Tomato Blight: Identification, Prevention, and Treatment

Any Amazon links below are affiliate links, which means I earn a commission at no cost to you if you click and finalize an order. As an Amazon Associate, I earn from qualifying purchases.

Tomato Blight: Identification, Prevention, and Treatment

Tomato blight is one of the most destructive problems you can face in the garden. It spreads quickly, damaging leaves, ruining fruit, and wiping out entire crops in a short time. Understanding the different types of blight, what they look like, and how they spread is key to protecting your tomatoes. Fortunately, with the right preventative steps and quick action, you can often save at least part of your harvest.

Below, I’ll cover the three most common types of tomato blight, how to identify them, and the best ways to prevent and treat them.

What Causes Tomato Blight?

Tomato blight is most often caused by different types of fungi, though other tomato diseases, like bacterial wilt, can cause similar-looking problems. In common gardening terms, “blight” usually refers to fungal infections that thrive in warm, wet conditions. Because spores are carried by rain, wind, and even insects, blight tends to be much more common outdoors than indoors, where plants are protected from rainfall and airborne spores.

Tomato Blight Identification

There are 3 main types of tomato blight, and unfortunately one or more can be present on your tomatoes at the same time.

Early Blight (Alternaria solani)

• Timing: Can appear any time during the growing season.
• First signs: Dark brown or black spots, often with concentric rings, starting on the lower leaves. A yellow halo may appear around the spots.
• Progression: Spots enlarge, leaves yellow and die, and stems or fruit can develop similar lesions.
• Impact: Weakens plants, reduces yield, can kill plants if severe.

Late Blight (Phytophthora infestans)

• Timing: Usually appears later in the season but can infect at any stage, including seedlings.
• First signs: Light brown, water-soaked spots on leaves that expand rapidly. A fuzzy white growth may appear on the undersides in humid conditions.
• Progression: Leaves collapse, fruit develop brown, greasy lesions, and plants can be destroyed within days.
• Impact: Extremely deadly to tomatoes and potatoes, can overwinter in potato tubers.

Septoria Leaf Spot (Septoria lycopersici)

• Timing: Common in mid-to-late season, especially during wet weather.
• First signs: Many small, circular spots with dark edges and light centers on lower leaves.
• Progression: Spots spread upward, leaves yellow and fall off.
• Impact: Rarely kills plants but reduces productivity.

Quick Symptom Comparison Table

Blight Type	First Signs	Typical Progression	Key Risk Factor
Early Blight	Dark spots with yellow halo on lower leaves	Spreads upward, leaf drop, stem lesions	Warm, moist
Late Blight	Water-soaked light brown spots, white fuzz underside	Rapid leaf collapse, fruit rot	Cool, wet
Septoria Leaf Spot	Many small dark spots with pale centers	Spreads upward, leaf yellowing and drop	Prolonged wet

The following video gives helpful tips for identifying each type of tomato blight (along with a few other common tomato diseases!)

How to Treat Tomato Blight

For prevention or early blight cases, you can try a homemade baking soda tomato blight treatment. Combine a tablespoon of baking soda, a teaspoon of vegetable oil, a dash of soap, and a gallon of water, then spray the plants on a regular basis.

A stronger tomato blight cure is fungicides, including Daconil and copper fungicide. There are also a few newer treatments that more gardeners are starting to use, including:

• Trichoderma-based fungicides – these are beneficial fungi that crowd out the bad ones, making them a good fit for organic gardens
• Hydrogen peroxide sprays – mixing 8 ounces per gallon of water can help stop an active infection fast – but don’t overuse or the plants could be damaged
• Potassium bicarbonate – works better than baking soda when you need to raise the pH on plant surfaces
• Microbial amendments like EM-1 – these boost beneficial microbes in the soil, which may help plants stay healthier overall

No matter what you use, if you find early blight or septoria leaf spot, you should at least prune off the affected areas if they’re small enough. Burn or bury the clippings far away from your garden, and sanitize your pruning shears and any other tools you used.

The previous methods will not work for tomatoes with late blight, which should be fully disposed of as outlined in the following section:

How to Get Rid of Tomato Blight

Depending on the type of blight you have and its severity, you may have to immediately dispose of the plant and cut your losses. Late blight can’t be treated, so pull up any plants afflicted by late blight and burn or bury them far away from your garden. You can remove any untouched tomatoes before disposing of the rest of the plant, since they are still edible if used quickly.

