How to Identify, Treat, and Prevent Late Blight in Tomatoes

Are your tomatoes struggling with Late Blight this season? In this article, gardening expert Jenna Rich looks at all you need to know about how to identify, treat, and prevent this potentially deadly tomato disease from attacking your plants this season.

late blight tomatoes

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We’ve all heard about “the blight,” and some confusing and conflicting information exists. Does it live in the soil? Can you get it from your neighbor’s garden? Some say it moves up the coast during rainy periods and can travel miles in the wind. Is it a bacteria, a disease, or virus?

As with anything in gardening, it is important to understand what is happening and why so you can better understand how to treat and prevent it in the future.

Understanding late blight and how it spreads is key because it can devastate your tomato and potato crop very quickly if proper care is not taken. Here you’ll learn how to quickly diagnose, treat, and prevent tomato late blight.

What is Tomato Late Blight?

Close-up of Late Blight tomato plants in a sunny garden. The tomato plant has sluggish, twisted, dry: brown and green leaves with rotting spots. The leaves are oval, green, with serrated edges. The fruits of the tomato are round in shape, covered with a red thin skin. The fruits have rotten spots and a sluggish, soft texture.
Tomato blight is caused by Phytophthora infestans, a fungal-like disease that occurs later in the season.

Tomato blight (also called late blight because it occurs later in the season than “early blight”) is caused by the oomycete Phytophthora infestans, which have fungus-like qualities. This pathogen attacks tomatoes, potatoes, and other nightshade crops.

Oomycetes (organisms of the phylum Oomycota) used to be classified as a part of the Kingdom fungi due to their filamentous growth pattern and the fact that they feed on decaying matter.

However, after years of research and studies, it has been determined that these organisms are actually more closely related to algae and green plants. Oomycetes are now included in the kingdom Protista.

Oomycetes are also called “water molds,” which makes sense because they usually show up during extensive wet periods and travel in water. Other water molds include damping off, root rots, and downy mildew.

How Tomato Blight Spreads

Close-up of the leaves of a Late Blight tomato plant in the garden. The plant has pinnately compound leaves, consisting of oval green leaflets with serrated edges. The leaves have purple-brown rotting spots and a white-gray bloom.
Oomycetes produce sporangia that cause brown spots on tomato plants.

Late blight infects your plants through structures similar to spores. They spread by wind and rain. Simply put, oomycetes produce sporangia that look like brown spots on the leaves or petioles of tomato plants.

Zoospores are produced asexually within the sporangia. Zoospores then use their flagellum to propel themselves and move quickly through water on wet surfaces like plant leaves and stems, finding nooks and crannies to sit in and infect. They stick to surfaces by releasing a gelatinous secretion and feed on decaying matter they find along the surfaces.

Did you know? Zoospores can travel up to 50 miles in the wind, infecting farms and gardens nearby! This is why, during rainy seasons with lots of high winds from storms, hurricanes, etc., you might hear about this disease “moving up the coast.” It’s quite literally being carried up the coast in various weather patterns!

What Crops Does Late Blight Affect?

Close-up of a Late Blight tomato plant in the garden. The plant has upright green stems with brownish-purple rotting spots. The leaves are compound pinnate, with oval green leaves with serrated edges, and brown spots due to a fungal disease. The leaves are drooping. The fruit of the tomato is round, medium in size, covered with a thin bright red glossy skin.
This pathogen affects tomatoes, potatoes, and related crops, including solanaceous weeds and petunias.

Late blight appears mainly on tomatoes, potatoes, and some ornamental relatives of these crops, like solanaceous weeds and even petunias. “Solanaceous” refers to anything in the nightshade family.

You may recall the Irish Potato Famine of the mid-19th century that led to mass starvation and destroyed most of the Irish potato crop. This was caused by the same oomycete pathogen that we see today!

There have been trials for many years to create “cultivars” that are disease-tolerant or less susceptible. These cultivars should be considered, especially if you live in a region where this problem is prevalent, like a moist, humid region.

Did you know? The term “variety” refers to a plant that has developed with little human interference, whereas a “cultivar” refers to a plant intentionally produced by selective breeding to allow growers to grow crops with a decreased chance of damaging viruses, diseases, and pathogens.

Ideal Disease Conditions

Close-up of rows of Late Blight tomato plants in a sunny garden. The plant has upright stems covered with compound pinnate leaves with oval green leaves with serrated edges. The fruits are round, with a thin glossy orange-red and pale green skin. The leaves are wilted, brown, rotten, drooping.
Late blight thrives in wet conditions and specific temperatures, rapidly infecting tomato plots.

