Early Blight vs. Late Blight on Tomatoes: Identification and Management

"Blight" is a term used loosely to describe rapid browning and death of plant tissue on tomatoes, but the two diseases commonly called blight are caused by fundamentally different organisms and require different management responses. Using copper fungicide for late blight is inadequate. Using a.

—- title: "Early Blight vs. Late Blight on Tomatoes: Identification and Management" slug: blight-on-tomatoes-early-vs-late hub: problems category: "Problem-by-host" description: "Early blight and late blight on tomatoes have different pathogens, different symptoms, and require different fungicides. Misidentifying them leads to failed treatment." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 9 scientific: "Solanum lycopersicum" —-

"Blight" is a term used loosely to describe rapid browning and death of plant tissue on tomatoes, but the two diseases commonly called blight are caused by fundamentally different organisms and require different management responses. Using copper fungicide for late blight is inadequate. Using a mefenoxam-based oomycide for early blight is overkill and unnecessarily expensive.

The ability to tell them apart — before spraying — is a practical skill that makes a measurable difference in outcomes.

Pathogen Distinction

This distinction drives all subsequent management decisions.

Per Penn State Extension, early blight is caused by Alternaria solani — a true fungus with chitin-based cell walls. Most conventional and organic fungicides have activity against true fungi.

Per Cornell Cooperative Extension, late blight is caused by Phytophthora infestans — an oomycete (water mold) with cellulose-based cell walls, more closely related to algae than to true fungi. The cell wall difference means fungicides targeting fungal chitin synthesis (most conventional fungicides) have limited effect on P. infestans. Specific oomycide-class products are required for effective late blight control.

Early Blight: Full Description

Symptom Pattern on Tomatoes

Per NC State Extension:

• Location: Begins on the oldest, lowest leaves; progresses upward as the season advances
• Spot shape: 1/4 to 1/2 inch, circular to angular
• Spot appearance: Dark brown spots with a distinctive bulls-eye or target pattern of concentric rings — this is the diagnostic feature
• Yellow halo: Chlorotic (yellow) zone surrounding the dark spot
• Stem cankers: A. solani can infect stems near the soil line, causing dark, sunken collar rot in seedlings
• Fruit infection: Occasionally infects fruit at the calyx end, causing dark, leathery lesions, but this is uncommon in home gardens

Timing and Conditions

Per Penn State Extension, early blight:

• Appears after plants begin flowering (not in seedling stage typically)
• Most active at 75–86°F
• Favored by alternating wet and dry conditions
• Most severe when plants are nutrient-stressed, particularly nitrogen-deficient

Early blight is essentially ubiquitous in northeastern gardens where tomatoes have been grown previously. A. solani spores persist in soil and plant debris for years.

Management of Early Blight

Per Cornell Cooperative Extension:

• Copper fungicides (OMRI listed): Applied at 7–10 day intervals from flowering; effective preventively
• Chlorothalonil (Daconil): Broad-spectrum protectant; conventional; apply at 7-day intervals in wet weather
• Mulch: 3–4 inch layer beneath plants prevents soil-splash inoculation of lower leaves
• Drip irrigation: Reduces leaf wetness duration
• Remove infected lower leaves: Primary local inoculum source; remove and dispose of (do not compost)
• Adequate nitrogen fertility: Nitrogen-stressed plants senesce faster and are more susceptible
• Crop rotation: 2–3 years out of tomatoes/potatoes reduces local inoculum

Late Blight: Full Description

Symptom Pattern on Tomatoes

Per Penn State Extension:

• Location: Can appear on any part of the plant — leaves, stems, fruit
• Leaf lesion: Pale green to grayish, water-soaked spots with poorly defined, irregular margins; the spots have a greasy, oily appearance
• Sporulation: In humid conditions, white to gray cottony mold appears on the undersurface of lesions — this is the single most reliable visual sign
• Stem lesions: Dark brown to black; may girdle the stem causing rapid wilting above the lesion
• Fruit infection: Firm, brownish-black lesions on green fruit; fruit remains firm initially (unlike bacterial soft rots); lesions expand rapidly
• Speed: The entire plant can be killed within 5–10 days of initial symptoms in cool, wet weather

Timing and Conditions

Per Cornell Cooperative Extension, late blight is favored by:

• Cool temperatures: 60–70°F (optimal for P. infestans sporulation and infection)
• Extended leaf wetness: 10+ hours of continuous moisture
• Overcast skies, fog, heavy dew, rain
• Temperature differentials between day and night

In the Northeast, late blight outbreak risk is highest in June–July during cool, wet periods and again in September. In hot (above 80°F), dry summers, late blight rarely develops.

Management of Late Blight

Per Penn State Extension, late blight requires specific responses:

Preventive (before symptoms appear):

• Monitor regional blight alerts (USABlight.org, your state Extension)
• Apply a protective oomycide-class product when late blight is reported in your county or adjacent areas:

- Chlorothalonil + cymoxanil (Curzate) combination - Mandipropamid (Revus) - Fluopicolide + propamocarb (Previcur Flex — commercial)

At first symptom detection:

• Apply an oomycide immediately — late blight spreads fast enough that 24-hour delay matters
• Remove and destroy heavily infected plant parts; do not compost; bag or bury
• Inspect and treat all solanaceous plants in the area (potatoes, peppers)

For home gardeners: Per Cornell Cooperative Extension, the most accessible options for home garden late blight management are:

• Copper + chlorothalonil combination: Provides more complete protection than copper alone but less than dedicated oomycides; practical for organic gardeners willing to apply at 5-day intervals
• Fixed copper (high-rate copper hydroxide or copper octanoate): OMRI listed; some late blight protection; must apply preventively and frequently in wet weather

Comparision Table

Common Problems

Frequently Asked Questions

Can both diseases be present at the same time?

Per Penn State Extension, yes. Early blight is nearly always present to some degree on late-season tomatoes in the Northeast. During a cool, wet period, late blight can develop on the same plant simultaneously. The presence of target-ring spots does not rule out late blight on other parts of the plant.

Does late blight spread from tomatoes to potatoes?

Per Cornell Cooperative Extension, yes — P. infestans infects both tomato and potato. Distance between plantings matters less than you might expect since sporangia are windborne. See also: Blight on Potatoes for the potato-specific guide.

Are any tomato varieties resistant to late blight?

Per NC State Extension, several varieties carry the Ph-3 resistance gene, which confers partial resistance to some P. infestans races. Varieties with documented late blight resistance include 'Mountain Merit', 'Defiant PHR', 'Iron Lady', and several heirloom-style varieties. Resistance is not immunity — these varieties tolerate infection better but can still be damaged in severe outbreak years.

How does the fungicide resistance problem affect late blight management?

Per Penn State Extension, P. infestans populations have developed resistance to mefenoxam (Ridomil) in some regions. Rotating oomycide classes (alternating mandipropamid, cymoxanil, and fluopicolide) and combining them with chlorothalonil protectants reduces selection pressure for resistance.

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Recommended gear: Best tomato varieties for the home garden — determinate vs indeterminate — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Sources

1. Penn State Extension — Early Blight of Potato and Tomato
2. Penn State Extension — Late Blight
3. Cornell Cooperative Extension — Tomato Late Blight
4. NC State Extension — Vegetable Gardening Handbook
5. Clemson HGIC — Tomato Diseases