Tomato Blight: How to Tell Early from Late and What to Do About Each

You search "tomato blight" and get two very different answers. One disease is manageable with copper spray and good hygiene. The other has wiped out entire crops across continents and is, honestly, often a death sentence for the plants it reaches. Both go by "tomato blight." Knowing which one you're dealing with changes everything about how you respond.

In the Pacific Northwest, this distinction matters more than almost anywhere else. Our cool, foggy coastal summers are genuinely ideal for the more dangerous of the two. If you grow tomatoes west of the Cascades, late blight is the one to understand before the season starts, not after you see it on your plants.

Two Diseases, One Name

"Tomato blight" is a casual term applied to two distinct diseases caused by two completely different organisms with different biology, different weather preferences, and very different prognoses. Early blight and late blight share almost nothing except that both start on leaves and both can kill your crop. Treating them the same way means losing plants you could have saved, or wasting time on a plant that is already past saving.

This article covers how to tell them apart, what each one does, and the honest truth about what you can and cannot do once either shows up. We also have companion guides on blossom end rot and tomato leaf curl if you're narrowing down what's happening on your plants right now.

What Is Early Blight?

Early blight is caused by the fungal pathogens Alternaria tomatophila and Alternaria solani. Despite the name, it usually appears in midsummer on mature plants. The "early" refers to the fact that it hits older tissue first.

Symptoms to Look For

The hallmark is the target-board pattern: concentric brown rings inside a larger brown spot, giving each lesion a ridged, circular appearance about a quarter to half an inch wide. Spots start on the oldest, lowest leaves and move upward over several weeks. Yellow tissue surrounds the spots; affected leaves eventually drop. Early blight can also cause dark, leathery, sunken lesions on fruit near the stem end.

Flip a leaf with early blight spots and check the underside. You'll see the same brown spots, but no cottony white fuzz. That absence is key.

What Causes It

Early blight thrives in warm, humid conditions, specifically 75 to 84 degrees F with extended leaf wetness. The fungus overwinters in infected plant debris and soil, splashing back up onto lower leaves during rain or overhead irrigation. Stressed plants are far more susceptible.

Here is the local context that matters most: according to the OSU/WSU/UI Pacific Northwest Plant Disease Handbook, early blight is uncommon west of the Cascades. Our summers typically run cooler than the 75-84 degree sweet spot this pathogen prefers. If you see blight symptoms in the coast range or Willamette Valley, odds are higher than you'd expect that it's late blight, not early blight.

What Is Late Blight?

Late blight is caused by Phytophthora infestans, not a fungus but a water mold (oomycete). This is the pathogen behind the Irish Potato Famine of the 1840s, which killed roughly a million people and drove another million to emigrate. That history gives you a sense of what shows up in a garden when late blight arrives.

Symptoms to Look For

Late blight lesions start as irregular, water-soaked greenish-gray greasy patches on leaves. They expand rapidly into large brown-gray dead areas that can encompass most of a leaf within days. Under humid conditions, the leaf underside at lesion margins develops a characteristic cottony white fuzz: the visible sporangia of the pathogen, actively producing spores. This white spore fuzz on the leaf underside is the diagnostic feature that separates late blight from everything else. If you see it, your identification is made.

Fruit develops large brown leathery lesions that often appear oily. The plant does not slowly decline the way it does with early blight. Late blight can collapse a plant entirely within days under cool, wet conditions.

Why It's the Pathogen That Changed History

Late blight spreads through airborne spores that can travel miles on the wind. A neighbor's infected plant, a farm across town, or a diseased transplant from a garden center can introduce the pathogen without any contact with infected soil or debris. You cannot eliminate late blight from your garden by cleaning up and rotating crops, because the next infection may arrive from the air. Prevention, variety selection, and rapid removal of infected plants are the only tools that actually work.

Early Blight vs. Late Blight: The Quick-Reference Comparison

When you're standing in the garden trying to make the call:

Where spots start: Early blight begins on the oldest lower leaves and moves upward slowly. Late blight can appear anywhere and spreads in all directions at once.

Spot appearance: Early blight produces concentric brown rings with a target-board pattern. Late blight produces irregular, water-soaked greasy patches that turn gray-brown with no ring pattern.

Leaf underside: Early blight shows brown spots, no fuzz. Late blight shows white cottony mold at lesion margins under humid conditions. White fuzz on the leaf underside means late blight.

Speed: Early blight spreads over weeks. Late blight can destroy a plant in days.

Weather: Early blight prefers warm days (75-84 degrees F) with moisture. Late blight prefers cool and wet, slows in heat.

PNW risk: Early blight is uncommon west of the Cascades. Late blight is the dominant threat in coastal PNW gardens.

Treating and Managing Early Blight

Early blight is not a death sentence. With consistent management, plants can produce a good crop even when the disease is present. The key is acting early and combining approaches rather than relying on any single tool.

Copper Fungicide: Prevention, Not Cure

Copper-based fungicides prevent new infections on healthy tissue. They do not reverse existing lesions or cure infected leaves. Apply before visible symptoms appear and on a 7 to 14 day interval, reapplying after rain. Organic gardeners can also use biological fungicides like Serenade Opti or Sonata (both Bacillus subtilis-based) for mild cases or as preventive applications. Read copper labels carefully; over-application builds up in soil over time.

