Bullseye Rings vs Greasy Patches: The One Difference That Tells Early Blight From Late Blight (and How to Stop Each)
Early blight forms bullseye rings on lower leaves; late blight makes greasy patches on new growth and kills plants in days. Here’s how to tell them apart and treat each correctly.
A gardener I know lost her entire tomato crop in late August — not from neglect, but from misidentification. She recognized the brown spots, assumed early blight (the blight she’d managed before), and applied a strobilurin fungicide on a weekly schedule. It did nothing. What she had was late blight, which is caused by an oomycete, not a true fungus, and shrugs off the strobilurin class entirely. By the time she realized the distinction, every plant was gone.
Early blight and late blight share a name and a host plant, but they are caused by completely different organisms, respond to different treatments, and behave on completely different timelines. Getting the ID right on day one is not a minor detail — it determines whether your response actually works.
This guide explains not just what each disease looks like, but why it looks that way — because understanding the mechanism is what makes the visual ID reliable even in unusual or early-stage cases. It also covers common plant diseases that can look similar, so you can rule out the alternatives.
The One Visual Rule That Separates Them
Early blight produces small, firm brown spots with a bullseye of concentric rings — like a target drawn on the leaf. Late blight produces large, spreading blotches with a greasy, water-soaked appearance, as though the leaf was pressed against a wet window. These two visual signatures come from completely different biological mechanisms, which is why they demand different treatments.
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Beyond shape, look at where and how fast the disease is moving:
- Early blight starts on the lowest, oldest leaves and moves upward over weeks
- Late blight hits new growth at the top of the plant first, and can kill the entire plant in 5–7 days under favorable conditions
If your plant is collapsing fast and the sick leaves are at the top, assume late blight until proven otherwise — the time cost of being wrong is your harvest.
Early Blight: Why the Lesions Form Those Rings
Early blight is caused by Alternaria solani and the closely related A. linariae — true fungi that overwinter in infected plant debris and soil. They wake up in spring and infect the lowest leaves first, which are most often hit by rain splash carrying spores up from the ground.
The bullseye pattern is not random. Alternaria grows outward in alternating bursts — rapid expansion, then a pause, then another burst. Each growth cycle lays down a ring of dead tissue. When the pathogen pauses, the plant’s immune response walls off that zone, creating the visible band. The yellow halo surrounding the lesion is the plant’s own response: it breaks down chlorophyll in the immediately surrounding tissue to try to contain the infection.
The conditions that favor early blight are hot and humid:
- Optimal temperature: 82–86°F, though infection can begin at 59°F
- Requires free water or 90%+ relative humidity for spore germination
- Leaf spots appear within five days of infection
- Disease slows significantly during extended dry stretches
On fruit, early blight creates leathery black lesions with raised concentric ridges near the stem end — the same ring pattern expressed in fruit tissue. On stems, look for oval to irregular dry brown areas with dark concentric markings.
Because early blight progresses slowly and plants can carry significant leaf loss without a yield reduction, doing nothing is often the right call for home gardeners. Plants can tolerate a lot of early blight without a meaningful drop in tomato production. Save your fungicide budget for when you genuinely need it.
Late Blight: Why the Lesions Look Greasy (and Why That Changes Everything)
Late blight is caused by Phytophthora infestans — and the first thing to understand is that it is not a true fungus. It is an oomycete, sometimes called a water mold, which is more closely related to algae than to fungi. This matters more than it sounds: the fungicides that work well on early blight — particularly strobilurin-class products in FRAC Group 11 — have essentially no efficacy on late blight. Spraying the wrong product will not slow it down.
The greasy appearance comes directly from how P. infestans infects. It releases zoospores — motile, swimming spores that require a film of water to move across the leaf surface. As the pathogen enters leaf cells, it digests the cell contents and causes cell walls to collapse. That collapse draws water from surrounding tissue outward, creating the characteristic waterlogged, oily look you see before the tissue turns brown. Under high humidity, the pathogen pushes sporangiophores back out through the stomata on the leaf underside, creating the white cottony growth that is late blight’s most reliable diagnostic signature.
Late blight thrives in cool, moist conditions — the opposite of early blight’s heat-and-humidity window:
- Ideal infection period: nights in the 50s to 60s°F, days in the low 80s, with rain, fog, or heavy dew
- Lesions appear within 3–5 days of infection
- Spores travel via wind for 5–10+ miles between gardens
- Complete defoliation of a tomato plant can occur within 14 days
P. infestans is the organism that caused the Irish Potato Famine of the 1840s — an event that killed over one million people. That history reflects the pathogen’s speed: this is not a disease to monitor and revisit. It is a disease to act on the day you see it.
