Tomato spotted wilt virus in potatoes

Tomato spotted wilt virus (TSWV) is common and widespread throughout Australia. It affects around 500 species including:

• crops
• ornamentals
• weeds.

In potatoes, it's particularly important that the seed crops grown for seed tubers are free of TSWV. Otherwise the virus can be passed on to successive crops in the seed tubers.

TSWV can cause smaller tubers, which reduces crop yield. Tuber distortions and internal necrosis triggered by the virus also affect crop quality and reduce marketable yield.

Symptoms of TSWV

Symptoms caused by TSWV can vary greatly according to the strain of virus and potato variety. The following symptoms can indicate TSWV.

On potato plants:

• Dead (necrotic) spots or patches on the leaves. These can appear as concentric rings separated by green tissue, which can lead to confusion with target spot (early blight).
• Stems that are severely affected might die – and occasionally the whole plant dies.
• Plants grown from infected tubers are severely stunted, with a rosette form of growth and dark green leaves (see Figures 1 and 2).

On potato tubers:

• Potato tubers can grow normally after TSWV infection, but may be small and distorted and show sunken, black necrotic spots.
• Internally, tubers may have hollow necrotic centres, dark shadowing and necrotic spots.
• These spots may appear as concentric rings and be visible through the skin (Figure 3). But it's also possible for infected tubers to show no external symptoms.

Spread of infection

TSWV is spread in potato crops by:

• planting infected seed tubers
• some species of thrips, which are small, flying insects about 1mm to 2mm long

Infected potato seeds (tubers) pass on TSWV

TSWV can be carried from one potato crop to the next through infected seed (tubers). But some potato varieties are much more likely to pass the virus on to the next crop than others. For example:

• Shepody – 84% carryover
• Russet Burbank – 32% carryover
• Atlantic – 16% carryover
• Coliban – less than 5% carryover

Thrips

Organisms that are responsible for spreading a disease are known as vectors of the disease.

Several species of thrips are well-established in Australian potato districts as vectors of TSWV.

In Victoria, TSWV is mainly spread by the:

• tomato thrips (Frankliniella schultzei) 
• onion thrips (Thrips tabaci)

But the most effective vector of TSWV is the western flower thrips (WFT), Frankliniella occidentalis (Figure 4). WFT has not yet been found to be a problem in Australian potato crops, since potato seems to be a non-preferred host plant.

WFT is particularly resistant to chemical pesticides. Growers must monitor their crops for both TSWV and thrips.

How thrips spread TSWV

Thrips transmit the disease from infected plants to healthy potato plants in the following ways:

• If thrips lay eggs on infected plants, the larvae that hatch from the eggs can pick up TSWV as they feed by puncturing the surface cells and sucking out the contents. Larvae must feed for more than 15 minutes to pick up TSWV from the plant.
• As the larvae mature into adults, they fly off and feed on other plants (including potatoes), spreading the virus as they do. Adult thrips must also feed for more than 15 minutes to infect a plant.

The adults tend to be more attracted to plants in flower, since they also feed on pollen.

Monitoring crops for thrips

Populations of thrips tend to increase during spring and summer, when conditions are warm and moisture levels are high. Numbers can build up rapidly. In large populations, the process of thrips feeding can damage the crop regardless of whether they have TSWV.

Detecting thrips by sight is difficult, as they tend to hide in the flowers. You can monitor your crops for thrips by shaking or tapping potato or weed plants (particularly flowers) over a:

• sticky trap
• container
• sheet of paper.

In susceptible areas, place sticky traps around the crop to monitor changes in the thrips population.

Managing TSWV and thrips

Viral diseases such as those caused by TSWV cannot be readily cured, but you can take the following measures to keep both the virus and its vectors under control.

Keep in mind that TSWV is most effectively managed by controlling the source of the virus rather than the thrips.

To prevent TSWV at the source:

• plant only certified seed
• control weeds in and around the crop
• avoid overlapping crops
• if possible, avoid planting the varieties that are most likely to carry over TSWV to other crops
• keep the property free of old crop residues and volunteer potatoes
• inspect the crop for infected plants and rogue them out (this is often only economically viable for seed crops).

To manage thrips:

• plant crops very early or late in the season, so that young, vulnerable plants are not exposed to peak thrips infestations
• keep the property free of old crop residues and volunteer potatoes
• control weeds (especially flowering ones) in and around the crop
• do not bring plant material (including cut flowers) onto the property unless it's been checked for western flower thrips
• monitor the crop for thrips and for changes in thrips populations.

Some other methods for controlling thrips include:

• chemical control – use chemicals very carefully. Thrips are hard to get at with sprays and can also be resistant to chemicals or only susceptible at some stages of their life cycle. Because some thrips species (such as WFT) are more resistant to chemicals than others, it's important to have the species identified before considering chemical control
• apply granular systemic soil insecticides at planting to protect young plants (though this is not always effective)
• apply insecticide sprays during periods of peak thrips activity, which you can monitor with sticky traps – because harmless species can also be active at these times, you must make sure the species is correctly identified before using insecticides.

Photo credits

Figure 3 courtesy of the Plant Virology Group at Agriculture Western Australia.

Figure 4 courtesy of Peter Gillespie, NSW Agriculture.