Tomato Blight: Early vs Late Blight — How to Tell Them Apart and Save Your Crops

Tomato Blight: Early vs Late Blight — How to Tell Them Apart and Save Your Crops

It is mid-July in Zone 7a. Your tomato plants looked great last week. Now you notice dark spots on the lower leaves and your stomach drops. You have seen this before. The plants that got hit last year didn't make it past August.

Here is the thing: not every dark spot on tomato leaves is the same disease. Early blight and late blight look similar to an untrained eye but are caused by completely different pathogens, thrive in different weather conditions, and require slightly different management approaches. Knowing the difference can save your crop.

There is also a third look-alike, Septoria leaf spot, that adds to the confusion. Let's walk through everything you need to know.

What Is Early Blight?

Early blight is caused by a fungus called Alternaria linariae. It is the most common tomato disease most home gardeners encounter. It is annoying, sometimes devastating, but usually manageable if you catch it early.

Here is what to look for:

• Starts on the lower leaves. This is your earliest warning sign. The fungus lives in the soil and on crop debris, so it splashes up onto the bottom of the plant first.
• Small brownish-black spots appear that grow to about a quarter to half an inch across.
• Concentric rings inside the spots. This is the classic tell. The lesions look like a target, with alternating dark and light rings. If you see that pattern, you are almost certainly looking at early blight.
• Yellowing around the lesions. A yellow halo usually surrounds each spot before the leaf turns completely brown and dies.
• Progresses upward. The disease moves from the bottom of the plant toward the top. As lower leaves die and drop off, your tomato plants end up with bare stems and exposed fruit.
• Fruit infection at the calyx. Spots can develop where the stem attaches to the tomato. They are sunken, leathery, dark, and often show the same ring pattern. These spots on the fruit itself are usually small but can expand.

Early blight favors warm weather (above 80 degrees F) and moderate to heavy rainfall. If you had a hot, rainy summer, early blight will likely show up. The spores are spread by wind and water splash, and they survive between seasons in infected crop debris left in the soil and in volunteer tomato plants that pop up from seed dropped the previous year.

What Is Late Blight?

Late blight is caused by Phytophthora infestans, a water mold (not a true fungus). This is the same pathogen that caused the Irish Potato Famine in the 1840s. It is no joke.

Here is how to spot it:

• Appears ALL over the plant, not just the lower leaves. Unlike early blight, late blight will hit young upper leaves, stems, and fruit simultaneously.
• Lighter tan colored lesions with a pale green or yellow halo. The spots tend to look watery or soggy, not dry and leathery.
• White, fuzzy growth on leaf undersides in humid conditions is a dead giveaway.
• Rapid progression. Under the right conditions, a plant can go from a few spots to completely ruined in just a couple of days.

Late blight thrives in cool, wet weather, which is the opposite of early blight's preferred conditions. A cool summer with persistent rain or fog in Zone 7a is a late blight risk. If you get one of those summers, expect trouble.

Late blight spreads incredibly fast through wind and water. Once it shows up in your garden, it can move to your neighbors' gardens within days. You cannot sit on this one.

Septoria Leaf Spot: The Look-Alike

Septoria leaf spot deserves a mention because it often shows up alongside blight and adds to the confusion.

Septoria lesions are smaller than either blight type. They have tan or light gray centers, often with dark edges. They tend to appear on lower leaves first like early blight, but the spots are much smaller and don't show the concentric ring pattern.

Septoria is more common than blight and usually less devastating. The management is similar but not identical, which is why getting it right matters. If you aren't sure whether you are dealing with blight or Septoria, the preventive steps below will help either way.

Prevention: The Best Defense

No garden is ever completely free of tomato diseases. But the right habits dramatically reduce your odds of losing plants to blight.

• Water at the base of plants. Never water overhead. Wet foliage is the number one driver of fungal and water mold infections. Drip irrigation or soaker hoses are ideal.
• Trellis or stake your tomatoes. Good airflow around plants makes it much harder for disease spores to establish. Caging tomatoes is fine, but staking or trellising gives the most air circulation.
• Rotate tomato locations. Don't plant tomatoes, peppers, or potatoes in the same spot year after year. The spores of both early and late blight survive in the soil. A minimum two-year rotation is the standard recommendation.
• Clean up all tomato debris at season's end. This is non-negotiable. Pull everything, bag it, and throw it in the trash. Do not compost infected material. Those spores will survive composting and come back to haunt you next year.
• Space plants properly. Crowded plants = poor airflow = perfect conditions for disease. Follow the spacing recommendations on your seed packet. If you are planting heirlooms that can get six feet wide, give them six feet.
• Mulch around plants. A two to three inch layer of mulch prevents soil-borne spores from splashing up onto lower leaves during rain or watering. This alone can significantly reduce early blight pressure.
• Choose resistant varieties when possible. This is your biggest advantage. More on this below.

