Everything you ever need to know about Tomato Blight

Huge thanks to Scot Nelson for providing these close-up pictures // CC By 2.0


Last year I learned a lot about growing tomatoes. One problem I ran into towards the end of the growing season was blight. So next time around I want to be prepared; I did some research to understand how this disease develops and some great ways to prevent it. In this post, I want to put together everything I learned, so that you can find all you ever need to know about Tomato blight in one place.

What causes blight in tomatoes? Blight can be separated into two different kinds; Early blight (caused by Alternaria solani) and Late blight (caused by Phytophthora infestans). This Fungus lives in the soil and prefers cool, moist conditions.

To get a better understanding of Blight, let’s take a close look at the causes of it in tomatoes.


Looking at the cause of blight

Early blight: is caused by Alternaria solani, a type of fungus that thrives in damp, warm conditions. This disease can easily be identified by the ring-shaped brown spots on the leaves. If kept unchecked early blight will continue to spread, diminishing the harvest or sometimes killing the plant. The same fungus is also known for tuber blight in potatoes. In the early stages, this disease starts with a small black or brown spot that will spread out. Most vulnerable are old leaves; once infected, other parts of the plant (like Stem, Fruit) can also be affected.

Early blight – Source // CC By 2.0

Late blight: is caused by another type of fungus called Phytophthora infestans. This disease is commonly referred to when talking about tomato blight. It’s a very violent fungal infection; infected plants are sure to die, and the disease will spread rapidly. The only thing that can be done against late blight is trying to delay it as much as possible and reacting quickly to the first onset. Unlike most other crop diseases, late blight will completely kill off your tomatoes, both plants, and fruits.

Let’s start by looking where the pathogens come from, so we can get a better idea of how to prevent them.


Late Blight – Source // CC By 2.0


Where does blight come from?

The fungus can come from various sources. In most cases, spores lay dormant in the soil and will start to develop once they come in contact with the plants. Esp. Watering and Rain will cause an impact that can transfer the spores onto the crops.

Early Blight infected plants will continue to spread the fungus around a year after the plants decay. Old tomato-debris is also a significant risk factor for this reason. Sometimes the plants or seeds you buy already had contact with the spores but don’t show any signs yet.

Most cases of Late Blight arise from airborne spores; Once an infected plant starts to produce spores, other gardens will begin to get contaminated, even if the nearest diseased plant is more than 10 miles away. Sometimes, spores can also come from debris in the soil. Late blight needs living material to survive and does typically not overwinter.

Early Blight starts to show up in warm conditions (> 15°C/59°F) during a wet period. These conditions can often be found early on in the growing season, hence its name. Late Blight needs high humidity but can affect plants throughout the whole growing season. It’s, however, more frequent towards the end of the season.


How does Blight affect our tomato plants

Early Blight: This disease survives in plant debris within or above the soil. Unlike late blight, this disease can over-winter, so it’s best to avoid recently infected areas. The wind or affected seeds can introduce new infections. Esp in warm wet conditions will expedite the rate of infections. Most likely, the disease will be transmitted from the soil onto your tomato plants.


The first symptom of infected plants is a small brown lesion. Concentric rings will start to form as the spot expands. At this point, the center of the disease can feel dry and dead. Eventually, the leaf will wilt and die. Stems and fruits (through the calyx) can also be affected by this disease. These infected areas feel leathery and often have the typical concentric rings. Usually, it takes about a week from infection to a visible state.

Infected Stem – Source CC by 3.0

Soon the fungus will start to produce Spores. It requires a wet night for the Conidiophores to be established; they will begin to produce spores as soon as daylight hits them the day after. Spores tend to spread during damp nights or are carried by wind and insects. Early Blight can damage your plants severely, but if identified early enough, there is a good chance for them to survive.


Late Blight: In most cases, Phytophthora infestans will over-winter in culled potato piles, infected tomato-fruits seedlings, or come windblown from warmer regions. The spores will penetrate the plant’s cell wall in damp conditions and start to wreak havoc. Sometimes the disease can stay dormant in the plant if the conditions aren’t good enough.

It takes about a week to go from infection to producing sporangia and starting to spread. The First Symptom of sick plants is irregular shaped watery lesions; Usually, a Halo will form around this area, shortly after, mildew will be visible on the underside of the leaf. As the disease progresses, the fungus will be visible on both sides, the lesions get more prominent, and the foliage dies. At this point, the infection has spread to other plants in the vicinity. The leaves are usually the first infected part of the plant, but the disease will also damage stems and fruits.

Blight infected Stem – Source // CC By 2.0

Late blight mostly damages Potatoes and tomatoes, but sometimes it can also spread to other members of the nightshade family. It’s near impossible to avoid the spores of this fungus if your garden gets infected. The best way to cope with this disease is to prevent it in the first place.


How to prevent Blight from happening

There are a lot of great ways to prevent blight from occurring. Sadly, it’s almost impossible to avoid the disease entirely; the goal is to delay its outbreak as much as possible. It’s certainly possible to finish your complete harvest before the first plant gets infected, so how do we do that?

