Advice
RHS Help & Advice
Virus diseases of tomatoes
There
are numerous viruses that affect tomatoes.
Some of the more common or damaging viruses are described here. Other possible hosts are as follows:
Strains of tomato mosaic virus (TMV): herbaceous plants, tobacco and potato. The most troublesome virus disease of tomatoes.
Cucumber mosaic virus (CMV): herbaceous and woody ornamentals such as lilies, delphiniums, primulas and passion flower. Cucumber (right) and other cucurbits (marrows and pumpkins) are particularly susceptible.
Tomato spotted wilt virus (TSWV): many species in the tomato family (Solanaceae) and herbaceous plants, including gloxinia, arum lily and dahlia.
Pepino mosaic: A recent problem, first detected in the Netherlands and the UK in 1999. It has so far been confined to commercial plantings, but gardeners who suspect that they have this problem should seek advice from the local DEFRA Plant Health Inspector.
Symptoms
Mottling or distortion of leaves and fruit, stunting of growth and poor quality and yield of fruit. As similar symptoms can be caused by more than one virus, the diseases can be most conveniently classified into groups under the type of effect seen.
Stunted and distorted leaves can also be caused by exposure
to hormone weedkiller (right), including the vapour from lawn treatments
that include these products. If this is the cause of the symptoms, plants may recover once the
source of contamination is removed.
Leaf symptoms and causal virus
Mosaic (TMV/CMV): scattered
patches of light green or yellowish-green discolouration.
Fern leaf (CMV, alone or with TMV): leaf
blades are reduced to half their width or are not formed at
all so that the midribs stand out.
Streak (TMV): brown
areas on the leaves which eventually shrivel. Dark longitudinal
streaks are also produced on leaf stalks and stems penetrating into the tissues
so that brown marks show in the pith when the stems are cut
open.
Spotted wilt (TSWV):
young leaves curl downwards soon after infection and the plant
ceases to grow. A bronzed appearance to the leaves, or the production of concentric
bronze-coloured rings. This symptom should not be confused
with bronzing of the fruit.
Pepino mosaic:
distortion and mosaic with a bubbling of the leaf surface.
Plants may be stunted, light green, or spiky.
Fruit symptoms and causal virus
Dry set (TMV): failure
to set fruit.
Bronzing: the most
serious symptom caused by TMV. Brownish patches develop beneath
the surface usually at the stalk end and give a bronzed patchy
appearance to the young fruit.
Streak (TMV plus a strain of potato virus X): slightly
raised brown spots in addition to bronzing.
Mosaic tomato spotted wilt (TSWV):
fruit ripens unevenly causing pale, often yellow, areas of
irregular mottling or distinct circles, contrasting with the
normal red of the ripening skin.
Pepino mosaic:
marbled pattern with areas of lighter skin.
Seedling symptoms
Tomato seed is occasionally infected by TMV. Young seedlings cease growing and develop narrow leaves with some mosaic. In the winter, stunting, distortion or yellowing along the veins of the leaves are the dominant symptoms rather than mosaic.
Transmission of viruses
Viruses require a ‘vector’ to introduce them into the plant. Insects are very common virus vectors, but for many of the tomato viruses it is often the growers themselves that inadvertently spread the problem. Transmission methods for viruses affecting tomatoes are as follows:
Cucumber mosaic virus:
spread by aphids, and occasionally by handling infected plants (mechanical transission).
Tomato mosaic virus:
mechanical transmission. This virus is extremely infectious, transmitted on fingers, tools and clothes during handling and trimming of plants. Also occasionally carried on seed (see above).
Pepino mosaic virus: mechanical transmission.
Tomato spotted wilt virus:
spread by thrips,
particularly the western flower thrips (WFT).
Potato virus X: mechanical transmission.
Control
Hygiene: If signs of seed infection are noticed before planting out, the affected plants should not be touched until planting out is complete, when they should be destroyed.
Any plant showing symptoms of spotted wilt should be destroyed as soon as the first symptoms are seen.
It is probably not worthwhile removing older plants infected with TMV, because considerable spread of the virus may have occurred already. TMV-infected plants may produce normal fruits later.
Plants that are obviously affected by virus should be handled last, after which hands and instruments should be washed thoroughly with soapy water.
Tools may be sterilised in Virkon S as an additional precaution (available to gardeners from farm suppliers).
In a greenhouse, TMV symptoms may be reduced by increasing temperatures slightly. Raising them so that they are frequently above 25°C (77°F) may also alleviate symptoms due to CMV.
At the end of the season all crop debris should be removed and destroyed. New plants should be grown on a fresh site, but if tomatoes have to be grown in the same situation every season then the growing medium should be changed completely before planting the next crop.
If symptoms reappear in successive seasons despite taking these precautions, it is possible that the virus is being harboured by another plant species, such as an ornamental plant. Avoid growing other types of plants with tomatoes in a greenhouse if maximum tomato yields and quality are desired. The growing of potatoes, including self-sown plants, in the vicinity of tomatoes, should be avoided.
Resistant cultivars: 'Cherry Wonder', 'Cumulus', 'Dombito', 'Dona', 'Estrella', 'Ida', 'Nimbus', 'Piranto', 'Shirley', and 'Sonatine' are resistant to TMV. These cultivars are resistant to currently known strains of the virus, although it is possible that new strains could develop.
Virus
testing kits are available from Forsite Diagnostics