Tomato Blight (Identification, Prevention and Treatment)

Written by: Luke Marion

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As an organic gardener, I often feel like I’m at a disadvantage when it comes to dealing with plant diseases like tomato blight. It’s frustrating to watch my tomato plants suffer, often being the first to show signs of trouble. This challenging condition doesn’t limit itself to tomatoes; it also impacts my zucchini, cucumbers, pumpkins, squash, melons, potatoes, and even beans!

Green tomatoes on the vine with tomato blight.

In this article, I’ll share how to recognize, prevent, and treat tomato blight effectively. This way, my garden can stay filled with healthy plants and productive tomatoes.

Garden Troubleshooting

Knowing how to troubleshoot issues in your garden is going to go a long way in preventing them in the first place. Knowing what tomato plant leaf curl means, or how to treat blossom end rot is a great place to start. 

From there, you'll want to check out our guide to understanding tomatoes and learn how to grow healthy, organic tomatoes. You should probably also know how to pick the right seeds for your location to maximize success.

What Is Tomato Blight

Tomato blight is a set of diseases that harm tomato plants. These diseases come from fungi, bacteria, and the environment. Knowing about tomato blight helps me grow healthy tomatoes. It lets me manage my garden and fight off diseases early.

Learning about blight can help us spot problems early, meaning we can act fast to prevent further disease. Understanding the disease's life cycle helps us predict when it might strike. Keeping plants healthy is also key as they’re less susceptible to the disease.

Understanding tomato blight helps me protect my plants from these diseases. Each part of learning about blight improves my garden and plant health. With the right knowledge and action, I can beat the challenges of blight.

Types of Tomato Blight

Knowing about tomato blight is key to managing diseases. There are three main types: early blight, late blight and Septoria blight. Each type has its own signs and effects on plants.

Early Blight

Early blight is a common disease affecting plants, particularly those in the nightshade family, such as potatoes and tomatoes. It's caused by a soil-borne fungus called Alternaria solani. The name "early" does not imply that the disease occurs only early in the growing season, but it does most commonly occur early in the season.

Symptoms of early blight usually start appearing on the lower, older leaves as small brown spots. These spots enlarge, often forming a pattern of concentric rings giving a "bull's-eye" appearance. The surrounding leaf area may turn yellow. If left uncontrolled, it can cause significant defoliation, usually starting at the base of the plant and moving upwards.

Early blight can also affect the fruits of the plant. On tomatoes, dark, sunken, leathery spots can form, usually (but not always) at the point where the fruit is attached to the plant.

Late Blight

Late blight is a serious and devastating plant disease caused by the pathogenic water mold Phytophthora infestans. It's most infamous for causing the Irish Potato Famine in the 1840s, but it also affects other plants in the nightshade family, such as tomatoes, peppers, and eggplants.

This pathogen prefers cool, damp, and humid conditions and can spread rapidly, especially in these environments. It is one of the few plant pathogens that can produce spores both asexually and sexually, leading to great genetic diversity and adaptability.

Symptoms of late blight typically appear later in the growing season. Initial symptoms include irregularly shaped dark spots on leaves, often surrounded by a pale "halo". As the disease progresses, it may cause the entire plant to wilt and die.

It can also cause dark, firm lesions on tubers and fruits. The fruit of plants affected with late blight should not be eaten.

Septoria Blight

Septoria leaf spot, also known as Septoria blight, is a common fungal disease that primarily affects tomatoes but can also occur on other plants. It is caused by the fungus Septoria lycopersici.

Septoria leaf spot usually appears on the lower leaves after the first fruit sets. The disease begins as small, water-soaked spots. These spots darken and enlarge to form circular or angular grey or tan areas that may be up to 1/4 inch in diameter. The center of these spots later turn white and tiny black specks, which are the fruiting bodies of the fungus, can often be seen in the white centers.

This disease does not typically affect the fruit directly, but by defoliating the plant, it can lead to sunscald on the fruit. Also, heavily defoliated plants will produce fewer, smaller fruit.

