How Do Tomato Plants Get Blight? & How to Protect Plants


It is heartbreaking to see your tomato plants infected with either early or late blight.  Unfortunately, blight is a reality, especially when wet, humid conditions persist in your garden.  If your tomato plants have blight, you are probably wondering how they got the disease.

So, how to tomato plants get blight?  Early tomato blight is caused by a fungus that spreads by spores on the wind, by human contact, or by water splashing infected soil onto the lower leaves of a plant.  Late tomato blight is caused by a fungus that spreads by spores on the wind or by surviving the winter in infected potato tubers.  Both early and late tomato blight can survive the winter, and can affect tomato and potato plants.

There isn’t much you can do to cure tomato plants that get early or late blight.  However, there are plenty of steps you can take to prevent them.  Let’s start with some background on both diseases, and then we’ll get into ways to prevent them in your garden.

How Do Tomato Plants Get Blight?

There are two types of blight: early tomato blight and late tomato blight.  Each one has some unique characteristics in terms of how it spreads, the symptoms it causes, and how to prevent it.

How Do Tomato Plants Get Early Blight?

Early blight is caused by one of two fungi: either Alternaria tomatophila or Alternaria solani.  Early blight spreads more rapidly in damp conditions, such as after rainy weather, watering your garden, or heavy morning dew.

Humidity levels over 90% are a risk factor for the rapid spread of early blight.  It can develop in temperatures between 59 and 82 degrees Fahrenheit (15 and 28 degrees Celsius), and the ideal temperature for the disease is in the range of 82 to 86 degrees Fahrenheit (28 to 30 degrees Celsius).

rain on tomato plant
Early blight is more likely to spread in wet conditions, such as after extended periods of rainfall.

The fungi that cause early blight can survive in the soil over the winter and infect plants the following spring.  They can also survive the winter in your compost pile.  This is more likely if you compost infected plants, or if your compost pile did not get hot enough to kill the fungi that cause early blight.

Early blight can also get into your garden on infected seeds or seedlings from a nursery.  When ordering seeds or seedlings, be sure to buy from a reputable company that guarantees their products!

tomato seedling
Get your tomato seedlings from a reputable nursery to prevent early blight!

Keep in mind that early blight also affects other members of the nightshade family or plants, such as potatoes and eggplant.  This means that planting tomatoes near potatoes or eggplants in the same year may allow early blight to spread between different plants.

It also means that planting tomatoes where you had potatoes or eggplants in a previous year can allow the disease to spread via the soil.

How Do Tomato Plants Get Late Blight?

Late blight is caused by the fungus Phytophthora infestans.  It spores spread on the wind, and it does well in cool, wet conditions, with an ideal temperature range of 60 to 70 degrees Fahrenheit (16 to 21 degrees Celsius).

Late blight is the more devastating of the two blights, able to spread rapidly and destroy entire crops.  In fact, it caused the Irish Potato Famine in the 1840s, which caused widespread starvation and migration.

The spores of the fungus that cause late blight spread on the wind from one plant to another.  There is no cure for infected plants.

potato late blight
Late blight caused the Irish Potato Famine in the 1840s, which caused widespread starvation and migration.

Late blight can also infect potatoes, so growing tomatoes and potatoes close together at the same time can encourage the spread of the disease.  The disease may also spread if you plant tomatoes in the same spot where infected potatoes were grown the year before.

What Are The Symptoms of Tomato Blight?

Early and late blight have different symptoms, but you should be on the lookout for both diseases in your garden.  Put regular inspections on your calendar, and if you see signs of either disease on any plant, remove it and destroy it immediately to prevent the further spread of disease.

Symptoms Of Early Tomato Blight

Early tomato blight, as its name implies, occurs early to mid-season, before late blight appears.  Symptoms of early tomato blight first appear on older leaves, lower on plant.  You will see small dark spots, which are round and brown, on these leaves.

The area around the leaves may turn yellow.  In some cases, the entire leaf may turn brown and fall off the plant.

early blight on tomato leaf
Some symptoms of early blight: brown spots on leaves, with yellow areas around the edges.

If enough leaves fall off, the fruit of the tomato plant will be exposed to too much direct sunlight.  This can cause sunscald, which may make fruit inedible due to blisters that turn white or gray.

