Identify, prevent and treat common tomato diseases 

Growing your own tomatoes can be easy and rewarding, however there is always the chance that a tomato plant will contract a disease. This could impact the success of your tomato harvest. There are many ways in which to prevent and manage tomato plant health to ensure you still get the best tomato crop, even when facing a disease or pest attack. During this article we are going to look at how to identify the most common tomato plant diseases in the UK and garden pests that can affect a tomato plant and its production rate. We will cover how to prevent and treat these common tomato problems too. 

A tomato garden filled with delicious red fruits is indeed a sight for soar eyes. However, growing this plant from seed to healthy produce can be challenging, considering tomatoes are sensitive to various diseases. They can affect any part of the plant, and if you do not take proper steps to prevent and eradicate them, they can ruin the entire crop. 

Common Tomato Diseases in the UK 

Learning about different tomato diseases and their causes will help you grow a beautiful and healthy garden. It is even more important if you are cultivating other vegetables nearby since some of them are susceptible to the same infections as tomatoes. Depending on the cause, most of them fall under one of the following categories:

• Fungal diseases
• Bacterial diseases
• Viral diseases

Tomato Blight 

Also known as late blight, comes from a microorganism which thrives in moist and cooler conditions.

Outdoor tomatoes are more so affected by late blight; however, I have found that it is just as easy for greenhouse or indoor grown plants to get late blight. This could happen if a door or window has been open on a windy day which would allow the traveling spores to enter and infect the plants. Wet and damp conditions create the perfect atmosphere for late blight to thrive. Last year my polytunnel crop was badly affected as I had left the door open on a few, extremely hot days to allow air flow in. When the door was zipped back up the moisture and humidity inside was very high which meant that the blight spread incredibly fast.  

Late blight starts to show in brown-black patches on the stems and foliage of the tomato plants. This then develops and spreads to all of the plant leaving it looking droopy and black throughout it’s growth. The fruits gain brown patches which can then mould and rot. If the plants are left, the growth eventually goes mouldy too. 

Prevention & long-term treatment 

Watering around the base of plants as close to and on the compost, avoid using the spray function on a hose to minimise splashing around the leaves and upwards as this distributes the blight spores, spreading the disease quickly. Late blight thrives in water patches, keeping leaves dry will ensure blight does not set in as easily. 

Eliminating this disease is a bit complicated. You would probably need to pull out and destroy all infected plants and weeds, and some spores could remain in the ground. Treating the plants with chlorothalonil and copper fungicide will help protect them. Still, to prevent this disease from getting anywhere near your garden, you should plant tomatoes somewhere where they can get a lot of sunlight, leave enough space between them, and avoid overwatering.

Although blight is like a fungus, it is more of a pathogen, fungus treatment spray won’t get rid of the problem. However, I have personally found that spraying a generic domestic fungicide has helped prevent and hold back the spread of late blight. Ideally spray as a precautionary treatment even if there are no signs of late blight, carrying out ‘top-up’ sprays every 2 – 3 weeks during the growing season to help keep blight at bay. 

Once plants begin to shoot up, remove lower leaves especially if starting to yellow. This helps keep air flow around the plants preventing congestion & damage which can lead to blight. Removing the lower leaves also pushes more energy up into the flowering shoots for better fruiting. Remove and destroy effected foliage or growth as the first signs of late blight shows to prevent and limit spread. 

If the blight attack was inside a greenhouse or covered space, always be sure to thoroughly clean down the whole area with warm soapy water and/or disinfectant to kill off any left-over spores. Cleaning after the plants have been removed and then again in spring before the next growing season begins will help prevent an issue with blight returning. If the plants were in pots that are re-used year on year, these will also need a good clean too. 

Septoria Leaf Spot

Septoria leaf spot is a tomato disease caused by the fungus Septoria lycopersici. It will first appear on the oldest leaves, shaped like small greyish-brown spots with dark edges. These spots usually show up after the blooming phase, when the first fruits begin to form. Over time, spots enlarge and start overlapping, causing leaves to become yellow and fall off. In some cases, you may even notice small fruiting bodies in the middle of these lesions.

Treatment and Prevention

Septoria affects only tomato leaves, which means you can slow its spreading by removing infected leaves during the growing season. At the end of the season, you should pull out and dispose of all infected plants. You can also treat plants with copper fungicide as soon as the blossom drops at 7 to 10-day intervals.

To prevent this infection, leave enough space between plants and keep them well-mulched. Once plants are established and start developing fruits, prune off the lower branches. Also, water plants at the base and avoid wetting the foliage. 

Fusarium Wilt

Fusarium wilt is a soilborne tomato disease, caused by Fusarium oxysporum. It infects the plant through the roots. The first symptoms will appear after fruits begin to mature. The infected part of the plant will wilt during the day and somewhat recover over the night. The disease will also cause a stunt growth and yellowing and browning of leaves. It favours warmer weather.

