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Spotted wilt & viruses

Affected leaf and thrips

Affected leaf and thrips

Affected fruit



Affected fruit

Affected lettuce

Affected lettuce

Affected fruit

Affected fruit

General information

Tomato spotted wilt virus (TSWV), first described from tomato in Australia in 1915, is the type or reference virus for the Tospovirus group of plant viruses. Tospoviruses cause major economic losses in a wide range of vegetable, field and ornamental crops throughout the world.

At least 16 tospoviruses are known worldwide and all are transmitted by thrips. An important reason for the increased importance of the group has been the distribution worldwide of the western flower thrips (Frankliniella occidentalis), a very efficient vector or carrier of TSWV and several other tospoviruses.

Capsicum chlorosis virus (CaCV) is a member of the watermelon silver mottle virus or serogroup IV group of Tospoviruses. Members of this group are widespread and damaging throughout Asia. CaCV was first described from Queensland in 1999 and occurs in all coastal and sub-coastal vegetable production areas in Queensland. The virus has also been found in north-eastern, coastal areas of New South Wales and at Kununurra in the Ord River area of Western Australia.

Iris yellow spot virus IYSV) is primarily a pathogen of onion and related allium species.

Cause

Three tospoviruses have been identified in Australia: tomato spotted wilt virus (TSWV), capsicum chlorosis virus (CaCV) and iris yellow spot virus (IYSV).

Symptoms

TSWV causes a range of symptoms, depending on the plant species affected. Symptoms are influenced by the age of plants, weather conditions and nutritional status. Symptoms include ringspots, mottling, chlorotic blotches and line patterns on leaves. Both leaves and fruit are often distorted with dark spots or ring patterns on fruit. Wilting and purpling of leaves can occur and necrotic lesions can develop on stems of affected plants.

The symptoms of CaCV are somewhat similar to those of TSWV but distinct differences do occur. Younger leaves develop marginal and interveinal chlorosis and are narrowed and curled often giving leaves a strap-like appearance. Older leaves are chlorotic and frequently develop ringspot and line patterns typically seen with TSWV. Fruit from infected plants is small, distorted and frequently has necrotic spots and scarring over the surface. Infected plants are stunted and do not develop the chlorotic blotching generally seen on young leaves of plants infected by TSWV.

On onion, IYSV causes eye-like or diamond-shaped spots on leaves and the seed stalk, which often bends as the spot dries causing the seed stalk to collapse.

How does it spread

All viruses in the Tospovirus group are transmitted by thrips, minute, slender insects belonging to the insect Order Thysanoptera. The thrips/tospovirus relationship is very specific and of the many thousands of species of thrips identified throughout the world, less than 20 species are known to transmit tospoviruses. The four vector (or carrier) species found in Australia are:

  • western flower thrips (Frankliniella occidentalis)
  • tomato thrips (Frankliniella schultzei)
  • melon thrips (Thrips palmi)
  • onion thrips (Thrips tabaci).

CaCV is transmitted by melon thrips and tomato thrips in  Ceratothripoides claratris, a thrips species . Transmission of IYSV appears to be exclusively by the onion thrips (Thrips tabaci).

Tospoviruses are not spread by other sap-sucking insects such as aphids, whiteflies and leafhoppers.

Transmission of tospoviruses can only occur if they are acquired from infected plants by first or early second instar larvae thrips. More mature thrips, including adults, may acquire the viruses but the viruses cannot complete their life cycle within the insect to allow transmission. The larvae can acquire the virus during feeding periods of less than 30 minutes.

Once acquired by immature thrips, the viruses circulate and multiply within the insect and are transmitted to plants as the adult thrips pierce and suck the contents of plant cells. Thrips remain infective for life but do not pass the virus to their offspring through the egg.

About five days are required from the time the virus is acquired from an infected plant until the thrips is able to transmit the virus to another plant. This allows time for the virus to move and multiply in the insect gut and salivary glands. Long feeding periods are not required for thrips to transmit the virus and efficient transmission occurrs in 5-10 minute feeding periods.

Tospoviruses are not spread in seed or on cutting and cultivation equipment. They are not spread by handling plants and do not survive in soil or decaying crop residues. Tospoviruses can be spread in infected plant parts used for propagation such as cuttings and bulbs.

Crops affected

It has one of the largest host ranges of any plant virus, infecting over 900 species of weeds, field crops, vegetables and ornamentals. Many hosts are in the potato (Solanaceae), aster (Asteraceae) and legume (Fabaceae) families.

Crops that frequently suffer major losses from TSWV isnclude lettuce, capsicum, tomato, potato, and ornamental crops such as aster, statice, calendula and dahlia. Other crop hosts that have sporadic losses include peanut, tobacco, chickpea, rhubarb, eggplant, celery and a range of ornamental species.

Weeds have a major role in the life cycle of both the virus and the thrips vectors. Many weed hosts of TSWV occur  and include sowthistle (Sonchus oleraceus), capeweed (Arctotheca calendula), thornapples (Datura spp.), cobbler´s pegs (Bidens pilosa), nightshades (Solanum spp.) and Jamaican snakeweed (Stachytarpheta jamaicensis).

CaCV infects capsicum, chilli, tomato, Hoya and peanut. It also infects several weed species including Ageratum conyzoides (Billygoat weed), which is a common and symptomless host throughout coastal Queensland.

IYSV is primarily a pathogen of onion and related Allium species. The virus occurs in the United States, Brazil, Israel, Japan and Europe and was first found in Australia in 2003.

Control options

Infected plants cannot be cured. Management aims to prevent or reduce the levels of disease in crops by removing or avoiding sources of virus infection and minimising spread by thrips.

Crop and farm hygiene 

Old, infected crops infested by thrips are a major source of virus and should be removed as soon as possible, particularly if young crops are to be planted nearby. Weeds along headlands and irrigation channels provide host plants for thrips and tospoviruses. Disease levels are often higher adjacent to these areas. Maintaining a buffer zone of at least 25 m, which is free of weeds between a virus source and a susceptible crop can considerably reduce virus levels.

Controlling thrips with insecticide 

Reducing thrips populations with appropriate insecticides can help reduce virus spread. However, insecticides are often of limited value in tospovirus control because there is significant spread from outside sources and only a short feeding time is required for transmission. Frequent use of insecticides may also lead to thrips populations developing resistance.

Use healthy planting material 

Viruses can be introduced in infected seedling plants which then provide a virus source through the life of the crop. Seedling production areas should be located well away from production areas, kept weed-free and a regular insect management system used. Thrips-proof netting or UV-absorbing plastic provides a higher level of protection for seedling production.

Resistant varieties

Varieties of capsicum and tomato resistant to TSWV are available. If using these varieties, care should still be taken with crop-farm hygiene and other preventative measures to prolong the useful life of the resistance sources.

Chemical registrations and permits

Check the indian Pesticides and Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this disease on the target crop in your state or location. Always read the label and observe withholding periods.

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