The first step is all about the plants themselves. Airflow is essential to prevent fungus-based diseases, so here are a couple of techniques that help against blight.

  • Single Stem growing: Focus on one main stem, when growing tomatoes. This helps to keep the plants upright and prevents leaves from touching the ground. Furthermore, the plant won’t grow all over the place, making it easier to space out the plants a bit more and reducing the risk of spreading diseases via contact. To achieve a single stem tomato, you have to remove all tomato suckers that grow in the junction between the stem and a branch. Another benefit of this type of pruning is reduced competition for nutrients within the plant, providing you with delicious and big tomatoes.
  • Pruning the bottom leaves: Another great way to improve ventilation, as well as reduce the chance of infection, is to remove any branches growing below the first fruit. Usually, about three branches grow underneath the first flower cluster. Those branches are the most vulnerable when it comes to contracting diseases from the soil. During Rain and watering, small particles from the earth can come into contact with the plant, but by removing any low hanging leaves, the chance will be significantly reduced. It’s also important to prune side branches if they stretch lower than the first flower cluster.
  • Spacing: plays another huge role in keeping air-flow to an optimum when planting tomatoes. It’s essential to give your plants some space to breathe. One example of doing so is to grow in rows (south to north) and removing any branches between the rows. This way, foliage won’t overlap. Between the rows, I usually leave a 20″ (50cm) gap. The less time it takes for the sun to dry off any moisture from the plants, the better.
  • Maximize sun exposure: Tomatoes love the sun. It’s advantageous if the early sun can reach your tomatoes. The best way to maximize sun exposure is to grow from north to south. This way, both morning and evening sun can hit your plants, plus the afternoon sun will shine straight down at the plants. This works excellent when planting in rows of 2.
  • Mulch: Good air-flow will naturally increase the amount of water your plants need. A great way to counter-act this is to spread some Mulch. Additionally, mulch will prevent material from the soil from splashing up while watering. Just make sure your mulch is disease-free and apply about 3″ (7.5cm) on top of your bed.
  • Applying Baking-Soda: Blight is sensitive to ph-levels. Using Baking-Soda to the leaves of your plants will help to defend any incoming spores. Simply mix 1.5 Table Spoons of Baking soda and vegetable oil with 1 gallon of water. Adding a bit of soap can help to distribute the oil properly. Apply the mixture to the whole plant in the morning or evening, but make sure there is no forecast of rain.
  • Water carefully: Make sure you water your plants gently, to avoid soil-debris from splashing up.
  • Consider Raised Beds: A raised position helps to improve ventilation. Raised Beds are a great way to reduce the chance of spreading soil-derived diseases, as you have full control of the soil inside the growing bed. There are a lot of additional benefits to raised beds, which I won’t mention here.

Make sure to keep your garden clean: If you are lucky enough to catch Blight early on, be sure to remove any plant-parts that show signs. You shouldn’t leave these parts in your garden, as they can still transmit the disease. Make sure to cut generously, so you don’t accidentally touch an infected area with your scissors. After finishing, disinfect your tools and gloves.

Consider Blight resistant plants: When facing a very contaminated soil and constant issues with Blight, it’s best only to grow resistant plants for a couple of years. This way, you can make sure that all the disease-caring plant debris are no longer infectious. Here are a few resistant ones to pick from:

  • Legend – red/large
  • Fantasio – red/medium
  • Iron Lady – red/medium
  • Jasper – red/cherry
  • Defiant – red/medium
  • Aunt Ginny’s Purple – red/purple & large
  • Lemon drop – yellow/small
  • Berry – red/cherry
  • Red Alert – red/cherry

There are a lot more blight-resistant tomatoes to choose from; they come in all shapes, sizes, and colors. Some plants are only blight-tolerant, which means they still produce lots of fruits after being infected. Make sure to pick the right tomatoes for your conditions, if you already struggle with blight pick completely resistant plants. On the other hand, if you just want some insurance, then tolerant plants might be enough.

There is also a helpful real-time Map of reported outbreaks. It can warn you in advance, once late blight is on its way to your garden. Make sure to apply some Baking-Soda-Solution to improve your crop’s defenses.


What should you do if you spot Blight in your Tomatoes

Removing Infected Tomatoes – Source // CC By 2.0

Water Carefully: Make sure you water your plants cautiously. Avoid any splashing water, to prevent soil from getting in contact with your tomatoes. Fungi needs wet conditions to thrive, an excellent way to slow down its growth is to keep your plant dry above the surface. Drip irrigation, for example, is great to bring water to your plants without running the risk of splashing.

Shelter your Tomatoes: Reducing the over-ground exposure to water is essential to keep your plants alive. Moving your plants in a Greenhouse or building them a roof will help to minimize exposure. Sadly this doesn’t work during extended rainy periods. Humidity plays another considerable part in the survival of the Fungus. Keeping away any direct water contact is essential, but this won’t kill off the Blight. It’s best to use this method in a dry period -Blight can be halted by dry weather- to protect your plants from short showers and dew.

