Tomato Pests and Diseases - What's Eating / Killing my Plants?

Although tomatoes are very easy to grow and are very rewarding there is a vast number of  pests and diseases that can attack your precious crop.  Fortunately for the amateur gardener, most of these are unlikely to cause a problem as most are easily preventable or simple to treat.

Tomato Diseases

If you have a nervous disposition the I would suggest that you do not read the next sentence. There are over 30 diseases that can afflict tomatoes, they can get early or late blight, either white or grey mould (or both), then there is the quirky stuff like corky root rot and curly top. However, it is unlikely that you will come across more than two or three of these and it is really only the commercial growers that need to worry about this long list. Also for the home grower, there are some very simple things that can be done to prevent disease in tomatoes.

Avoiding Tomato Diseases

As healthy plants are much more likely to resist disease and other problems, giving your plants the right soil and fertilizer along with regular watering, you will find that you will have a mainly trouble-free time growing your tomatoes.

If growing outdoors keep the plot free of weeds and debris where insects and diseases can flourish.

Rotate crops each year so that soil-borne diseases cannot get established, if growing indoors (e.g. in a greenhouse) change the soil each year.

Keep your gardening tools and equipment clean, this stops the spread of disease (and the tools last longer as well)

Remove any unhealthy leaves as soon as you see them, and remove any unhealthy plants if needed.

Do NOT compost any diseased plant or leaves

Do not water the foliage of your plants, as most diseases thrive in damp conditions.

Leaf Mould

This is a fungal infection that attacks the foliage of tomatoes and is extremely common amongst tomatoes grown in greenhouses. You may sometimes see this fungus referred to as Fulvia fulva or Cladosporium fulvum, but it is now known as Mycovellosiella fulva.  The main symptoms of tomato leaf mould are yellow patches on the upper side of the leaves, with a pale, greyish-brown mould (sometimes purplish) on the underside of the infected leaf.  In severe outbreaks, the mould may also be seen on the upper surface of the leaf.  Eventually, the infected leaves will turn brown and shrivel but tend not to drop.  Occasionally the flowers and fruit may be attacked but this is very uncommon, an infected plant will give reduce yield and lack vigour in growth.

The tomato leaf mould fungus produces a huge number of microscopic spores on each affected leaf, which can be spread by air currents, insects, hand and clothing.  These spores are resistant to dry conditions can easily overwinter on the surfaces of the greenhouse structure and on any debris left in the greenhouse, the disease develops and spreads rapidly in warm high humidity conditions.  Plants that have had the soil dry out and then soaked tend to suffer more severe attacks.

Leaf Mould - Non-chemical control

By providing plenty of ventilation, especially at night, to reduce the humidity in the air, and only watering in the early morning avoiding getting the leaves wet, will greatly reduce the risk of infection.  Do not let the soil dry out as this will weaken the plants and make them more susceptible to attack.  If at all possible keep the temperature below 21°C (70°F).  In order to increase airflow remove some of the lower leaves once the fruit has set. At the end of the season remove all plant debris and clean the greenhouse structure of a disinfectant such as jeyes fluid (tar oils) always follow the manufactures instructions on usage.

Leaf Mould - Chemical control

There are some copper based fungicides that are approved for use on tomatoes ( e.g. bordeaux mixture) however these do not carry specific recommendations for dealing with tomato leaf mould, follow the manufactures instructions for dealing with tomato blight (potato blight).  If it is towards the end of the cropping period then treating with chemicals will not have any effect on preventing yield loss, but may still be useful in that the number of spores will be reduced hence lowering the number to be removed at yearend clean up.

Grey Mould

Grey Mould (Botrytis) affects many plants not just Tomatoes.  It is a fungal disease that thrives in excessively moist conditions, such as in an unventilated greenhouse. The spores are around all the time and are air-borne and infect a plant through a wound, (such as caused by removing the side shoots).  Grey Mould will affect all upper parts of a plant (leaves, stem and fruit).  The symptoms of Grey Mould are patches of grey fur, underneath which the plant tissue rots.  As the infection matures black sclerotia about 2mm across are produced, which fall to the ground and act as a place for the spores to overwinter.

As there are no fungicides are approved for use against grey mould by amateur gardeners,  prevention is the only real way to fight this disease.  So keep the greenhouse well ventilated, do not overcrowd the plants, remove all infected  leaves, fruits or plants immediately.  The use of jeyes fluid (tar oils) as a soil sterilant may kill the sclerotia as part of winter hygiene in the greenhouse.

Water or Ghost Spot

This is caused by the spores of Grey Mould infecting the fruit and then drying off as the conditions get warmer and drier. It shows as transparent rings on the stems and the fruit and does not cause any real damage.  To prevent it follow the advice above for preventing Grey Mould and also avoid getting water on the setting fruit.

Blight

This is the same as potato blight, so if growing outdoors avoid growing them together or near each other, also avoid growing where potatoes grew the previous year. You are much less likely to get blight on our tomatoes if you grow them in a greenhouse.  Infected plants develop dark brown to black patches on the leaves and may also have brown patches on the green fruit, more mature fruit will decay rapidly.  Destroy all infected plants as soon as you see them.

Because the risk of infection is so dependent on specific combinations of temperature and rainfall that periods of high risk (blight infection periods or Smith Periods) can be predicted accurately. Gardeners are able to access these warnings but must rely on a more restricted range of protectant fungicides containing copper (Bordeaux Mixture or Fruit and Vegetable Disease Control) since the more effective systemic products are not approved for amateur use.  When wet weather is forecast from June onwards, protectant sprays are advisable, especially for outdoor tomatoes.