Fusarium wilt will usually affect one half of the leaf or a plant and move to the other side as the infection progresses. The internal tissue of the stem will change colour into coppery red or black, and the plant will die within a week.

Treatment and Prevention

Unfortunately, there is no way to eliminate this disease. If plants in your garden get infected, the only solution is to remove and destroy infected plants.

The first step to prevent this infection is to purchase resistant tomato variants. Like most fungi, fusarium spores can persist in soil for years. Because of that, it is essential to plant tomatoes in sterile, well-draining soil. In case drainage is poor, you can also build raised beds.

Verticillium Wilt

Verticillium wilt is a disease similar to fusarium wilt. However, it is caused by a different fungus, named Verticillium dahliae. The first symptoms of this infection will also appear on the lower leaves, which will become yellow, curl up, and fall off. Unlike fusarium, verticillium thrives in cold conditions and usually attacks the entire plant.

Treatment and Prevention

Since there is no cure for this disease, you should pull out and dispose of infected tomatoes. Verticillium spores can survive in the soil for years, even if there are no host plants. Thus, make sure you choose resistant tomato variants for your garden and plant them in sterile soil.   

Anthracnose

Anthracnose is a fungal infection caused by Colletotrichum phomoides, which usually attacks tomato fruits. In the initial stage of this disease, you will notice small bruised spots on the skin of the fruits. As the infection progresses, these bruises become larger, darker and mushy, and there is also a chance other fungi will infect the fruit interior. In some cases, the entire fruit will rot and fall off.

Treatment and Prevention

Anthracnose develops in warm, humid conditions, and spreads swiftly during heavy rains. To avoid this infection, plant tomatoes in well-drained soil, avoid overwatering the plant, especially once fruits start to form, harvest the fruits as soon as they are ripe, and regularly clean up the debris.

If you live in areas where the humidity is high, you should treat the infection with fungicides. Try using sulfur powders or liquid copper. You can use them as prevention when the first foliage begins to develop and apply them at 7-day intervals. If you prefer an organic option, you can also try applying neem oil. 

Leaf Mould

Leaf mould is another fungal infection that thrives in humidity. The cause of this disease fungus Passalora fulva, also referred to as Fulvia fulva. It usually affects the plants spaced too closely in conditions of poor air circulation.

Like most fungal diseases, leaf mould will first attack the lower leaves and work its way up. It starts as pale green or yellow spots. In high humidity, spots will spread on the bottom of the leaves, which will become velvety and turn grey.

Treatment and Prevention

Leaf mould is similar to other fungal diseases, which means you can start treating it with preventive fungicides. You can either use liquid copper or chlorothalonil. Try increasing air circulation between plants by pruning and spacing them. Keep the foliage dry and always water plants at the base.

Powdery Mildew

Powdery mildew is one of the infections affecting only foliage. It is caused by Oidiopsis taurica fungi, which will appear as a white or grey powder sprinkled on tomato leaves. Some plants infected with powdery mildew may also have pale yellow spots.

Mildew spores block the photosynthetic process of the plant. That causes leaves to dry and fall off in the later stages of infection. Another effect is stunt growth. 

Treatment and Prevention

When it comes to mildew, the good news is that it will not affect the fruits. Still, you should harvest the crops as soon as possible. You can treat the plant with biofungicides or sprays such as neem oil to stop it from spreading further.

It usually affects underwatered or undernourished plants, so provide them with proper care to prevent this infection. Regular watering, mulching, and abundant nutrition will help avoid issues such as mildew.

Bacterial Speck

Bacteria speck is a cold-weather disease, caused by Pseudomonas syringae tomato bacteria. It affects all parts of the plants and in the early stage, you will recognize it by small black or brown spots, surrounded by a yellow halo. Over time, spots grow larger and overlap, causing foliage to yellow and fall off. In severe cases, spots will also appear on fruits.

Nonetheless, bacterial speck is not fatal to the plant, and it fortunately stops progressing as soon as the weather becomes warmer.

Treatment and Prevention

You can control this disease with copper-based sprays, but you will not be able to eliminate it. Sometimes it will become resistant to copper, so you might have to get rid of it before it turns into breakout. This usually happens after heavy rains.

Moreover, bacterial speck can survive in debris and even seeds. To prevent it from attacking your garden, start planting tomatoes when the weather warms up, and start practising crop rotation every three years.

Bacterial Canker

Bacterial canker is considered to be one of the most destructive plant diseases. Clavibacter michagensis bacteria, which causes this disease, is predominantly a greenhouse issue. It can attack plants in any growing stage, and sometimes even certified disease-resistant seeds will harbour it.