For more information, check out this article on sunscald from the University of Maryland Extension.

Early tomato blight can also affect the stems and fruit of tomato plants directly.  In this case, you will see concentric rings that look like targets on the stems and fruit.

The rings on the stem may eventually turn brown, sunken, and dry.  The spots on the fruit will start at the stem and become dark, leathery, and sunken.  Infected fruit might even drop off the plant.

As mentioned before, early blight can also affects potatoes and eggplant.  However, this is more likely caused by the fungus Alternaria solani.

For more information, check out this article on early tomato blight from the University of Minnesota Extension.

Symptoms Of Late Tomato Blight

Late tomato blight appears mid to late-season.  First, you will see small, green, water-soaked spots appear on the leaves of your tomato plant.

Eventually, these spots will grow and turn brown.  The disease will then spread rapidly to the tomato stem and fruit of your tomato plants.

Once this happens, you will see dark lesions on the stems and brown spots on the fruit.  The entire plant can turn black and die within a few days of infection.

late blight on leaf
Symptoms of late blight on the leaf of a plant.

If humidity levels are high, you may see a white, powdery, cotton-like fungus on infected leaves, fruit, and stems.

late blight on tomato stem
Symptoms of late blight on the stem of a plant.

For more information and pictures, check out this article on tomato late blight from the Cornell University College of Agriculture.

How Does Tomato Blight Spread?

Both types of blight are more likely to spread in wet conditions, but the method of disease transmission varies a bit between the two.

How Does Early Tomato Blight Spread?

As mentioned before, early tomato blight can survive over the winter in soil, manure, or compost piles.  Rain or watering can cause infected soil to splash up onto the lower leaves of a tomato plant and cause infection.

You don’t have long once the disease strikes – the spores of the fungus that cause early blight can germinate in two hours!

Early blight can also spread if the leaves of a plant come into direct contact with infected soil.  To avoid this, don’t lay your tomato plants on the soil in your garden when transplanting them.  Instead, keep them in a container until they are ready to bury in the soil.

In addition, early blight can spread if the leaves of an infected plant come into contact with another plant.  To prevent this, leave enough space between plants and avoid overcrowding.

If your plants grow more than you thought they would, consider pruning to prevent this method of disease transmission.

Finally, the spores of the fungus that cause early blight can spread on the wind or by human contact.  Beware of wind after rainy or damp weather if a warm front moves in!

wind sock
Wind is a major factor in spreading early tomato blight between plants, or even between gardens!

For more information, check out this article on tomato early blight from the University of Maine Extension.

How Does Late Tomato Blight Spread?

Late tomato blight cannot survive in soil in isolation, since it needs living tissue to survive.  This means that it cannot survive on tomato seeds or on tomato plants over the winter.

However, late blight can survive the winter underground if it lives in infected potato tubers.  If you see volunteer potato plants growing in your garden in the spring, remove them to prevent the spread of late blight to your other plants.

If you see volunteer potato plants in the spring, destroy them, since they could be carrying late blight, which can spread to your tomato plants.

Late blight is more likely to come to your garden from infected tomato transplants, such as those purchased from garden centers or online.  The spores from the fungus that cause late blight can also spread on the wind.

Hairy nightshade is a weed related to tomato plants which can also carry late blight.  In addition, petunias can carry late blight in certain conditions.

One spot of late blight on an infected plant is capable of producing thousands of spores in 5 days or less, so be on patrol twice a week to check for symptoms!

For more information, check out this article on tomato late blight from the University of Minnesota Extension.

How Long Does Tomato Blight Last In Soil?

As mentioned before, tomato blight needs living tissue to survive.  So, the only way late blight can last in soil over the winter is by surviving underground, inside an infected potato tuber.

Late blight cannot survive on a tomato seed, since a tomato seed that has not sprouted is not yet living tissue.  Late blight also cannot survive the winter on an infected tomato plant, since freezing kills the entire plant.

For more information, check out this article on tomato late blight from the University of Massachusetts.

Early blight, on the other hand, can survive at least one year in the soil, and potentially for a few years.  The best way to avoid spreading early blight in your garden is to practice crop rotation – more on this later.

