Blight is a fungal or bacterial disease that turns tomato leaves brown, yellow, or spotted, and spreads fast in warm, wet conditions

Blight on tomato plants is caused by fungi or bacteria that attack the leaves, stems, and fruit. The two most common types are early blight (caused by the fungus Alternaria) and late blight (caused by the pathogen Phytophthora infestans). Both thrive in humid air and wet foliage, which is why they spread rapidly during rainy seasons or when you water plants from overhead. Early blight typically appears first on lower leaves as brown spots with concentric rings, while late blight shows up as water-soaked patches that turn gray or purple and spread upward through the plant.

The disease spreads through water droplets—either from rain, overhead watering, or even morning dew—and through soil splashing onto leaves. Once spores land on a wet leaf, they germinate within hours and begin infecting plant tissue. Warm temperatures (65 to 80 degrees Fahrenheit for early blight, 60 to 70 degrees for late blight) combined with high humidity create ideal conditions for the fungus to multiply. If left untreated, blight can defoliate an entire plant in two to three weeks and ruin your harvest.

Key Takeaways

  • Early blight and late blight are two separate fungal diseases with different appearance and spread patterns, but both need dry foliage and good air circulation to prevent.
  • Overhead watering and wet leaves are the primary way blight spreads, so water at soil level in the morning and allow foliage to dry quickly.
  • Removing lower leaves on healthy plants and spacing plants far enough apart for air to flow between them cuts blight risk significantly.
  • Fungicide sprays work best as prevention before blight appears; once the disease is established, removing infected leaves and improving conditions is more effective than spraying alone.

How wet foliage creates the conditions blight needs

Blight spores cannot infect a dry leaf. They need standing water or high humidity on the leaf surface to germinate and penetrate the plant tissue. When you water tomatoes from above, or when rain falls and leaves stay wet for hours, you are creating the exact environment the fungus needs to take hold. Morning dew alone can be enough if humidity stays high all day.

This is why overhead sprinklers and soaker hoses that wet the foliage are a major risk factor. Drip irrigation or soaker hoses that deliver water directly to the soil at the base of the plant keep leaves dry and cut blight risk sharply. If you must water from above, do it early in the morning so the sun dries the foliage quickly. Avoid watering in the evening, when leaves will stay wet overnight—that is when spores germinate most readily.

Poor air circulation traps moisture around leaves

Plants crowded together or planted in a spot with little air movement cannot dry out quickly after rain or watering. Stagnant air around the foliage keeps humidity high and allows moisture to linger on leaves for the hours the fungus needs to infect. Tomato plants need space to breathe.

Space plants at least 24 to 36 inches apart, depending on the variety—check the seed packet or plant label for the specific spacing your variety needs. Prune lower branches and remove leaves that touch the ground; these are the first to get splashed with soil-borne spores. As the plant grows, remove leaves below the first fruit cluster to improve air flow and keep foliage away from soil. A fan running nearby on a still day can also help dry foliage and reduce humidity around the plant.

Soil-borne spores splash onto lower leaves

Blight spores overwinter in soil and plant debris. When rain or overhead watering splashes soil onto the lower leaves of your tomato plant, it carries those spores directly onto the foliage. This is why early blight almost always starts on the lowest leaves and works its way up the plant.

Mulching around the base of the plant with 2 to 3 inches of wood chips or straw creates a barrier that reduces soil splash. Keep mulch a few inches away from the stem so it does not trap moisture against the plant. Remove any fallen leaves or plant debris from the ground around your tomato plants, since these can harbor spores that will splash back onto the plant during the next rain.

Warm, humid weather accelerates the disease

Temperature and humidity work together to speed up blight. Early blight moves fastest when daytime temperatures are between 65 and 80 degrees Fahrenheit and humidity is above 85 percent. Late blight prefers slightly cooler, wetter conditions—60 to 70 degrees with high humidity. Both diseases can appear within a few days of the right weather, especially if spores are already present in your garden from last season.

You cannot control the weather, but you can control how your plants respond to it. Proper spacing, pruning, and watering technique reduce how vulnerable your plants are when humid weather arrives. Some tomato varieties are naturally more resistant to blight than others—look for varieties labeled "EB" (early blight resistant) or "LB" (late blight resistant) when you buy seeds or transplants.

Fungicide sprays prevent blight but do not cure it

Fungicides work best as a preventive measure before blight appears. Copper-based fungicides and sulfur sprays are available at garden centers and work on both early and late blight. Spray every 7 to 10 days starting in mid-summer, or sooner if you see the first signs of disease. Always spray in the early morning or late evening when bees are not active, and follow the label directions exactly—more is not better and can damage the plant.

Once blight is visible on leaves, fungicide sprays slow the spread but do not stop it. At that point, your best move is to remove the infected leaves when ready—cut them off with clean pruners and throw them in the trash, not the compost pile. Improve air circulation, stop overhead watering, and allow foliage to dry quickly. If the disease is severe and spreading fast, you may need to remove entire branches or even the whole plant to prevent it from spreading to nearby tomatoes.

Resistant varieties and crop rotation lower your risk

Some tomato varieties have been bred to resist early blight, late blight, or both. Look for seed packets or plant labels that list "EB" for early blight resistance or "LB" for late blight resistance. Resistant varieties are not immune—they can still get blight in severe conditions—but they slow the disease down and give you more time to manage it.

Crop rotation also matters. Blight spores survive in soil and plant debris over winter. If you grew tomatoes in the same spot last year and blight was a problem, plant tomatoes somewhere else this year, at least 3 feet away from where they grew before. Do not compost tomato plants that had blight; throw them in the trash instead. After three years of growing tomatoes in a different location, the spores in the old soil will have died off and you can safely return to that spot.

Frequently Asked Questions

Can I eat tomatoes from a plant with blight?

Fruit from a plant with early blight is safe to eat if the fruit itself has no spots or damage. Early blight primarily attacks leaves and stems, not the fruit. Late blight can infect fruit directly, so do not eat tomatoes with soft, brown patches. If only the leaves are affected, harvest the fruit and remove it from the plant before the disease spreads further.

What is the difference between early blight and late blight?

Early blight appears as brown spots with concentric rings on lower leaves and spreads upward slowly over weeks. Late blight shows up as water-soaked, gray or purple patches that spread rapidly and can infect fruit. Late blight is more destructive and harder to control. Both need wet foliage to spread, but late blight thrives in cooler, wetter conditions and can destroy a plant in days.

Should I remove all the leaves on my tomato plant to stop blight?

No. Removing too many leaves weakens the plant and reduces photosynthesis, which the plant needs to produce fruit. Remove only the infected leaves and the lowest healthy leaves (below the first fruit cluster) to improve air flow. Prune gradually over a few weeks rather than stripping the plant bare at once.

Will blight spread to other vegetables in my garden?

Early blight and late blight are specific to tomatoes and potatoes. They will not spread to peppers, beans, squash, or other garden vegetables. However, if you grow potatoes nearby, blight can spread between tomatoes and potatoes, so keep them separated and do not touch potato plants after handling tomatoes with blight.

Is it too late to save my plant if blight covers half the leaves?

It depends on how much fruit is already on the plant. If the plant is heavily loaded with developing tomatoes, remove all infected leaves, improve air circulation, and stop overhead watering—the fruit may still ripen. If the plant is early in the season with little fruit, removing it entirely prevents the disease from spreading to other tomatoes and gives you space to plant something else.