily, Lisianthus, Stocks.
Garden Flowers: Anemone, Aster, Begonia, Campanula, Chrysanthemum, Dahlia, Delphinium, Foxglove, Fuchsia, Geranium, Gladiolus, Hydrangea, Impatiens, Larkspur, Lobelia, Lupin, Nasturtium, Nicotiana, Paeony, Pelargonium, Penstemon, Petunia, Poppy, Primula, Snapdragon, Sunflower, Viola, Wallflower, Zinnia.
Weeds: Bindweed, Buttercup, Chickweed, Clover, Dandelion, Groundsel, Knotgrass, Nightshade, Plantain, Shepherd's Purse, Stinging nettle.
There are many species of thrips worldwide, of which 8 are carriers of TSWV. Of these only two occur in the UK, although the possibility of others being introduced is very real. The Onion Thrips is of little importance in this respect, but the more recently introduced Western Flower Thrips has rapidly established itself as a major pest of glasshouse crops and is an efficient vector of TSWV.
Only the lava can acquire the virus by feeding on an infected plant, but the disease is passed on by the more mobile adult which remains infectious all its life. Adults can live for up to 60 days, laying hundreds of eggs during this period. Should the adult alight on a suitable food plant, there is little incentive for it to move on and virus transmission from plant to plant may be correspondingly slow. Should the plant be unsuitable or unpalatable, however, the thrips may make several attempts to feed before moving to a new host. If this sequence is repeated, the disease can spread much more rapidly.
Thrips are tiny insects, up to 2mm long and very thin. The most familiar species in the UK are Cereal Thrips and Onion Thrips, both being relatively large dark coloured species and hence easily seen with the naked eye. They are also known as thunderflies as they often take to the air before a thunderstorm, and are carried considerable distances by even light winds. For such small insects they cause a surprising degree of irritation by simply walking over one's skin. Curiously, the irritation always seems to be at some distance from the offending party, so that casual attempts to swat or brush away the source of the annoyance are often ineffective.
Western Flower Thrips are smaller and paler and more apparent from the damage they cause than visible in themselves.
Describing the symptoms of TSWV poses problems as each plant species seems to have its own particular set of symptoms which vary according to the conditions under which the plant is growing. Symptoms are also much better documented for crops of major economic importance than for minor crops such as sweet peas, so that most books merely list all the possible sympoms, the majority of which are not relevant in this particular case.
Classic symptoms include necrotic streaking of stems and petioles which becomes more obvious as the disease progresses. This is typical of a systemic infection which will also cause severe stunting. If a young plant is infected, it may not exhibit any of the above symptoms, but merely turn yellow and die.
Few plants seem to have any resistance to TSWV, and such resistance as has been discovered seems relatively inefectual. It has been postulated that this may be due to the fact that the genus Tospovirus, to which TSWV belongs, has only recently evolved from a family of viri which normally infect animal hosts, so that plants have not yet had time to evolve an effective defence mechanism.
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© 2005 P E M Rowland