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

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?

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

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?

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

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

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

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

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.

Greenhouse Growing Tips

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

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.

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.