Verticullium Wilt

This is a soil-borne fungal disease that affects the roots of the plants, the first signs of infection are the wilting of the top leaves in hot weather and then the lower leaves start to turn yellow.  After a while, the whole plant wilts and becomes permanently limp.  If you cut through the stem just above soil level you will see brown staining in the internal tissues.  There is no cure for infected plants and they should be carefully removed and destroyed, avoiding spreading the infected soil.  So again prevention is the byword here keep weeds under control, keep your boots and tools clean to avoid spreading infected soil.

Stem Rot

This is caused by the fungus didymella, which infects the stem of the plant just above soil level.  The infection normally shows up as dead rotten patches that can girdle the stem.  Infected plants then wilt and die.  If the disease is caught early enough then it may help to spray with captan. although it is probably better to remove and destroy the infected plant.

Tomato Viral Diseases

Worldwide there are over 20 viral diseases that can affect the tomato plant, for which there is no cure.  Any infected plant should be removed and destroyed.  These viral infections cause a wide variety of mosaic patterns and distortions to the leaves, stunted growth and marbling patterns on the fruit.

Spotted Wilt

This is the most serious of the viral diseases affecting tomatoes as it spreads rapidly and can destroy a crop.  The young top leaves of the plant turn brown with concentric rings appearing on them as well.   This virus is spread by thrips so controlling these will help to prevent the spread of this virus.

Mosaic Virus

This shows as pale green or yellow mottling on the leaves which may also curl and distort.  Another symptom is that the fruit will fail to set or will have a bronzed patchy appearance.

Enation Mosaic Virus

This virus causes the leaves to be so badly distorted that they will become long thin curled and twisted threads, mainly affecting the leaves at the top of the plant.

Pepino mosaic virus

This is a relatively new virus in Europe and in the UK has quarantine status, see the Defra website for more information.

Tomato Environmental Issues

There are a number of physiological problems that can affect tomatoes, which are caused by adverse environmental conditions rather than by disease or pests.

Blossom End Rot

Blossom end rot shows as a circular patch, varying in colour from greenish-brown to black, at the end of the fruit that had the flower (the end furthest from the plant). As this patch increases in size, it becomes sunken so that the fruit has a flattened appearance at the affected end. The blackened patch varies greatly: in some fruits, it is only 1cm (1/2in) across, while in others it is 2.5cm (1in) or more in diameter.

Blossom end rot is due to a lack of calcium reaching the fruit and the main reason for this is lack of water flow through the plant. Tomatoes that are grown in pots or growbags (where there is limited root space) are more likely to suffer from this, as they are more likely to have the soil around the roots dry out and hence stop the flow of water through the plant.

To avoid this problem keep the soil evenly moist at all times, and during hot days it may need watering two or three times during the day, which is better than one double dose.  Keep the air humidity down by proper ventilation.

Once a fruit has blossom end rot there is no cure for it.

Greenback

The fruits have hard green patches around the stalk which never ripen, this is generally caused by either too much sun, excessively high temperatures and/or a poor feeding regime.

Blotchy ripening

Parts of the fruit remain orange, yellow or pale green and never ripen, it is also known as whitewall, the causes are the same as for greenback.

Split Fruit

This is the result of irregular watering, either a lot of water has been applied after a dry spell in the greenhouse or heavy rain follows a drought outdoors.

Catfacing

This is the result of irregular temperatures during the fruit setting period and is identified by the corky scars and irregular concavities in the fruit.

Tomato Pests

Moth

The green or pale brown caterpillars of the tomato moth feed on the leaves and fruit causing a lot of damage. Remove the caterpillars by hand.

Eelworm

Both the potato root eelworm and the rootknot eelworm can affect tomato roots, the infected plant will have stunted growth, discoloured leaves and be severely wilted.  Plants infected with eelworms will usually have tiny cream-coloured cysts on the root.

Red Spider Mite

Tomatoes grown under glass are very susceptible to red spider mite. The mites lay their eggs on and feed on the underside of the leaves producing a reddish mottled look. In heavy infestations, you may see fine silk webbing on the plants, and the leaves lose most of their green colour and dry up or fall off. Heavily infested plants are severely weakened and may die.

There is no chemical control of red spider mite that can be used on edible plants.  Although plants sprayed with plant oils, plant extracts or fatty acids have shown signs of reduced infestation.   Biological control in the form of a predatory mite (Phytoseiulus persimilis) which feeds on the eggs and active stages of glasshouse red spider mite is an attractive alternative to using insecticides as it avoids resistance problems and the risk of spray damage to the plants.

Whitefly

Whitefly is a common sap-feeding pest, mainly of houseplants and greenhouse plants. Whiteflies excrete a sticky substance (honeydew) on foliage, which allows the growth of sooty moulds. The adult flies, which look like tiny moths, lay their eggs on the underside of the leaves.  Because Whitefly have become resistant to most pesticides biological control is often the most successful.  This is achieved by introducing a tiny parasitic wasp, Encarsia formosa, which attacks the Whitefly nymphs. The parasite is available by mail order from the suppliers of biological controls. It is important to introduce the parasite before plants are heavily infested as it cannot give instant control. Parasitised nymphs turn black so it is easy to monitor the progress of the control. As Encarsia is killed by most insecticides, do not use it in conjunction with chemical sprays.  Hanging yellow sticky traps among the plants will help to catch the adult flies.  If you do want to spray then you will need to apply frequent doses of plant extracts, plant oils, fatty acids or urea/mineral lattice.