Sanitation Is Your Best Tool

Remove affected leaves as soon as you spot them and bag them for the trash. As plants mature, strip the lower 12 inches of foliage to reduce soil-splash points. Mulching the base of plants cuts splash of soil-borne fungal spores onto lower leaves. Use drip irrigation rather than overhead watering. Rotate all nightshade family crops, including potatoes, eggplant, and peppers, on a three to four year cycle since early blight material can persist in soil.

Late Blight: When to Fight and When to Let Go

The honest answer here is the uncomfortable one. Once late blight is established on a plant, there is no cure. No fungicide will reverse the infection, heal damaged tissue, or eliminate the pathogen from infected tissue. If you are spraying a plant that is already showing symptoms, you are likely delaying the inevitable while it produces more spores.

Confirmed Late Blight? Here's What to Do Right Now

Remove the infected plant immediately. Do not shake it or carry it through the garden. Slip it carefully into a large plastic bag, seal it, and put it in the trash. Do not compost it. Both the Royal Horticultural Society and the PNW Plant Disease Handbook recommend bagging and trashing as the only safe disposal method. Do not save seed from blighted plants.

If you have healthy plants nearby, start or intensify a preventive copper spray program on them now. You are protecting what is still clean. Late blight spores travel by wind, so alert any neighbors growing tomatoes or potatoes. Giving them a heads-up may save their crop.

Can Fungicide Still Help?

Preventive fungicide, started before symptoms appear during high-risk weather windows, can slow spread and protect healthy plants. Copper-based products are the primary organic option; conventional options include mancozeb. The PNW Plant Disease Handbook is direct: cultural controls alone cannot prevent late blight in a wet coastal season. Preventive applications before symptoms appear are the most reliable approach when conditions favor an epidemic.

Greenhouse-grown tomatoes are substantially safer from late blight because the structure keeps foliage dry. If late blight is a recurring problem in your garden, a low tunnel or hoop house is worth the investment.

The PNW Angle: Why Late Blight Is Our Biggest Tomato Challenge

Coastal Oregon and Washington are essentially a late blight paradise. Cool days, cool nights, frequent fog, wet maritime summers, and temperature swings that drive condensation onto leaf surfaces: all of it creates the moisture window that Phytophthora infestans needs. Hot, dry weather checks the pathogen. We rarely get enough of it at the right time to protect a crop without intervention.

Why Our Climate Is Late Blight's Favorite

August and September on the Oregon coast bring exactly the combination that triggers epidemics: daytime temperatures in the 60s, coastal fog overnight, and enough rain to keep foliage wet. By the time late blight is visible, conditions may have been favorable for days. The pathogen moves faster than your diagnosis schedule. Treat late blight as a baseline assumption every season.

The Early-Plant Strategy

The most effective PNW-specific strategy is timing. If you transplant tomatoes into a low tunnel or hoop house in late March or April rather than waiting for outdoor planting, fruit can set and ripen before late blight peaks in August and September. An early-planted, covered crop in coastal Oregon can be largely harvested before the worst disease pressure arrives. Start seeds indoors in January or February, harden transplants carefully, and build even a basic low tunnel with wire hoops and plastic sheeting. That investment is modest compared to losing an entire harvest to one wet week in August.

Resistant Varieties Worth Growing in the PNW

Variety selection is one of the few tools that genuinely reduces late blight risk before the season starts. Resistant varieties do not offer immunity, but they show significantly slower disease progression under infection pressure, which can mean the difference between a partial harvest and total crop loss in a bad year.

Legend is the standout for PNW coastal gardens, bred by Oregon State University specifically for late blight resistance in Pacific Northwest conditions. If you grow one resistant variety, start here.

Mountain Magic earns consistent high marks for both flavor and resistance, tending to ripen before the worst of the season. Defiant PHR is a mid-size slicing type with PHR (Partial Host Resistance) tolerance, meaning it slows the disease's progress rather than stopping it. Iron Lady is a compact determinate with strong resistance to both blights, practical for tunnel growing. Plum Regal is the go-to paste type for blight-prone areas. Small-fruited types like Matt's Wild Cherry often ripen fast enough to outpace late blight even without formal resistance ratings.

All of these varieties still benefit from good cultural practices in a wet year. Resistance reduces risk; it is not a license to skip the rest of the work.

Prevention Is the Real Strategy

The most important tomato blight work happens before either disease arrives. Once late blight is established, your options narrow to removal and damage control. Prevention is the strategy, not a backup plan.

Space plants for airflow. Use drip irrigation rather than overhead watering. Mulch generously at the base. Strip the lowest 12 inches of foliage as plants mature. Sanitize pruning shears between plants. Start a copper spray program before symptoms appear and reapply every 7 to 14 days and after rain. Never compost diseased material; double-bag and trash it. Rotate all nightshade family crops on a three to four year cycle.

Plant early under cover. Choose resistant varieties. Keep foliage dry. Remove infected plants without hesitation. Done consistently, these practices give you the best realistic chance of a good harvest in coastal Oregon or Washington, where late blight is a near-annual threat.

For more on what else can go wrong with tomatoes, see our guides on blossom end rot and tomato leaf curl causes and fixes.