Tomato Blight: How to Identify, Prevent and Treat This Plant Disease

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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

• Watering Practices: I water my plants in the early morning. This lets any extra moisture evaporate before night, controlling humidity. Learn more about garden watering strategies here. Also, check out this post on how to reduce your garden water bill.
• Sunlight Exposure: My tomato plants get full sunlight most of the day. This keeps their leaves dry and boosts their health.
• Mulching: Using organic mulch keeps soil moisture in and reduces soil splashing on the leaves. This is key for preventing disease. Learn five ways to build organic garden soil here.

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.

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!

• 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. 

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.

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.

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.

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

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.

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

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.

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.

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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