Tomato spotted wilt virus

3 SYMPTOMS of TSWV on tomatoes — purpling and ringspots.

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Tomato spotted wilt virus affecting tomato fruit

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BY DR CRAIG WEBSTER DEVELOPMENT OFFICER, DPIRD

Tomato spotted wilt virus (TSWV) is a serious disease of many vegetables including tomato, capsicum and chilli but many other vegetables and weeds can also be infected (over 900 hosts are known).

These include common weeds like nightshade and cape weed, which often don’t show very strong symptoms. These plants provide a local source of the virus that allow it to spread into nearby crops. Symptoms of the virus vary but include necrosis, purpling and ringspots on the fruit and new leaves of infected plants. The ringspots are the

3 ADULT western flower thrips feeding on bean leaves have caused the mottled effect.

most common symptom in capsicum and chilli and are seen on the fruit and leaves. These symptoms impact on fruit marketability and can lead to losses of 100%, especially when young plants are infected before fruiting. In tomatoes, ringspots are only seen on fruit and symptoms on leaves but may be absent in resistant varieties. The virus is spread by thrips which, apart from spreading the virus, also directly cause damage to plants they feed and lay eggs on. Many species of thrips can spread the virus but in Western Australia, the main ones are the western flower thrips (WFT), tomato thrips and onion thrips. The rate that they spread the virus is different for each species so it is important to know which ones are present. Others, such as the plague thrips, are not known to be able to spread the virus. Adult and larvae thrips are usually found inside flowers or on the underside of leaves and cause silvery flecked scars with small black faecal spots. Thrips cannot be identified by eye and require specialists to identify the species.

Thrips larvae are small and creamy yellow.

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3 1: Signs of thrips feeding including silvery scars with black spots on underside of capsicum leaves. Thrips larvae are small and creamy yellow. 2: Symptoms of TSWV on chilli — irregular ripening, ringspots. 3: Symptoms of TSWV on capsicum leaf — ringspots and yellowing caused by the virus.

For thrips to spread the virus they have to hatch on an infected plant. Older thrips cannot acquire the virus. Once they feed on an infected plant they incubate the virus for about five days as the virus moves through the thrips body before it can transmit. Once infected with the virus the thrips remain infectious for the rest of their life and can spread the virus to many plants. Temperature affects the rate at which they develop. Only actively feeding larvae and adults can be killed by insecticides, so several treatments may be needed to kill all stages of thrips as they develop. Many populations of western flower thrips have become resistant to many insecticides, so it is important to rotate chemistries.

3 SYMPTOMS of TSWV on tomatoes — yellow ringspots and irregular ripening.

Effective control measures

Control source reservoirs, such as weeds and alternative

crops. Often chilli plants or weeds display few symptoms but act as good sources for thrips to breed on and acquire the virus before spreading to capsicum and tomatoes

Use care when spraying weeds

with herbicide — insects can migrate off dying weeds into crops. You may want to treat weeds with insecticides at the same time to prevent this

Inspect crops and remove plants with symptoms when

seen. These can be treated with insecticide to prevent the insects moving the virus, then buried or burned

Check seedlings are free from thrips or symptoms when

planting, or use seeds instead as the virus is not seed borne

Leave a fallow break between

crops and avoid sowing crops close together to prevent TSWV and thrips moving from older to younger crops

Use yellow or blue sticky traps to monitor for thrips in

crops — especially when plants are flowering and are most attractive to thrips

Take action to control thrips numbers before they become

established — especially before flowering. When they enter flowers they are hard to reach as they remain hidden

Insect proof netting on

entrances (0.192mm size) can prevent thrips movement into greenhouses

When spraying use small

droplets (<100 microns) as this is more effective — use water sensitive paper to monitor droplet size

Spray in the early morning or

late afternoon as leaves will remain wet for longer and provide longer exposure to the thrips Don’t mix insecticides — the chemicals may be dangerous or deactivate each other. It will also encourage resistance to develop I nsecticides only work when

insects are actively feeding

(larvae or adults) but not when pupae are present. Spraying sequentially to kill all cycles is effective but depends on temperature With daytime average temperatures of 10–20°C, sprays should be six days apart, when daytime average temperatures are 20–30°C, sprays should be 3–5 days apart.

MORE INFORMATION

Contact Craig Webster, on 0499 997 563 or email craig.webster@dpird.wa.gov.au or go to https://ausveg.com.au/infoveg/infoveg

search/management-of-thrips-and-tomatospotted-wilt-virus/

This project has been funded by Hort Innovation using vegetable industry levies and contributions from the Australian Government with co-investment from the Queensland Department of Agriculture and Fisheries; Victorian Department of Economic Development, Jobs, Transport and Resources; the Northern Territory Department of Primary Industry and Resources; Western Australia’s Department of Primary Industries and Regional Development; and the University of Tasmania. It is supported by a second smaller project led by the New South Wales Department of Primary Industries and similarly funded by Hort Innovation using vegetable industry levies and contributions from the Australian Government and the NSW Department of Primary Industries.

Department of

Primary Industries and Regional Development