This disease shows up later in the season in most regions because the oomycete pathogen prefers wet conditions and temperatures between 60° and 78° with around 90% humidity for 3-5 days. 

If these conditions occur, late blight can infect a whole plot of tomatoes within 10 hours to 2 days. Keep an eye on your forecast so you can be prepared. Consider checking your plants for symptoms about every 2-3 days.

Did you know? Oomycete spores can survive on living plant material, in volunteer plants or fallen leaves, remaining dormant until conditions are right. This is why cleaning up plant debris is so important. When conditions are right, the pathogen is known for producing lots of spores at rapid speed, spreading far distances.

Identifying Late Blight in Tomatoes

If you suspect your garden is infected, here are the quickest ways to identify this disease.

Symptoms

Close-up of tomato leaves infected with the fungal disease Late Blight. The plant has pinnately compound leaves, blue-green in color, consisting of oval leaflets with serrated edges. leaves have irregular brown rotting spots.
Late blight exhibits brown spots on leaves and stems, lesions on petioles, and white fuzzy growth underneath leaves.

The main symptoms of tomato blight are brown, gray, or purplish spots on leaves or stems and can include shriveled-up and dried leaves. You may also see discolored lesions on the stems and petioles. Sometimes referred to as stems, petioles are the stalk that grows off the main stem of the plant’s supporting leaves.

If you flip the leaves over, you will likely see white fuzzy growth on the underside. These fuzzy spots are destructive spores. Leaves will continue to brown or die altogether. If you don’t take action, spots will spread and infect all foliage and fruit very quickly.

Refer to Cornell’s blog post for some great close-up photos that may help you visually identify symptoms.

How to Properly Dispose of Infected Plants

Utilization of a tomato plant affected by a fungal disease Late Blight. Close-up of female hands in white gloves plucking a tomato plant from the soil in a garden. The tomato plant has green, pinnately compound leaves and oval, unripe fruits covered with a thin, shiny green skin. The leaves are brown, rotten, withered, drooping, dry.
Removing infected plants from the garden is crucial to completely eliminate the pathogen.

Once you’ve identified the pathogen, it’s essential to completely kill it before it spreads.

Remember, this pathogen feeds on living plant or animal matter. If you add infected plants to your compost pile with a continuous supply of living matter, spores can easily move back into your garden when conditions become ideal.

Never put infected plant debris in your compost pile!

To fully kill the fungus-like organism, pull out infected plants on a hot and sunny day and lay them out in the sun, bag them, or cover them with a tarp so they cannot travel and spread. Next, throw them in the trash or burn them.

Pro Tip: If you have confirmed this disease in your tomato patch and have potatoes growing nearby, you should keep a close eye on the potatoes as they will likely also be affected.

How to Avoid Late Blight

Close-up of young tomato seedlings on the counter in the garden center. The seedlings have upright pale green hairy stems and pinnately compound leaves consisting of oval green leaflets with serrated edges.
Obtain tomato plants from trusted seed suppliers or reputable local sources to prevent late blight.

Diseases are often brought in from the outside. The best thing you can do is start your tomato plants from well-known, reputable seed suppliers or buy plants from reputable local sources. Inspect plants before adding them to your garden for any signs of disease, such as leaf, petiole, and stem lesions.

Disease Prevention Tips:

  • Throughout your growing season, keep pruning shears clean between sessions, as spores can transfer from plant to plant.
  • Check plants 2-3 times a week for any lesions or white fuzzy spots that contain spores.
  • Practice regular pruning to encourage airflow.
  • Pay attention to weather alerts and even local cooperative extension office notifications so you can be on the lookout for symptoms.
  • Remove any solanaceous weeds around your tomato and potato patch, as they can be carriers of the disease.
  • Only water with drip lines or soaker hoses at the base to avoid splashing and wet plants/leaves. Do not use overhead irrigation on tomatoes. Mulching at the base of plants can also prevent splashback from the soil level.

Prolonged heat will slow the spread of late blight, but keep in mind that, once present, the pathogen will start spreading again once the weather cools.

YouTube video
There are several tips you can follow to prevent most diseases from striking your tomatoes.