The foliage of the infected plants becomes yellow, curls up, and gradually falls off, while stems develop light stripes that turn dark over time. Sometimes, the disease affects the surface of the leaves and fruits covering them with dark, raised lesions with white edges. In most cases, this infection will result in a total crop loss.

Treatment and Prevention

Copper-based products may slow down the spreading of bacterial canker. Still, planting disease-free seeds in sterile soil is the most effective way to avoid this disease. Bacteria can survive for several months in soil without a host, and indefinitely in debris. Like most tomato diseases, it spreads through water, so proper soil drainage and watering are of the utmost importance.

Mosaic Virus

Mosaic virus can stunt growth, cause leaves to grow in misshapen forms, and reduce the quality and quantity of crops. Aside from tomatoes, it attacks other plants such as cucumbers, potatoes, peppers, and even tobacco and roses. 

If the plant is infected with this virus, the first noticeable symptom will appear on the leaves as light green and yellow markings that resemble a mosaic. In the later phases of the disease, fruits become lighter in colour, and the mosaic structure may show up on their skin as well.

Treatment and Prevention:

There is not much you can do for the plant infected with mosaic virus. The only way to treat it is to remove it from the garden before the virus spreads. Pre-treating plants with neem oil can help prevent it.

Mosaic virus usually enters the plant through cuts and open wounds on leaves and stems, so you should always sterilize gardening tools before and after handling the plant. If you are a smoker, it is also a good idea to wear gloves while working in the garden, since the virus can spread through tobacco products as well. Of course, the chances to happen something like that, are pretty slim, yet it is better not to risk it.

Tomato fruit splits 

Lack of hydration can lead to the tomato fruit losing plumpness, making the skin have less elasticity with a dryer and harder skin that can then lead to splitting.² The fruits then become susceptible to other diseases such as moulds, rot and blight from the damaged and exposed tissue caused from the fruit skin splitting.  

Regular watering in the morning or early evening when the day is at its coolest can help keep plants healthily hydrated each day. 

However, not too much water at any one time- I recommend enough to make the soil damp to the touch but not soaking wet.  

Blossom End Rot 

A dark patch or circle can appear at the base of the fruit, where the flower blossom has pushed away from the main stem to create the fruit. This appears if the tomato plant has a calcium deficiency. Usually this can be salvaged, and any parts of the fruit which are not affected are still ok to use simply by cutting off the black ends when harvested and ready to eat. 

Calcium shortage in soil or compost is not the main issue here, it is linked to the lack of water the plant is receiving. Water carries calcium up from the roots into the shoots, stems and fruit of the plant and is integral in the role it plays to ensure all nutrients are being successfully delivered to all areas of the plant, especially those furthest away from the roots/main stem such as flowers or the fruit in this case. 

To aid calcium uptake and prevent blossom end rot in tomato fruit, simply ensure your plants aren’t drying out too much or too regularly.  

Similarly to blight, watering can be key to keeping this disease under control. Just enough water to moisten the soil rather than soaking the compost or letting the compost dry out too much in this instance can keep the tomato fruit healthy and the nutrient uptake running well, preventing blossom end rot from either appearing or continuing. If the greenhouse is humid this can also prevent the plants uptake in water from the roots, the humid air keeps foliage damp tricking the plant into thinking it has enough water when in truth it’s not the correct volume required for healthy shoots, flowers and fruit. Keep the growing area well-ventilated in the daytime to prevent high humidity.² 

But, on windy days it is best not to ventilate to prevent any late blight spores entering inside.  

How to prevent Tomato diseases 

There are a few gardening tricks which can help prevent tomato plants from being attacked by diseases or pests. Simple solutions which help give you a healthier plant with a better harvest as well as preventing common problems. 

Staking & tying in tomatoes 

This aids the plant stem to keep upright, strong and stable, otherwise if left, it can get top heavy and flop over causing damage to the plant. The damage caused can encourage disease to set in whilst it is under stress or has open ‘wounds’. If the tomato plant falls over whilst in fruit, this could also affect the health and success of the fruit produced. Adding in stakes or a frame that the tomato plants can be tied to will aid them as they grow. This creates stability allowing plants to focus on growing, rather than holding themselves up or having to repair themselves following any damage and reserves their energy for producing good, healthy fruit. 

Pinching out side-shoots 

Once the tomato plant has grown on from a young plant moving into more established growth, little green shoots develop where the base of a leaf meets the main stem. These are called side-shoots, which look like a mini tomato plants.  

Pinch side shoots out by squeezing your thumb and first finger against the base and stem of any side shoots, then twist and pull them off to remove them. 

What does ‘pinching out’ of tomato side-shoots do? This focuses the plant’s energy into producing more flowers rather than more foliage growth, creating more fruit trusses on the main, stronger stem for a better and increased tomato crop yield. 

Feeding tomato plants 

Regularly feeding your tomato plants will enhance root health, increasing growth rate and aid the production of flowers to produce more fruit. 

