What blight is and why it spreads so fast

Blight is a fungal disease that turns tomato leaves brown, yellow, or spotted, then kills the plant from the ground up or top down depending on which type you have. Early blight starts on lower leaves as brown rings with concentric circles. Late blight looks like water-soaked patches that spread rapidly, especially in cool, wet weather, and can destroy an entire plant in days.

Both types spread through water — rain splash, overhead watering, or even your hands and tools moving between plants. The fungus lives in soil, on plant debris, and on seeds. Once it takes hold, it moves fast because tomato leaves stay wet longer in humid conditions, and the spores germinate in as little as 12 hours.

The reason blight matters is that it cuts off the plant's ability to photosynthesize and move nutrients. A plant covered in dead leaves cannot ripen fruit, and the disease can kill it before harvest. Prevention stops this before it starts.

Key Takeaways

  • Water the soil at the base of plants in the morning, never the leaves, because wet foliage is where blight spores germinate.
  • Remove the bottom 6 to 12 inches of leaves on each plant once it is established, since early blight starts there and spreads upward.
  • Space plants far enough apart that air flows between them, and prune suckers to open up the canopy so leaves dry quickly after rain.
  • Sterilize pruning tools between plants by wiping the blade with a cloth soaked in 10 percent bleach solution, because the fungus spreads on contaminated equipment.
  • Choose disease-resistant varieties labeled with an EB or LB code on the seed packet, which means they resist early or late blight.

Water only the soil, never the leaves

The single most important step is to keep water off the foliage. Blight spores need moisture to germinate, and a wet leaf is an open door. Use a soaker hose or drip line that delivers water directly to the base of the plant, staying below the leaves entirely.

Water early in the morning, ideally before 9 a.m., so any accidental splash on leaves dries quickly in the sun. Avoid watering in the evening, when leaves stay wet all night. If you water by hand, hold the hose low and aim at the soil only — do not let the spray reach the foliage.

After rain, wait until the leaves are completely dry before working in the garden. Walking through wet plants spreads spores on your clothes and tools. On humid mornings when dew covers the leaves, give the plants an hour to dry before you prune or tie them up.

Remove lower leaves and thin the canopy

Early blight lives in soil and splashes onto the lowest leaves first. Once a plant is 12 inches tall and established, remove all leaves within 6 to 12 inches of the ground. This removes the most vulnerable part and stops the disease from climbing upward. Repeat this pruning every two to three weeks as the plant grows, always keeping the bottom clear.

Prune suckers — the shoots that grow in the angle between the main stem and branches — to open up the canopy. Fewer leaves means better air circulation, and leaves that dry faster are less likely to host the fungus. Aim for a plant you can see through, not a dense wall of foliage.

Remove any leaf that shows spots, yellowing, or browning as soon as you see it. Do not wait. The longer a diseased leaf stays on the plant, the more spores it releases. Bag the leaves and throw them in the trash, not the compost pile — the fungus can survive composting.

Space plants and improve air flow

Crowded plants stay wet longer because air cannot move between them. Space tomatoes at least 24 to 36 inches apart, depending on the variety — check the seed packet for the mature width. If you planted them closer, thin seedlings early or transplant extras to another spot.

Stake or cage plants so they stand upright and leaves do not touch the ground or each other. Leaves that rest on soil or on neighboring plants trap moisture and create perfect conditions for blight. A plant supported vertically also makes it easier to remove lower leaves and spot disease early.

If you grow tomatoes in rows, orient the rows so prevailing wind blows across them, not along them. Wind dries foliage and carries spores away. In a garden with still air, consider a small fan on humid days, though this is less critical if you follow the other steps.

Sterilize tools between plants

Blight spores stick to pruning shears, knives, and your hands. If you prune a diseased plant and then move to a healthy one without cleaning, you transfer the fungus directly. Wipe the blade of any cutting tool with a cloth soaked in a 10 percent bleach solution — one part household bleach to nine parts water — after each plant.

Wash your hands and change your shirt if you have been working with a plant you suspect has blight. The fungus can live on fabric for hours. If you are pruning many plants, sterilize tools every few cuts, not just between plants.

At the end of the season, clean all tools thoroughly and store them dry. Do not leave pruning shears in a damp shed where the fungus can survive the winter.

Choose resistant varieties

Seed packets and plant labels list disease resistance codes. Look for EB (early blight resistance) or LB (late blight resistance). Resistant varieties are not immune — they still need good care — but they slow the disease down and give you more time to catch it before it spreads.

Common resistant varieties include 'Mountain Magic', 'Iron Lady', and 'Defiant PhR' for early blight, and 'Katahdin', 'Elate', and 'Plum Regal' for late blight. Check your local seed catalog or nursery for what grows well in your region.

Resistant varieties are especially worth growing if blight has been a problem in your garden before, or if you live in a humid area where the disease is common.

Mulch and clean up debris

Blight fungus overwinters in dead leaves and plant debris on the soil surface. After you harvest, remove all plant material from the garden — stems, leaves, fruit that fell, everything. Do not leave tomato plants standing in the bed after frost kills them.

explore 2 to 3 inches of mulch around the base of plants during the growing season, but keep it 6 inches away from the stem. Mulch prevents soil from splashing onto leaves during rain, which is how early blight reaches the foliage. Wood chips or straw work well.

At the end of the season, do not compost tomato plants or leaves if they showed any sign of disease. Throw them in the trash or burn them if local rules allow. The fungus can survive cool compost piles.

Frequently Asked Questions

Can I use fungicide to prevent blight?

Yes, but only after you have done the steps above. Fungicides work best as prevention, not cure. Copper-based or sulfur-based fungicides can be sprayed on healthy plants every 7 to 10 days starting in mid-summer, especially if blight has been a problem before. Follow the label for mixing and safety. Organic options include neem oil and bacillus subtilis products.

What if I already see spots on the leaves?

Remove the affected leaf when ready and bag it. If only a few leaves show spots, you caught it early and can stop it by removing diseased foliage, improving air flow, and watering only the soil. If half the plant is spotted, the disease is advanced and you may lose the plant. Prune off all diseased leaves, sterilize your tools, and monitor the rest of the plant daily.

Does late blight need different prevention than early blight?

Late blight spreads faster and thrives in cool, wet weather, so prevention is even more critical. The same steps explore — water the soil, remove lower leaves, improve air flow, and sterilize tools. Late blight can kill a plant in three to five days, so if you live in a cool, humid region, choose late-blight-resistant varieties and consider preventive fungicide sprays starting in early summer.

Can I save seed from a plant that had blight?

The fungus does not live inside the seed itself, so seed from a diseased plant is safe. However, if you save seed from a plant that was infected, you may carry spores on the seed coat. Soak seeds in a 1 percent bleach solution for one minute, rinse well, and dry completely before storing.

Why do my tomatoes get blight every year?

The fungus survives in soil and on plant debris, so it returns if you do not clean up thoroughly at the end of the season. Rotate tomatoes to a different bed each year if possible, or replace the top 6 inches of soil where tomatoes grew. Choose resistant varieties, space plants wider, and water only the soil. If blight is severe in your area, consider growing tomatoes in containers with fresh potting soil each year.