How Do You Get Rid Of Tomato Blight In The Soil? (Tips To Prevent Tomato Blight)

There is not much you can do besides destroy plants that have been infected with early or late blight.  However, there are lots of steps you can take to help prevent tomato blight from spreading in your garden.

Buy Disease Resistant Tomato Plant Varieties

One of the best ways to prevent early or late blight is to buy tomato plants that are resistant to these diseases.  Usually, this will be indicated in a seed catalog or on a seed package.

Often, hybrid seeds are bred specifically for disease resistance.  If you are an heirloom tomato seed enthusiast, you may want to be careful about planting them near your other plants, since they may be susceptible to blight.

For more information, check out my article on the pros and cons of hybrid seeds and my article on heirloom seeds.

Leave Enough Space Between Plants

Another good way to prevent the spread of blight and other diseases in your garden is to space your plants far enough apart.  That way, the leaves from different plants are less likely to touch and transmit diseases.

Also, if the soil is carrying fungi or other diseases, splashing water is less likely to infect more than one plant at a time.  If you are not able to watch your plants closely during the season, proper spacing gives you a little more time before diseases spread.

Use Crop Rotation

Crop rotation is another gardening method you can use to avoid the spread of disease.  Crop rotation means that you do not plant the same crop (or related crops) in the same place two years in a row.

It is even better if you can wait three or four years before planting tomatoes in the same place again.  Remember that other nightshade crops, including potatoes, peppers, and eggplants, are related to tomatoes.

Avoid planting these crops close together in the same year.  Also, avoid planting them in the same part of your garden multiple years in a row.

For more information, check out my article on planting potatoes and tomatoes together.

Support Your Tomato Plants

I don’t mean this in the sense of being a cheerleader, although I suppose that doesn’t hurt!  I mean this in the sense that you should help your tomato plants to grow vertically.

tomato stakes
Tomato stakes are one great way to prevent blight and other diseases from spreading in your garden.

To do this, let your tomato plants climb up on stakes, cages, or trellises, depending on how much space you have and your gardening style.

For more information, check out my article on supporting tomato plants, my article on tomato cages, and my article on trellises.

Inspect Your Plants Regularly

You should schedule some time once or twice a week to closely inspect your tomato plants.  Check to see if any of them appear to have any symptoms of early or late blight (discussed earlier).

These regular inspections will give you an advance warning of a disease outbreak in your garden.  You can plan on doing these inspections at the same time that you tie your tomatoes, when you water them, or at any other time that is convenient.

Prune Off The Lower Leaves And Branches Of Tomato Plants

As mentioned earlier, splashing water can put disease-carrying soil in contact with the leaves of your plant.  This is much less likely if you remove the lower leaves and branches of your tomato plant.

It may be hard to cut pieces off of your tomato plant, but it is better for 90% of the plant to survive than for the whole thing to die without producing any fruit!  Remember that a single infected plant can start a domino effect that affects all the tomato plants in your garden.

Water Your Plants From Below

When watering your plants, don’t get the leaves wet by watering from above.  Instead, water plants low, close to where the stem meets the ground.

garden hose
Water your plants from below, and water them in the morning. Avoid keeping the leaves wet for too long!

Also, water your plants in the morning, not at night, when they are likely to stay wet for longer.

Finally, do not work with plants when they are wet, such as after rain or heavy morning dew. Diseases are more likely to spread in wet conditions, and fungus is more likely to grow when leaves stay wet for too long.

Sanitize Your Garden Tools

After pruning, digging, or raking, make sure to sanitize your garden tools with alcohol or soap and water.  Any bacterial or fungal infections that live in the soil or in plants can spread on the tools you use in your garden.

Get Rid Of Infected Plants Immediately

If you do end up with infected plants in your garden, your regular inspections should uncover the problem quickly.  Remove the infected plants immediately and destroy them.

Do not compost the plants, since some diseases can survive the winter in compost piles.  Although your compost pile may get hot enough to kill the disease, this is not a sure thing.

Sterilize Your Soil

If an outbreak of blight is severe enough, it may make sense to sterilize your soil by using the sun.  For more information, check out my article on how to sterilize your soil.

Conclusion

By now, you have a better idea of how each type of tomato blight is spread, plus what symptoms to look for.  You also know how to take the proper steps to prevent the spread of these diseases in the first place.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions or advice about tomato blight, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

Recent Content