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Early Blight: How to Control Early Blight of Tomatoes | The Old Farmer's Almanac

Early Blight

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How to Control Early Blight of Tomatoes

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Early blight of tomatoes is a common issue that many tomato gardeners face. If left unchecked, it can destroy your tomato plants and severely hamper your harvest. Here’s how to watch for signs of early blight and keep it in check.

What Is Early Blight?

Early blight is a fungal pathogen of tomatoes. It also affects plants like peppers, potatoes, eggplants, and other members of the nightshade family.

This fungus first attacks the lower leaves of tomatoes, potatoes, peppers, etc., and moves up the plant. It is often confused with Septoria leaf spot, Verticillium and Fusarium wilts, and bacterial leaf spot. 

Early blight tomato infection is caused by the fungus Alternaria tomatophila and Alternaria solani. Two different species but the same genus. 

How Does Early Blight Spread?

During wet or humid conditions, like warm days with significant rainfall or morning dew, the fungus that causes early blight can replicate rapidly. Spores can be produced as quickly as 3-5 days after spots first appear. 

These spores are then ready to infect more areas of your plant or drift to an adjacent victim. If you are curious about the details of this fungus’ life cycle, check this summary from the National Center for Biotechnology Information

Alternaria spores can be spread by wind, people and animals, and by contaminated tools. Lower leaves commonly become infected when in contact with contaminated soil or when soil is splashed onto the leaves by rain or irrigation.

High humidity levels, rain, and wind contribute to spreading Alternaria spores. In other words, warm, humid summers with evening thunderstorms, like many of us experience in June, July, and August. The spores can overwinter in the soil and in contaminated plant material.

Early blight can cause cankers that girdle the plant. The Alternaria fungus also releases phytotoxins, toxic chemicals to the host plant—your tomato or potato. The result is struggling or even dead plants, and a reduced harvest.

Identification

Early blight begins on lower, older leaves in midsummer when temperatures warm. Watch for these signs when walking in your garden:

  • Small dark spots may appear on older leaves near the soil. Spots are brown and round and may develop concentric rings. 
  • Early blight spots range from the size of a pencil eraser to almost as big as a dime.
  • Leaf tissue surrounding the spots may turn yellow.
  • Eventually, infected leaves will turn brown completely and wither. They may fall off or remain attached to the plant as dried-out husks.
  • Stem infections are often oval in shape and also present in concentric rings.
  • Fruit can be infected and develops dark, leathery spots with raised ridges. Infection happens near the stem, and the fruit may drop off. 
Early blight affecting the stem and leaves of a tomato plant. Photo by AmBNPHOTO/Shutterstock
Early blight affecting the stem and leaves of a tomato plant. Photo by AmBNPHOTO/Shutterstock

Is It Early Blight or Another Problem?

As mentioned, several tomato diseases are commonly mistaken for each other. Sometimes, your tomatoes may have more than one issue going on. 

Here’s a quick primer, and if you want more information on how to avoid misdiagnosis, check this page from NC State Extension

  • Early blight: ¼ to ½ inch brown spots on leaves. Oval spots on stems. Fruit is affected near the stem. Many spots will have concentric rings like a bullseye. Starts on lower leaves.
  • Septoria leaf spot: Much smaller spots on leaves than early blight. Spots are usually round, about 1/16 of an inch, and often light gray in the center.
  • Fusarium and Verticillium wilts: Wilting is the key indicator. Stems will show brown fungus inside the water-carrying tissues. Leaf yellowing is common. Fusarium may wilt only one branch or one side of the plant at first.
Control and Prevention

Early blight is a fungal disease, and for the home gardener, mass application of fungicides is not practical or advised. Your best option is to apply some Integrated Pest Management principles to limit the fungus’ spread and survival.

Prevention and Monitoring

Early blight is common in tomato gardens. While it may be impossible to prevent it completely, several measures can limit the spread and intensity of early blight.

  • Closely inspect any transplants you purchase and bring home. If starting plants from seed, disinfect your containers and tools to kill any potential fungus from the previous season.
  • Alternaria are soil-borne fungi, so cleaning your equipment is essential. Snippers, cultivators, shovels, trowels, and any tool used on your tomatoes, potatoes, eggplants, and peppers should be disinfected before moving to another area of the garden. 
  • Give your tools a quick scrub to remove soil, then dip or spray them with a mild bleach solution. 

Choose Tolerant/Resistant Plants

Some tomato varieties exhibit more tolerance to early blight. Unfortunately, this tolerance doesn’t mean complete immunity, but rather that the tomato will “shrug off” and be less affected by the fungus. 

Many hybrids and even a few heirloom varieties are available that are less susceptible to Alternaria fungi and blight.

Treatments for Early Blight

Early blight cannot be cured, and cultural methods such as those below are the most effective for the home gardener. Fungicidal treatments for early blight are costly, environmentally damaging, labor-intensive, and thus not practical. 

Keep your plants growing vigorously

Fertile, rich soil and lots of sunlight can go a long way to helping tomatoes (and potatoes, peppers, and eggplants) keep marching when early blight starts to show up.

Compost will help your tomatoes to thrive, without the lack of fruiting that too much artificial fertilizer can cause.

Irrigate from below

If you irrigate your garden, try to avoid overhead systems for your tomatoes. Alternaria thrive in wet, humid conditions. The combination of damp foliage and soil splash from overhead sprinklers can worsen a fungus problem. 

Use drip irrigation or, if you water by hand, take care to water at the base of the plant and not from up high. 

Mulch, mulch, mulch

Mulch is your friend. From weed suppression to soil protection and eventual breakdown into the soil’s organic matter, mulch is hard to beat. In the case of early blight, mulching reduces soil splash from rain or irrigation. It provides a barrier to fungus movement from the soil to the lower leaves.

Prune your tomatoes

Keeping the lower leaves from touching the soil, and promoting airflow through the plant, especially near the ground, can significantly reduce the likelihood of many tomato diseases becoming established. Remove those bottom leaves that aren’t doing much, and let in some fresh air.

Remove infected tissues early

Leaves showing signs of early blight should be pruned off and disposed of. Bury them deeply or burn them. Slowing the progression of the fungi can provide the plant an edge to keep growing up top and have time to give a batch of tasty tomatoes. 

Removing the entire plant in the fall after your harvest will eliminate a significant spore source for next spring. Spores can overwinter in the dead vegetation they infected.

Rotation is important

For early blight, rotating where you grow tomatoes and related crops can provide a considerable advantage to your plants.

Alternaria fungi overwinter in the soil, so grow something from a different plant family for two years before returning tomatoes, potatoes, or other Solanaceae to that plot. 

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