Diagnostic Table: Match Your Symptoms to the Right Response
| Symptom You See | Disease | Urgency | First Action |
|---|---|---|---|
| Small brown spots with bullseye rings, yellow halo | Early blight | Low–moderate | Remove affected leaves; apply 2-in straw mulch |
| Large greasy or water-soaked patches, no rings | Late blight | High — act today | Apply copper or FRAC 40/43 fungicide immediately |
| Disease starts on lowest, oldest leaves | Early blight | Low | Monitor; prune lower 6–8 inches of foliage proactively |
| Disease starts on young top-of-plant growth | Late blight | High | Inspect underside for white mold; spray within 24 hours |
| White cottony growth on leaf undersides | Late blight (confirmed) | High | Apply fungicide; alert neighboring gardeners |
| Spreads slowly over weeks in hot dry weather | Early blight | Manageable | Maintain airflow; fungicide only if more than one-third defoliated |
| Rapid plant collapse after cool wet period | Late blight | Emergency | If more than 50% infected: remove, bag, and bin the plant |
| Fruit: black raised ridges near the stem end | Early blight | Harvest unaffected fruit | Reduce overhead watering; harvest remaining fruit early |
| Fruit: greasy olive-brown lesions anywhere on fruit | Late blight | Discard fruit | Bag and bin all affected fruit — do not compost |
How to Treat Each Disease Correctly
Treating Early Blight
Start with cultural controls before reaching for a fungicide:
- Apply a 2-inch layer of straw mulch around the base — this breaks the soil-splash cycle that carries spores from the ground onto lower leaves
- Proactively remove the lowest 6–8 inches of foliage on mature plants, which is where infection concentrates
- Switch to drip irrigation; overhead watering keeps foliage wet and accelerates spore germination
- Space plants 24–36 inches apart to allow airflow through the canopy
When NOT to spray: If fewer than one-third of your plant’s leaves are affected, hold off. Plants can carry a substantial early blight load without meaningful yield loss. Home garden plants routinely produce well even with moderate early blight.
When to spray: Start at first lesion when nights are consistently above 65°F and rain is forecast, or when more than one-third of the plant is defoliated and disease is spreading fast.
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→ Track My HarvestFRAC rotation for early blight (rotate to prevent resistance):
- Chlorothalonil / Daconil — FRAC M5 (7–10 day intervals, 0-day PHI)
- Copper fungicide — FRAC M1 (organic-approved; 7–10 day intervals)
- Mancozeb — FRAC M3 (7–10 day intervals, 5-day PHI; rated “very good” by Clemson Extension)
- Strobilurins (FRAC 11) — effective but limit to 2 consecutive applications or 5 per season
Treating Late Blight
The most important thing to know: standard strobilurin fungicides (FRAC Group 11) do not work on late blight. You need products specifically labeled for oomycetes.
FRAC groups that work on late blight:
- FRAC 40 — dimethomorph (Acrobat 50WP), mandipropamid (Revus): the backbone of late blight rotations
- FRAC 43 — fluopicolide (Presidio 4FL, 2-day PHI): effective in high-pressure situations
- FRAC 21 — cyazofamid (Ranman 3.6SC, 0-day PHI): fast-acting in rotation with FRAC 40
- FRAC M (chlorothalonil, mancozeb) — tank-mix these with a specific late blight product for best results
Spray intervals: 7–10 days as a preventive; tighten to 5–7 days once late blight is confirmed in your area or on your plants.
Organic option: Fixed copper fungicides (Kocide, Bonide Copper Fungicide) are the only OMRI-labeled products with adequate efficacy against late blight. Apply on a 7-day schedule during cool wet weather — copper is preventive, not curative once infection is established.
When to stop and remove: If more than half the plant is covered in late blight lesions, fungicides will not save it. Pull the plant, seal it in a garbage bag, and put it in the trash — not the compost heap, where spores can survive.
Resistant Varieties Worth Growing
Resistance is the most sustainable long-term defense. Look for seed catalog designations: “EB” for early blight, “LB” for late blight.