Type of Blight

Causing Agent

Key Symptoms

Impact on Plants

Early Blight

Alternaria solani

Dark brown spots with concentric rings, yellowing leaves

Early leaf drop, reduced yield

Late Blight

Phytophthora infestans

Water-soaked lesions, mushy appearance on leaves and fruits

Rapid plant decline, potential crop loss

Septoria Blight

Septoria lycopersici

Affects the leaves, primarily, with dark circular or angular grey or tan areas.

Defoliates the plant, leading to sunscald on the fruit.

What Causes Early Blight?

Early blight, caused by the fungus Alternaria solani, is a big threat to tomato plants. This fungus thrives under certain conditions. It's important to know what causes early blight to manage it well. 

The following environmental factors can greatly increase the risk of early blight. By noticing and changing these conditions, you can lower the disease risk in your garden.

Cause

Description

Impact on Plants

Warm, Wet Conditions

High humidity and temperatures between 75-86°F create optimal conditions for fungus growth.

Facilitates rapid spread of early blight.

Moisture on Leaves

Water left on leaves overnight encourages fungal spores to germinate.

Increases the risk of infection.

Overcrowding

Plants that are too close reduce airflow, trapping moisture.

Heightens chance of disease outbreak.

Tomato Plant Stress

Water stress and nutrient shortages weaken plants, making them susceptible to infection.

Increases vulnerability to early blight.

What Causes Late Blight?

Late blight is a major threat to tomato plants, caused by Phytophthora infestans. This pathogen spreads quickly and can destroy crops. Knowing what causes late blight helps us fight it better.

Key causes include:


  • Cool, wet and humid conditions that favor fungal growth.
  • Plant leaves touching the ground are exposed to spores.
  • Overcrowded and unpruned plants.

Knowing how late blight works helps us find ways to stop it.

Stage of Infection

Symptoms

Impact on Plant

Initial Infection

Water-soaked lesions on leaves

Leaves begin to wilt and decline

Advanced Stage

Dark brown, decaying spots

Overall plant health deteriorates

Fruit Infection

Circular, dark spots on tomatoes

Fruit becomes inedible

How to Identify Tomato Blight

It's key to spot tomato blight symptoms early to keep my plants healthy. Catching it early means I can act fast and reduce damage. Knowing the signs of early and late blight helps my tomatoes grow well.

Signs of Early Blight

Looking for early blight signs, I check the lower leaves of my tomatoes. Early blight starts with dark brown or black spots with a yellow ring around them. These spots can make leaves dry up and fall off, weakening the plant and spreading disease.

Signs of Late Blight

Late blight signs are more serious. They show up as light brown, wet spots that grow fast. If ignored, they can destroy the whole plant. Spotting these signs early can save my tomatoes and keep my garden healthy.

Tomato Varieties With Some Resistance to Blight

Choosing the right tomato variety is key to keeping them healthy. I look for blight-resistant tomato varieties that fight off diseases. These include ‘Defiant’ and ‘Brandywine’ which are great against early and late blight.

Tomato Variety

Resistance Type

Characteristics

Defiant

Early & Late Blight

Medium-sized fruit, disease-resistant, great flavor

Brandywine

Early Blight

Large fruits, rich taste, traditional heirloom

Celebrity

Early Blight

Compact growth, versatile use, reliable producer

Sun Gold

Early & Late Blight

Sweet cherry tomatoes, excellent for snacking

Environmental Control

Environmental control measures can greatly lower the risk of blight. Here’s what I focus on:

How to Treat Blight 

Knowing exactly how to treat infected tomato plants can be confusing with so much varying information out there. But rest assured, it's actually very easy to treat any form of blight, as long as you catch it early and treat it until it's gone.

Start by cutting off any infected leaves. This stops the disease from spreading and lets healthier parts grow. Keeping a close watch on your plants helps you catch and remove sick parts quickly.

Using organic fungicides is also important. These products help fight off infections. Applying them regularly during the growing season helps protect your plants from early blight, late blight, Septoria blight and even powdery mildew.