Avoid working with wet plants: Working with wet plants are perfect conditions for the disease to spread. It’s best to wait until the sun dries up your crops before you start caring for them.

Keep Scouting: It’s vital to look out for early signs of an infection. Check on your plants daily to make sure they’re all healthy. After Blight occurred in one of your plants, keep on monitoring your other endangered crops. This way, you have a chance to act immediately, which in turn will improve the survival rate of your plants and might prevent your tomatoes from being wiped out.

Fertilize to keep plants alive: This works great when dealing with Early Blight. In most cases, plants will survive the infection, but giving them some fertilizer helps the plant to regain its strength. Just be careful not to over-fertilize with potassium. In one of my recent posts, I did go over multiple organic methods to fertilize a garden bed.

Get rid of infected foliage: Infected plant parts should be removed. Generously cut off the diseased leaves. Similarly, consider cutting stems that got infected (with early blight, it’s sometimes better to keep them) and remove fruits that start to show signs. Reduce the chance of spreading by disinfecting your pruning scissors after each cut. Collect all the cut parts and remove them from your garden. Eiter burn them or throw them in the trash. Don’t compost them; they will continue to spread the fungus. Afterward, make sure to clean your gloves and tools.

Keep your growing bed ventilated – Source // CC By 2.0

Give your healthy plants a proper pruning: Foliage is crucial for plants to be healthy, but too much can be detrimental to them. Pruning excess leaves will help reduce the chance of getting infected. Your leaves will dry faster after the sun rises, and ventilation will improve. Furthermore, there won’t be any direct contact between the plants, which slows down the spread of the disease. Remove all leaves below the first flower cluster and prune the branches growing between the rows.

Prevent over-wintering: Late Blight can survive in infected plants for quite some time. Esp. when growing from harvested seeds. If you want to germinate your own seeds, ensure your plants weren’t infected at the time you collected your fruits. On the other hand, sometimes plants can be affected and not show any signs. Buying certified disease-free seedling is a good idea if your area currently has problems with blight. Early blight can over-winter in the same manners. However, it can also stay alive in the debris ontop and within your soil. So make sure to properly clean your growing bed, if you suffered from an outbreak.

Use organic Fungicides: The Baking-Soda Fungicide I shared earlier (in “How to prevent Blight from happening”) also works as a treatment. Just make sure to increase the concentration of Baking-Soda and Oil to 3 tablespoons a gallon. Alternatively, you can use a Coper/Sulfur based solution. They can be bought in most home improvement centers or online. Follow the directions on the label, because the concentration might vary between brands. It’s usually enough to apply organic Fungicides once a week.

Chemical Fungicides: If nothing seems to work, chemical fungicides can be a last resort measure. Chlorothalonil tends to work pretty well with tomatoes. But there are also a couple of effective alternatives. Make sure to follow the instructions on the container. One disadvantage of chemical fungicides is the waiting time until your fruits are safe to eat. Furthermore, chemical agents tend to affect and kill things that are beneficial to a garden, so handle them with care.

Know when to abandon your plants: This is every gardener’s nightmare. Sometimes, it’s best to cut your losses by ditching your plants. Usually, once 5-10% of your foliage is infected, it’s impossible to prevent the disease from wiping out the rest of your crops. It’s best to salvage as much as possible by picking any unripe and -apparently- uninfected fruits. Place the tomatoes in a warm room, using a cardboard box (Tomatoes produce Ethylene; this gas helps them to ripen). It’s best to spread them out evenly, stacking them increases the chance for the disease to spread if one of them is infected. Check on them from time to time, to cull any diseased fruits.

Consider Crop Rotation: Growing a different crop family after an outbreak can help to regenerate your garden. Usually, contaminated plant debris will start to decay. It’s best if you give your growing bed 3-4 years before you grow the same crops again. Crop Rotation is an excellent way for the soil to regain nutrients and reduce the chances of infections and pests.


Related Questions:

Can you eat tomatoes with blight? Eating Tomatoes from infected plants is fine to do. Just make sure to give them a good wash beforehand. Contaminated Fruits are still edible if picked immediately after the disease is visible. You can cut out any lesions and eat the rest. Once the disease reached an advanced stage -visible leathery rot- it’s best to get rid of the tomato. At this point, you sure didn’t want to eat it anyway.

How dangerous is Blight in Tomatoes? Blight spreads incredibly fast. Whole fields can be destroyed in a matter of days. This disease caused all the big potato-famines in the 19th century. It’s essential to act immediately after your garden gets infected to ensure as little damage as possible. Early blight can reduce or kill your harvest and also over-winter in the soil if kept unchecked, making it nigh impossible to grow any nightshades in the garden bed the following year. Late blight, on the other hand, is a lot more destructive and kills off most of the plants each year. Any Gardeners nightmare is a relatively early infection, as it can reduce the harvest to zero without any treatment.