Tomato Spotted Wilt and Impatiens Necrotic Spot - Viruses spread by thrips

By Roger Jones, Plant Virologist, Plant Protection Branch, South Perth Western Australia.

Recently the western flower thrips, Frankliniella occidentalis, an insect that feeds on many horticultural crops and weeds, was found in Western Australia. Its appearance here means that diseases of plants caused by viruses that are transmitted by thrips may become more serious problems for the State's vegetable and ornamental industries.

Tomato Spotted Wilt Virus

Tomato Spotted Wilt Virus (TSWV) has been found affecting ornamental and vegetable crops in Western Australia since the 1920s. The first record of the virus in the State appears to be for tomato in 1923.

There were records for TSWV on tomato, potato, lettuce and tobacco in the 1930s, while in the 1950s and 1960s it was first recorded infecting begonia, cornflower, chrysanthemum, dahlia, delphinium, lupin, poppy, petunia, nasturtium and Scarborough lily. Subsequently, marigolds, calendula, aster, helichrysum and capsicums have also been found infected in the State. Other introduced and native plants and many weeds are probably infected also but have not been tested.

Impatiens Necrotic Spot Virus

Recently the two distinct forms of the virus ('lettuce' and 'Impatiens' strains) have been given different names, with the 'Impatiens' strain being classed as a distinct virus. This has been called Impatiens Necrotic Spot Virus (IMNSV).

TSWV occurs on ornamentals and vegetables but IMNSV occurs mainly on ornamentals. It is unclear whether IMNSV is present in Australia. Only TSWV has been found so far, but few tests to separate the two viruses have been done.

Symptoms

Symptoms of TSWV and IMNSV vary from species to species and variety to variety. There are many variants of these two viruses and these cause variable symptoms in the same plant species. The sensitivity of varieties within the same plant species is also variable.

The expression of symptoms is influenced by environmental conditions, nutritional levels and the age and stage of the plant when it becomes infected. Infected plants grown at high temperatures may be symptomless, but develop strong symptoms when moved to a cooler environment.

Plants grown from infected cuttings tend to show symptoms more readily than the mother plants they came from.

Plants diseased with TSWV and IMNSV show one or more of the following types of symptoms:

irregular necrotic (dead) spots on leaves,

discoloured veins,

black or purple stem streaks,

chlorosis (yellowing), chlorotic blotching, chlorotic or necrotic ringspots and line patterns on leaves,

leaf distortion and deformation,

dropping of leaves or shedding of buds,

dieback and leaf collapse, and

stripes on petals.

Also common are:

stem browning,

generalised necrosis and

an overall stunting of the plant.

A combination of symptoms can result in stunted growth and death of the plant. Yield can be greatly reduced and quality badly affected. However, symptomless infection can also occur.

Transmission by thrips

TSWV and IMNSV are unusual among plant viruses, since they are the only ones to be transmitted by thrips. The traditional thrips vectors of TSWV in Australia are the onion thrips (Thrips tabaci) and the common blossom thrips (Frankliniella fusca).

In May 1993 the western flower thrips (Frankliniella occidentalis) was found for the first time in Western Australia. Western flower thrips has previously not been recorded in Australia, but has been present in New Zealand for more than 50 years. Its appearance in Western Australia means that TSWV is likely to become a more serious problem for the vegetable and ornamental industries in the future, rather than being only a sporadic disease problem as in the past. Also, IMNSV is likely to become widespread.

is a very efficient vector of TSWV and IMNSV,

has a very wide host range,

develops resistance to insecticides easily, and

can reach very high numbers on its host plants.

Its introduction to southern European countries in recent years led to a dramatic upsurge in losses caused by TSWV and IMNSV in vegetables and ornamentals.

Spread

TSWV and IMNSV are spread from plant to plant by thrips. Most spread occurs at warmer times of year, which is when the thrips populations tend to build up.

Only the larval stage of the thrips life cycle can acquire the virus, while only the adults transmit it. Thrips larvae must feed on an infected plant for 20 to 30 minutes to become carriers. Once they have picked up virus, they transmit virus to the healthy plants they feed on for the rest of their adult life (30 to 45 days).

Only a few thrips are needed to effectively transmit virus throughout a crop, well below the number of thrips needed to cause significant feeding damage.

Alternate hosts occur among cultivated and wild plant species. These act as sources for spread by thrips to susceptible plants. The role of native species in Western Australia is unclear, but some are likely reservoirs of TSWV and IMNSV.

Vegetatively propagated crops effectively pass TSWV and IMNSV from one generation to the next.

TSWV is not transmitted through pollen or seed.

Depending on the crop or variant (strain) of virus involved, infected plants can remain symptomless, or have a long latent period before symptoms are expressed. Symptomless hosts cannot be rogued out of a crop and thus act as a source for virus spread.

Detection

Diagnosis of TSWV and IMNSV is difficult. A reliable diagnosis can be made by combining the results of several tests - enzyme-linked immunosorbent assay (ELISA), indexing using a range of indicator plants and electron microscopy.

Control

An integrated program is needed to control TSWV and IMNSV. None of the measures mentioned below can solve the problem on their own; a combination of them can more effectively restrict virus spread and consequent losses.

To control TSWV and IMNSV:

vegetatively propagate from healthy stock plants,

grow plants from seed where possible,

rogue plants for virus symptoms on a regular basis,

destroy infected material immediately,

control potential weed hosts, and

control populations of thrips within the crop and on surrounding crops and weeds.

Thrips management is important. Only a few thrips are needed to spread the two viruses. If thrips are present, an effective insecticide spray program is needed to keep populations at a minimum and reduce the rate of virus transmission.

Rotate insecticides to prevent the build-up of resistance by the thrips.

Keep surrounding areas free of weeds, since many weeds are hosts to both the viruses and thrips.
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