Greenhouse Growing Tips

Close-up of young tomato plants growing in a greenhouse on a raised bed. Each plant is tied with a white rope to the ceiling for vertical growth. The tomato plant has an upright pale green stem and pinnately compound leaves, with bright green oval leaflets with serrated edges.
Greenhouse-grown tomatoes are less affected due to protection from wind and rain.

Greenhouse-grown tomatoes are less affected by this disease because they have protection from wind and do not receive rainfall. Although tomatoes inside a protected growing area still might become infected, the infection will likely be delayed.

Ensure good airflow with fans and proper spacing between tomato plants. Not only does ample space between plants help with airflow, but spores can easily travel to a whole string of plants if leaves are touching.

Also, practice proper pruning by removing suckers, lower leaves, and excess leaves as your plants grow to give them better airflow. Fungal diseases tend to affect older leaves first, so prune them from the bottom up as your tomato plants grow. Sufficient airflow helps prevent lots of diseases, including powdery mildew.

If you have many tomatoes and are growing in a greenhouse where plant disease is present, consider changing your clothing before entering another tomato area, as spores could transfer from your clothing.

Fall Cleanup

Cleaning plant debris in the garden near the rows of tomatoes. Close-up of a gardener with a large rake raking dry plant debris into one pile. The gardener is dressed in gray trousers, a red shirt and high ribbed boots. Rows of tomatoes grow in the background.
Prevent disease by removing and burning infected plants or adding them to a hot compost pile.

At the end of each season, you have a critical opportunity to set yourself up for a disease-free spring. In the fall, remember to:

  • Pull out any infected plants and burn them or add them to an active and hot compost pile. The pile must be active and hot enough to truly kill the pathogen, so if you are going this route, you must really know how hot composting works.
  • Remove any fallen leaves from infected plants.
  • If your potatoes were infected, dig them up and do not save them to plant out next year. Blight can survive in tubers, especially in regions that do not receive frost. You would continue the disease cycle if you planted these tubers.
  • Destroy any volunteer tomato or potato plants that pop up in paths or compost piles.

You can help prevent this disease in future years by practicing proper crop rotation. Try not to plant tomatoes, potatoes, or other members of the Solanaceae (nightshade) family for at least 2 years after the disease appears in that plot.

Frequently Asked Questions

Can late blight survive in the soil or on growing supports?

Late blight requires live plant material to survive, so if you live in a region that receives frost, you should not need to worry about carrying blight over from year to year.

Spores will not survive on support cages or stakes as long as they are cleaned between uses, so have no worries about reusing these items year after year.

If you have grown tomatoes in a greenhouse that will not frost over the winter, you may want to be cautious of any dropped leaves that you think came from infected plants.

Are infected tomatoes dangerous for humans to consume?

Although the infected fruit is not the prettiest, you can cut off the infected area and eat the remainder of the fruit. Blights cannot be transferred to humans as it is a plant pathogen.

However, infected fruits should not be used for canning of any sort. For preservation, you should always select the healthiest fruits for the best results. 

Are there specific tomato varieties that are less susceptible to late blight?

Yes, you can select varieties more resistant or tolerant of late blight than others. On many seed company websites, you can filter for specific disease resistance (look for the code LB). Below are a few cultivars bred for disease resistance.

SaucePlum Regal
Juliet
Slicer/BeefsteakDefiant PhR
Galahad
Mountain Merit
CherryCherry Bomb
Mountain Magic

Keep in mind that if conditions are perfect, you may still experience disease pressure even when growing a so-called resistant cultivar. Always check your plants for symptoms so you can catch them quickly, and your plants have a chance to recover if blight appears.

Where can I get more information on late blight?

Contacting your local cooperative extension office is a great starting point. There is likely an agent assigned to your county or region who can look at photos of suspected blight to help you diagnose the pathogen and answer any questions you may have.

Also, most state cooperative extension offices track crop diseases, so it’s a good idea to reach out if you have confirmed there is a pathogen present in your garden.

Can I spray my plants with something to prevent late blight?

There are some OMRI (Organic Materials Review Institute) approved fungicidal sprays that you can look into if you think that is best for your garden. If you are certified, check with your local certifiers to find out what products are approved for you.

Final Thoughts

This disease can be very destructive to tomatoes, so it’s important to know how to identify, treat and prevent it. Some years, crops are at higher risk due to various weather conditions, so have a plan in place in case trouble arises.

Be sure to buy seeds and plants from reputable sources, water properly, and protect your plants from wind and rain when possible. Take advantage of local resources and talk to your neighbors.

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