Resistant to both diseases:
- Mountain Magic (cherry) — Cornell and NC State Extension rate it outstanding for both early and late blight
- Mountain Merit (large round) — strong dual resistance, good yield
- Plum Regal (roma) — excellent sauce tomato with dual resistance
Strong late blight resistance:
- Defiant PhR — bred specifically for late blight pressure; “PhR” signals Ph-resistance
- Iron Lady — triple resistant: early blight, late blight, and Septoria leaf spot (Cornell breeding program)
- Lizzano and Golden Sweet (cherry) — late blight resistant with excellent flavor
One important caveat on late blight resistance: no tomato variety has complete immunity. Resistance reduces severity and slows the pathogen’s spread — it does not make plants untreatable. Even resistant varieties benefit from preventive copper applications when late blight pressure is high in your region.
For a broader look at how disease resistance is bred into tomato lines and what to look for in the seed catalog, the BadaBing cherry tomato is a recent example of stacking resistance to four major diseases simultaneously.
Prevention: One Set of Cultural Habits, Two Spray Strategies
The same cultural practices protect against both diseases, because both require moisture to infect and spread:
- Mulch heavily: 2 inches of straw or shredded leaves suppresses soil splash and regulates temperature
- Water at the base: drip irrigation or soaker hoses keep foliage dry where spores land
- Space and support: 24–36 inches between plants; cage or stake for airflow through the canopy
- Rotate crops: a 2–3 year break between solanaceous crops (tomatoes, potatoes, peppers) in the same bed reduces overwintering Alternaria inoculum in the soil
- Clear debris in fall: both pathogens overwinter in plant material; removing all foliage and fruit after the season breaks the cycle
Where the strategies diverge is in when to start spraying preventively. For early blight, begin only at first lesion or when weather turns hot and humid. For late blight, start before symptoms appear: once nights drop into the 50s–60s°F after a stretch of wet weather, apply a preventive copper spray. Because late blight spores travel miles on the wind, by the time you see the first greasy patch, your plant is likely already infected at multiple sites. If late blight is reported in your county or region, treat immediately — do not wait for a visible lesion.
For a full protocol on keeping fungal and fungal-like diseases at bay throughout the season, see our guide to preventing and treating fungal infections.
Frequently Asked Questions
Can late blight spread from a neighbor’s potato plants to my tomatoes?
Yes — and it happens regularly. Phytophthora infestans infects potatoes and tomatoes equally well, and its spores travel by wind for 5–10+ miles. If late blight is confirmed in potato fields or home gardens nearby, treat preventively even before you see any symptoms on your plants.
Does copper fungicide work on both blights?
Yes, copper is effective as a protectant against both early blight and late blight — meaning it prevents infection but does not cure active disease once inside the plant. For early blight it is one solid option among several. For late blight in organic systems, it is the primary tool. Apply on a 7-day schedule beginning when conditions turn favorable, not after you see lesions.
Can I eat tomatoes from a plant with blight?
Fruit from early blight-affected plants is safe to eat as long as the fruit itself shows no lesions. Tomatoes with late blight lesions — the greasy olive-brown patches on the fruit surface — should be discarded, as the tissue decays rapidly. Fruit harvested without visible lesions from a late blight plant is safe to eat.
My whole plant wilted and died in a few days — was that late blight?
Possibly, but check for Fusarium or Verticillium wilt, which cause rapid whole-plant collapse through the vascular system rather than via leaf lesions. If the dying leaves showed greasy patches and white mold on the underside before or during collapse, late blight is likely. If leaves turned yellow and wilted with no obvious spots, split the stem at the base: brown discoloration inside the stem tissue points to wilt disease, not blight.
Key Takeaways
Early blight builds concentric rings because Alternaria grows in alternating bursts — the pattern is biological, not coincidental. Late blight creates greasy water-soaked patches because Phytophthora infestans is an oomycete that releases swimming spores into water films and digests cell walls. These two mechanisms require completely different fungicides: FRAC M and M-series multi-sites for early blight; FRAC 40, 43, and 21 specifically for late blight. If you take one thing from this guide: white cottony mold on the underside of leaves is late blight’s signature — if you see it, act the same day.
Sources
- Late Blight of Tomato and Potato — University of Minnesota Extension
- Tomato Late Blight — NC State Extension
- Late Blight of Tomatoes — WVU Extension
- Early Blight in Tomato and Potato — University of Minnesota Extension
- Tomato Diseases and Disorders — Clemson HGIC