What plant feed is best for tomatoes and when?  

In the spring, once the tomato seedlings have been potted on and then also within the first 4 weeks of being planted out use a generic plant feed such as Growmore or Miraclegro multipurpose liquid plant food. Only choose feeds that state they are suitable for use with vegetable/edible plants. This will give the plant a boost in growth, strengthening it from root to shoot whilst it establishes and settles into its final position. 

After 6 weeks of being planted out into growbags, large pots or a border, begin to feed using a tomato feed. This can be found usually in a red bottle. Tomato food has been specially produced to give the plants a boost in phosphorus and potassium. Both of which promote good root health and, most importantly better fruiting quality. Continue to feed every 2 weeks until your plants have finished for the year. I use a liquid based tomato feed which is easy to pop in a watering can and apply directly onto the soil where the tomatoes are planted. 

There are tomato varieties available that have been bred to be ‘disease’ resistant, check out online seed suppliers for more information on this.  I have found that some of these varieties can still be prone to diseases especially blight, but they tend to contract it later or any damage is minimal which makes it worth a try.  

Other Tomato Problems – Pests 

Keeping pests at bay also helps to prevent disease from setting in. Many insects and living things love to nibble on tomato foliage especially when there is young growth. The damaged plant tissue from this can encourage disease to set in. Alongside this, insects are moving parts meaning that they can transfer and spread disease from plant to plant as they work their way through the garden or greenhouse either eating as they go or just moving about generally.  

Slugs and snails 

Tomato seedlings and the young foliage of a plant are very tasty for slugs and snails. If prevention is not put in place and these pests are left to run riot, their feasting can stump the further growth of a tomato plant. This can lead to the plant rotting off if the foliage has been eaten heavily and the damage caused is too much for recovery. 

Slug and snail barriers are a good way to stop these slimy pests from reaching your tomato plants. I use a thick layer of cleaned and crushed eggshells around the base of each plant which works about 80% of the time for me. Fine grit, copper rings and wool based products are also fairly successful natural and wildlife friendly slug barriers. 

Another option is to place beer traps in pots or in the ground near to where your tomato plants are going to be planted out to mature. I use this method alongside the eggshell barriers, both together make a big difference to my slug and snail defence in the garden. 

Hand picking slugs and snails off plants works well too, especially in conjunction with my above favoured methods. This is best completed at night-time (with a head torch!) when they are most active in the garden, keeping the snails and slugs in a pot ready to put on the bird table the following morning, or if you know anyone with chickens then slugs and snails are a great snack and protein sauce for them. 

Aphids, white fly and black fly 

The worst offender out of the above three pests are aphids. They are usually found in clusters running up the ends of new growth, stems and leaves. Aphids suck the sap from the tomato plant stems and foliage which can stunt growth. Aphids also secrete a substance called honeydew which makes the plant sticky, this can encourage sooty moulds to develop which can cause the plants to rot off. Aphids are good for the garden and biodiversity, so it is recommended that they are only treated if they are affecting the plants badly or the infestation is quite big. 

A natural way to treat aphids is to apply a lady bird larvae biological control. This can be bought online and in some specialist garden centres. The larvae and mature lady birds love to feast on aphids keeping them under control. 

White fly is also a problem in warmer, more humid places such as greenhouses or polytunnels where they can thrive.  

They act in a very similar manor to aphids, sucking the sap from plant stems and foliage and excreting honeydew, as mentioned previously, can promote growth of sooty moulds making the tomato plants suffer or rot off. 

Whitefly can be treated with a biological control too, using a parasitoid wasp will help keep them under control in a greenhouse, grow-house or garden.² 

Black fly is not as common on tomato plants; however they can spread from other plant hosts nearby if they are looking for somewhere to live & thrive in their 100’s. 

Both aphids, white fly and black fly can be treated using a watered down washing up liquid in a spray bottle that can be applied easily to the infested areas. The pests find it hard to grip the stems of plants when the grease of the washing up liquid has been applied. 

Another more natural way to prevent pests in the garden and around your tomatoes is by companion planting with Marigolds. French Marigold plants are the best as their scent is the most pungent out of all the Marigold varieties however African Marigolds or Calendula will also help to some extent.  

Summary 

Tomato plants can be easy and joyful to grow, knowing the potential diseases, problems and pests that they are susceptible to before you have a go at growing them can help ensure your tomato success.  

The main takeaway here and a lesson that I have learnt over many years of growing my own is to prepare. Keep growing areas clean and tidy, washing down greenhouses, canes, tools and pots at the end of each season helps to eradicate any lingering diseases ready for the following growing season. Watering, feeding and maintaining tomato plants regularly during their growing and fruiting seasons is key to optimal plant health and in turn a good and successful tomato harvest.