Second, make sure you thoroughly disinfect anything that came into contact with the tomatoes, including stakes, shovels and other tools, greenhouse materials, and more.

Tomato Blight Soil Treatment

There are no known soil treatments for late blight aside from time and freezing temperatures. However, early blight and septoria leaf spot can both potentially be treated using soil solarization during the summer. Also known as tarping, this method heats the soil, killing early blight and septoria leaf spot fungi along with most other living organisms. Keep in mind that it does take at least 6 weeks and can only be done in late spring and summer, but by the following year you should be able to plant tomatoes again.

How to Prevent Tomato Blight

You can take a few measures to avoid tomato blight.

First, consider picking a blight resistant variety. While they may still have problems if the conditions are right, resistant varieties will often last longer than traditional ones. Mountain Magic and Celebrity Hybrid are both good, tried-and-true options. In addition, there are some other newer varieties with strong resistance to blight, which you may be able to find through garden retailers. These include:

• Iron Lady – holds up well against several U.S. blight strains
• Stellar – has Ph-2 and Ph-3 resistance genes, which give it extra protection
• Defiant PhR – newer variety getting a lot of praise for its field performance
• Mountain Merit – determinate type with solid late blight resistance
• Brandywise – an heirloom cross that keeps the flavor but adds disease resistance

If possible, try growing your tomato plants indoors, either in greenhouses or even in your home. Keeping them inside lessens their chance of picking up diseases, since they’re exposed to fewer outside air and soil particles and their moisture and temperature can be controlled.

If you can’t plant indoors, plant your tomatoes in areas where tomatoes, peppers, eggplants, and potatoes haven’t been planted for at least four years. Be careful to avoid using any soil or compost that might have held plants touched by blight. Using good crop rotation principles will help.

Pick your tomato companion plants carefully and avoid plants like potatoes, which also suffer from blight.

Make sure your tomato plants are correctly spaced so they get plenty of airflow and stay dry.

Consider staking or trellising the plants and pruning them regularly to make sure they continue to get plenty of air. Prune the bottom leaves when the plant is fruiting so they won’t touch the ground.

You should also make sure you’re watering them correctly. Water at the base of the plant to keep excess water off the leaves, and try to keep the water from splattering soil everywhere, since this can stir up fungi. You can mulch directly around the base to further help with soil splash.

Most important of all, keep an eye on your tomatoes. If you notice any signs of blight, identify the type you have and treat your tomatoes as quickly as possible.

Tomato Blight FAQs

Quick Reference

In summary:

• Early blight often begins on the lower leaves, with dark concentric rings surrounded by yellowing tissue.
• Late blight produces pale, water-soaked spots that darken quickly, sometimes with fuzzy white fungal growth in humid weather.
• Septoria leaf spot shows numerous tiny spots with dark edges and tan or gray centers, often with tiny black specks in the middle (the fungal fruiting bodies).

For mild early blight or septoria leaf spot:

• Try a homemade baking soda spray: mix 1 tablespoon baking soda, 1 teaspoon vegetable oil, a dash of soap, and 1 gallon of water. Apply regularly.
• Use approved fungicides like Daconil or copper fungicide.
• Prune affected leaves and stems, and dispose of them far from your garden. Sanitize tools after use.

For late blight:

• Remove and destroy the entire plant—burn or bury away from the garden.
• Harvest uninfected fruit and use immediately.

How to Prevent Tomato Blight:

1. Choose blight-resistant varieties.
2. Grow in greenhouses or indoors where possible to avoid rainfall and airborne spores.
3. Practice four-year crop rotation for tomatoes, potatoes, peppers, and eggplants.
4. Space plants for airflow, stake them, and prune lower leaves.
5. Water at the base, avoid splashing soil, and mulch around plants.
6. Remove and destroy infected material promptly.

Enjoy Your Tomatoes

You should now know how to prevent tomato blight, along with the signs of blight to watch for so you can perform one of the treatment options early on.

Want to learn more? I’ve written a complete tomato growing guide, along with posts on:

• How to Grow Tomatoes in Pots
• Growing Tomatoes from Seed
• How to Transplant Tomatoes
• 35 Tips for Growing Tomatoes
• Signs of an Overwatered Tomato Plant
• Why Marigolds & Tomatoes Make Great Companions