Ecology, epidemiology

- Conservation, sources of viruses

The wilt virus disease of tomato ( Tomato spotted wilt virus , TSWV) is, like the cucumber mosaic virus ( Cucumber mosaic virus , CMV), one of the plant virus whose natural host range is the most extent. It infects a large number of hosts , cultivated or not: more than a thousand have been inventoried, belonging to at least 86 botanical families of dicotyledons for the most part. It is therefore easily preserved in the environment of tomato crops, in particular on various weeds such as Amaranthus spp., Anagallis arvensis, Capsella bursa-pastoris, Chenopodium amaranticolor, Convolvulus arvensis, Fumaria officinalis, Oxalis corniculata, Picris echioides, Poa annua, Solanum nigrum, Sonchus spp., Stellaria media, Taraxacum officinale and Veronica spp.

The families with the most host species are asteraceae (247) and nightshade (172). The latter include Cyphomandra betacea, Lycopersicon hirsutum, L. pimpinellifolium , 15 species of Solanum, including S. aethiopicum, S. muricatum, S. quitoense, Nicotiana tabacum, N. acuminata, N. alata, N. glutinosa, Physalis heterophylla, P. minima, P. peruviana, P. ixocarpa, Browalia sp., Datura stramonium, Dubosia leichardtii, Hyoscyamus niger, Lycium procissimum, Salpiglossis sp., Schizanthus sp. and Streptosolen jamesonii .

Many cultivated plants harbor this virus: 166 species have been identified in France since the emergence of TSWV in 1987, belonging to 34 families, including 7 among monocots:
- vegetable, aromatic or industrial species (eggplant, pepper, potato, tobacco, lettuce, endive, bean, broad bean, pea, melon, cucumber, squash, spinach, cabbage, artichoke, chard, beet, celery, parsley, lavandin , coriander, tarragon, basil, sage );
- ornamental species (daisy, anemone, arum, begonia, marigold, chrysanthemum, dahlia, zinnia, cyclamen, gladiolus, gerbera, lily, petunia, ranunculus, jasmine, impatiens ).

It is also found on certain food crops in hot areas: Vigna sp., Groundnut, chayote, pineapple

These numerous plants are reservoirs for viruses which make the control of TSWV extremely difficult.


- Transmission, dissemination

TSWV can be transmitted by several species of thrips according to the persistent mode (circulatory-propagative). At least 10 species of thrips have been recognized as vectors: Frankliniella fusca (Hinds), Frankliniella occidentalis (Pergande), Frankliniella schultzei (Trybom), Franklinella intosa (Trybom), Frankliniella tenuicornis (Uzel), Thrips tabaci (Lind.), Thrips palms (Karny). Scirtothrips dorsalis (Hood) has been identified as a vector in India, and Thrips setosus (Moulton) in Japan. Frankliniella bispinosa is a vector more recently involved in Florida, where it is highly present in tobacco crops, and as efficient as F. occidentalis in transmitting TSWV. It should be noted that the transmission efficiency of TSWV by different species of vector thrips is not the same.

In France , only Frankliniella occidentalis seems to intervene. The latter is a very efficient vector, much more than Thrips tabaci . Native to the western United States and Canada, F. occidentalis , dispersed throughout the North American continent, then in Europe from 1985. This thrips is currently considered to be the main vector of this virus in very large geographical areas spread over five continents, especially in protected crops in countries with temperate climates (United States, Europe ). Indeed, it is very prolific and difficult to control.

Only the larvae are likely to acquire the virus, in at least 15 minutes. Inoculations take place mainly during the feeding of adults. In F. occidentalis as in T. tabaci , the females accumulate more virus but it is the males which transmit it with the greatest efficiency. The larvae bite the epidermal cells, inject saliva causing the lysis of the cell content, and aspirate it. Five to fifteen minutes is sufficient to complete the acquisition, and the latency period lasts at least four days. The virus is retained during the insect's moult and could multiply in the latter. Thrips, which are small in size, travel short distances. They are sometimes carried away by ascending air currents which passively carry them several hundred meters, or even more in certain situations. Adults, whose lifespan varies from 30 to 45 days, are viruliferous until their death. Thrips do not transmit TSWV to their offspring. Symptoms may appear 7 to 14 days after inoculation.

In addition to being vectors of viruses, thrips are insect pests and as such cause various symptoms on tomatoes. You will also find information on their biology there.

TSWV is transmissible by mechanical inoculation , but not by casual contact. It is also affected by seeds and vegetative reproductive organs in several plants. Its transmission by seeds is controversial: if old results report that it would be transmitted at a rate of around 1%, the virus being present on the teguments but not in the embryo, recent work would invalidate this property. Be wary of plants that may have been contaminated in the nursery.

In nature, the risks of infection of a plant by TSWV depend in particular on the attraction exerted by the latter on thrips vectors. Indeed, these insects have preferences for their diet, depending on the season and the local climate.