What blight is and how to spot it on your tomatoes

Blight is a fungal disease that kills tomato leaves, stems, and fruit. Two types attack tomatoes: early blight and late blight. Early blight appears first on lower leaves as brown spots with concentric rings, like a target. The spots grow larger, the leaf yellows around them, and the leaf drops off. Late blight looks different — it starts as water-soaked patches on leaves and stems, often with a white mold on the underside, and spreads much faster, especially in cool, wet weather.

Early blight usually shows up mid-season when plants are established and leaves touch the soil. Late blight is more serious and can destroy an entire plant in days. Both spread through water — rain splash, overhead watering, or wet hands moving between plants. If you see these signs, the disease is already present and you need to act within days to stop it from spreading to the rest of your garden.

Key Takeaways

  • Early blight starts on lower leaves as brown rings; late blight appears as water-soaked patches and spreads much faster in cool, wet conditions.
  • Remove infected leaves when ready and dispose of them in the trash, not the compost pile, because the fungus survives in plant debris.
  • Water only at the soil level, never on leaves, and water early in the day so any splash dries quickly.
  • Fungicide sprays work best as prevention before disease appears; once blight is visible, they slow it but do not stop it completely.
  • Space plants far enough apart that air flows between them, and prune lower leaves once the plant is established to keep it away from soil splash.

Remove infected leaves and stop the spread when ready

The moment you see blight, remove every infected leaf by hand. Cut or pinch off the entire leaf, including the stem that holds it. Do not leave any part of the diseased leaf on the plant. Put these leaves in a sealed bag and throw them in the trash — do not add them to a compost pile, because the fungus survives in dead plant material and will spread to your next garden.

If blight is on the stem or fruit, cut out the diseased section with a clean knife, cutting well into healthy tissue. Sterilize the knife between cuts by wiping it on a cloth soaked in rubbing alcohol or a 10 percent bleach solution (one part bleach to nine parts water). Wash your hands after handling infected plants, because you can carry the fungus to other tomatoes on your hands or clothes.

After removing leaves, prune the lowest 6 to 12 inches of healthy leaves from the plant as well. This removes the leaves closest to soil splash, which is where blight usually starts. It also improves air flow around the base of the plant, which helps prevent the disease from taking hold in the first place.

Change how you water to prevent the fungus from spreading

Stop watering from above. Overhead watering — whether by sprinkler or by hand — splashes soil and fungal spores onto leaves. Instead, water only at the soil level, using a soaker hose or drip line that delivers water directly to the roots. This keeps leaves dry, which is the main condition the fungus needs to grow.

Water early in the morning, between 6 and 8 a.m. if possible. Morning watering means any water that does splash onto leaves will dry quickly as the day warms up. Evening watering leaves leaves wet all night, which gives the fungus hours to establish itself. If you must water in the evening, do it at least three hours before sunset so leaves have time to dry.

After watering, do not walk through the garden or brush against wet plants. Wet leaves are more vulnerable to infection, and moving through them spreads fungal spores from plant to plant. Wait until the foliage is completely dry before you prune, harvest, or tie up vines.

Use fungicide spray to prevent blight before it appears

Fungicide works best as prevention, not as a cure. If you spray before blight shows up, the fungicide creates a protective coating on leaves that stops the fungus from taking hold. Once blight is visible, fungicide slows the spread but does not eliminate the disease — you still have to remove infected leaves.

Copper fungicide and sulfur fungicide are the most common options for home gardeners. Copper is effective against both early and late blight and is approved for organic gardening. Sulfur works well for early blight but is less effective against late blight. Follow the label directions exactly — spray every 7 to 10 days during the growing season, or more often if you have had blight in past years or if the weather is cool and wet.

Spray in the early morning or late evening when bees are not active. Coat both the top and bottom of leaves, because the fungus can live on either side. Do not spray when the temperature is above 85 degrees Fahrenheit, because the fungicide can burn the leaves. Stop spraying two weeks before you plan to harvest, even if the label allows it, to avoid any residue on the fruit you eat.

Space plants and prune for air flow

Blight thrives in humid, still air. When tomato plants are crowded together, leaves stay wet longer and air cannot circulate to dry them. Space plants at least 24 to 36 inches apart, depending on the variety — check the seed packet or plant label for the recommended spacing. This sounds far apart, but it gives air room to move between plants and lets you water at soil level without splashing one plant onto another.

As the plant grows, prune side branches and lower leaves to open up the center. Remove any leaves that touch the ground or hang below the lowest fruit. Prune off branches that cross or crowd each other. The goal is to see light and air moving through the plant, not a dense wall of foliage. This is especially important for indeterminate varieties, which grow tall and can become very dense if left unpruned.

If you grow tomatoes in the same spot year after year, blight fungus builds up in the soil and plant debris. Rotate tomatoes to a different bed each year, at least 3 feet away from where you grew them before. If rotation is not possible, remove all plant debris at the end of the season and replace the top 6 inches of soil.

Choose resistant varieties if blight is a recurring problem

Some tomato varieties are bred to resist early blight, late blight, or both. If you have had blight in past years, growing a resistant variety is the most reliable way to prevent it. Look for varieties labeled with EB (early blight resistance) or LB (late blight resistance) on the seed packet or plant label.

Resistant varieties do not may provide you will never see blight, but they make the disease much less likely and less severe. Common resistant varieties include Mountain Magic, Iron Lady, and Defiant for early blight resistance, and Katahdin and Elate for late blight resistance. Check with your local extension office or seed catalog to find varieties suited to your climate and growing season.

Frequently Asked Questions

Can I save a plant with late blight, or should I pull it out?

If late blight is caught very early — just a few spots on one leaf — aggressive removal of infected leaves plus daily fungicide spray may slow it. But late blight spreads so fast that most gardeners find it faster to remove the plant entirely, dispose of it in the trash, and focus on protecting nearby plants. If you pull the plant, do not compost it.

Is it safe to eat tomatoes from a plant with blight?

Fruit itself is usually safe to eat if it has no visible spots or damage. The fungus lives in leaves and stems, not in the fruit. Wash the tomato before eating. However, if the plant is severely infected, it may not produce ripe fruit, and the stress of disease can make the plant stop growing altogether.

Can I use the same stakes and cages on a new plant if the old one had blight?

Yes, if you clean them first. Wipe stakes, cages, and any tools with a cloth soaked in a 10 percent bleach solution. Let them dry completely before using them on a new plant. This removes fungal spores that may have landed on them during the season.

Does rain cause blight, or does it just make it worse?

Rain does not cause blight, but it spreads it. The fungus needs moisture on the leaf surface to infect the plant. Heavy rain or frequent wet weather creates ideal conditions for the disease to take hold and spread quickly. In dry summers, blight is much less common even if the fungus is present in the soil.

How long does the blight fungus live in soil?

Early blight fungus can survive in plant debris in the soil for one to two years. Late blight fungus survives in infected potato plants and some weeds, and can persist longer. This is why removing all plant material at the end of the season and rotating crops to a different bed are so important for breaking the disease cycle.