What to Do When You See Symptoms

If you spot blight on your plants, act fast. Here is the protocol:

1. Remove affected leaves immediately. Use clean pruners. Start with the most severely affected lower leaves first, since early blight starts at the bottom.

2. Sanitize your pruners between plants. Wipe them with a mild bleach solution or rubbing alcohol. You don't want to carry spores from an infected plant to a healthy one.

3. Bag the infected material and trash it. Do not put it in your compost pile. Composting doesn't get hot enough to kill blight spores. The spores will survive and infect your garden next year.

4. Apply a copper-based fungicide to remaining healthy foliage. This won't cure infected leaves but can protect healthy ones from getting infected. Copper fungicide works best as a preventive measure, so apply it early.

5. In severe late blight cases, remove the entire plant. If the infection is widespread and the plant is clearly declining, pull it out. Bag it. Trash it. Delaying this decision can cost you your entire garden.

Common Mistakes That Worsen Blight

Most gardeners who lose tomatoes to blight made one or more of these errors:

• Putting infected leaves in the compost pile. This is by far the most common mistake. Blight spores survive home composting and end up spread all over your garden when you use that compost next spring. This single mistake can set your garden back a year.

• Overhead watering in the evening. Evening watering means leaves stay wet all night. That's exactly the extended moisture period spores need to germinate and infect. If you water overhead, do it in the morning so foliage dries by afternoon.

• Waiting too long to act. Blight moves fast in humid summer weather. By the time you notice symptoms, the spores may have already spread to nearby plants. If you see early blight on a few leaves, remove those leaves that same day.

• Ignoring the lower leaves. Early blight starts on the bottom. Those lower leaves are your canary in the coal mine. Check them regularly, especially during warm, wet stretches.

Resistant Varieties for Next Year

If blight hit you hard this year, the most important thing you can do is choose different varieties next time. Here are some that hold up well:

• Defiant PHR. This variety was specifically bred for early blight resistance. It has consistently outperformed most other varieties in blight conditions. If you only pick one thing to remember, make it this one.

• Mountain Magic. Good overall disease resistance, including to both early and late blight. A solid choice if blight is a recurring problem in your area.

• Hillbilly. Another heirloom that shows good blight resistance, which is unusual for heirlooms.

• The heirloom tradeoff. Most heirloom varieties lack blight resistance. This is an important thing to know going in. Heirlooms have their own charms, but if you live in a humid, blight-prone area like Zone 7a, you should weigh that tradeoff carefully. There is nothing sadder than watching your beloved Brandywine get taken out by blight when Defiant would have kept producing all season.

When to Cut Your Losses

There comes a point where a plant is beyond saving. Here is how to recognize it:

If 70% or more of the foliage is gone, the plant simply cannot produce enough photosynthesis to support fruit. You might get a few more tomatoes, but the plant is dying and it is still spreading spores to the healthy plants around it. Pull it.

Late blight spreads faster than early blight. If you confirm late blight, there is rarely a good reason to try saving an individual plant. The pathogen is too aggressive and too contagious. One infected plant can wipe out a whole garden in under a week during wet weather. Better to remove it immediately and protect your neighbors.

It feels bad to pull a dying plant. You invested time, effort, and hope in it. But holding on to a sick plant is like holding on to a sick family member who might infect everyone else. It isn't sentimental. It's practical.

Final Thoughts

Tomato blight is frustrating, but it is also predictable. If you know what to look for, how to prevent it, and when to act, you can keep most of your crop healthy even in bad blight years.

The key takeaways:

• Watch your lower leaves first. They tell you what's coming.
• Water at the base. Keep foliage dry.
• Clean up at season's end. Trash infected material. No compost.
• Rotate locations. Give the soil a break.
• Pick resistant varieties next year. Defiant PHR if blight is your main problem.
• Act fast. Blight doesn't wait for you to be ready.

Your tomatoes don't have to disappear in July. A little knowledge goes a long way.

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— C. Steward 🍅