Though you can use organic fungicides like neem oil or potassium bicarbonate, or even copper-based fungicides, they’re expensive. I have used a solution made from ingredients you likely already have in your kitchen for years, and it’s always done the trick.

Baking Soda Treatment for Early Blight, Late Blight & Powdery Mildew

Baking soda works by creating an alkaline environment on the leaf, and fungi cannot colonize the surface of the leaf since they need a neutral pH (around 7.0) to survive and thrive. 

This spray of water and baking soda will change the leaf's pH from around 7.0 to around 8.0. This change is enough to kill and prevent all blight spores!

Supplies & Ingredients Needed

  • 3 Tablespoons Baking Soda - Nothing fancy, just Arm and Hammer Pure Baking Soda will do.
  • 1 Tablespoon Vegetable Oil - Whatever vegetable oil you use in your home for cooking will do. Olive, avocado, canola, etc. This helps the spray to stick to the leaves.
  • 2 Drops Dish Soap - The dish soap helps emulsify the mixture. Don't skip this ingredient! I like to use a free and clear option to avoid unnecessary chemicals and dyes on my plants (we're growing food we want to eat, after all).
  • Spray Bottle -  You'll need some kind of sprayer. Since you'll likely need to reapply this multiple times, I recommend a pressurized pump sprayer

Step-by-Step Directions

  1. Mix 3 tablespoons baking soda with 1 gallon of water. 
  2. Mix in 1 tablespoon vegetable oil, or cooking oil of your choice.
  3. Mix in 2 drops of dish soap. 
  4. Gently stir to combine (while avoiding too many bubbles from forming).
  5. Add solution to your spray bottle and spray on tops and bottoms of leaves until they're dripping with the solution.

Pro Tip: Begin with 3 tablespoons of baking soda and apply once weekly for three weeks. If fungus is still growing or spreading, increase to 3 ½ Tablespoons and continue spraying once per week for an additional three weeks.   

WARNING: Spray in the morning or evening, as spraying in the heat of the day can harm your leaves. It’s ideal to spray in the early morning to avoid wet leaves sitting overnight.

FAQ

What are the main symptoms of tomato blight?

Tomato blight shows up in different ways, depending on the type. Early blight causes dark brown spots with rings on lower leaves, which may die off. Late blight leads to water-soaked lesions that turn light brown, potentially killing the plant.

How can I prevent tomato blight in my garden?

To stop tomato blight, plant tomatoes with enough space for air to move around them. Rotate crops yearly and avoid watering overhead to keep leaves dry. Good sunlight and managing humidity are also key to prevention.

Can certain tomato varieties resist blight?

Yes, some tomatoes like ‘Defiant’ and ‘Brandywine’ are made to fight early and late blight. Choosing these can lower the chance of infection.

What should I do if my tomato plants are infected with blight?

If tomatoes show any signs of blight, cut off and discard any infected leaves to stop the disease from spreading. Use the baking soda spray weekly, watch plants closely and act fast at first signs of disease.

What environmental conditions contribute to the development of tomato blight?

Blight loves heavy dew and cool, wet weather. These create high humidity that helps fungi grow. 

What is the difference between early blight and late blight?

Early blight, from Alternaria solani, hits older leaves with dark spots and a yellow ring. Late blight, from Phytophthora infestans, causes fast plant decline and is known for its severe effects, like the Irish Potato Famine.

How can I identify tomato blight early?

Spotting tomato blight early means checking for leaf spots, wilting, and plant health. Looking for dark spots or water-soaked lesions helps to catch problems early.

How can moisture affect tomato plants and their susceptibility to blight?

Too much moisture from dew or bad watering helps fungal diseases like early and late blight grow. Keeping leaves dry and ensuring good airflow can reduce these risks.

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Comments

Thanks for the information I will write down the recipe

Carol Hensel

Thank you. Now I know what to do.

Jerry

Can you tell me why my Yukon gold potato plants are producing tomato looking fruit on them

Asa

Thank you for this very informative post . I will keep this recipe handy in case I grow these plants. (I would love to grow veggies!)

Kathy

Thanks for info & recipe!

Shawn

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