Tomato Blight Identification: Visual Symptoms Guide

Tomato blight appears as dark, water-soaked lesions on leaves that rapidly expand into brown or black spots with irregular edges. Early blight shows distinctive target-like concentric rings in lesions, while late blight features white fuzzy growth on leaf undersides in humid conditions. Both types cause yellowing around affected areas and can quickly defoliate plants if untreated.

Spotting tomato blight early saves your entire crop. As a gardener, you need to distinguish between the two main types—early blight and late blight—because their visual signatures differ significantly. This guide provides precise visual identification points so you can confirm whether your plants are affected and take immediate action.

Immediate Visual Identification: What to Look For First

When examining suspect plants, focus on these critical visual markers that distinguish blight from other common tomato issues:

• Leaf lesions - Dark spots with water-soaked appearance that expand rapidly
• Pattern progression - Starts on lower leaves and moves upward
• Underside examination - White fuzzy growth indicates late blight in humid conditions
• Stem lesions - Dark, sunken areas that can girdle stems
• Fruit symptoms - Firm, brown, greasy-looking spots that rot fruit

According to the Cornell University Plant Disease Diagnostic Clinic, misidentification leads to 68% of failed treatment attempts among home gardeners. Proper visual diagnosis is the critical first step before implementing any control measures.

Early Blight vs. Late Blight: Visual Comparison

Feature	Early Blight (Alternaria solani)	Late Blight (Phytophthora infestans)
Initial leaf spots	Small, dark spots with concentric rings (target-like)	Irregular water-soaked spots that rapidly enlarge
Spot color	Brown to black with yellow halo	Dark brown to purplish-black
Underside appearance	No fuzzy growth	White fuzzy mold in high humidity
Progression speed	Slower (weeks to defoliate)	Extremely rapid (days to defoliate)
Fruit symptoms	Leathery, sunken spots near stem	Large, firm, brown greasy spots

How Tomato Blight Develops: Visual Timeline

Understanding the progression helps catch blight before it destroys your crop. The University of California Integrated Pest Management Program documents this typical visual timeline:

Early Stage (Days 1-3)

• Small (1/8 inch), dark spots appear on older leaves
• Early blight: Concentric rings begin forming in spots
• Late blight: Water-soaked appearance without defined edges
• Yellow halo develops around affected areas

Middle Stage (Days 4-7)

• Spots rapidly expand to 1/2 inch or larger
• Early blight: Target pattern becomes clearly visible
• Late blight: White fuzzy growth appears on leaf undersides in morning
• Leaves begin yellowing and dying from spot outward

Advanced Stage (Days 8-14)

• Leaves completely yellow, then brown and die
• Defoliation begins from bottom of plant upward
• Stems develop dark, sunken lesions
• Fruit develops large, rotting spots
• Entire plant may collapse within days for late blight

Conditions That Trigger Blight: When to Be Most Vigilant

Tomato blight doesn't appear randomly. The USDA Agricultural Research Service identifies specific environmental conditions that trigger outbreaks:

• Late blight - Thrives in cool (60-70°F), wet conditions; spreads rapidly during rainy periods
• Early blight - Prefers warm (75-85°F), humid weather; worsens after fruit set
• Critical moisture factor - Leaves must stay wet for 2+ hours for infection
• Seasonal patterns - Late blight appears earlier in season; early blight later

Gardeners in coastal regions report late blight appearing 2-3 weeks earlier than historical averages according to 2024 monitoring data from the National Plant Diagnostic Network. This shifting timeline means vigilance is needed earlier in the growing season.

Differentiating Blight From Similar Tomato Issues

Many gardeners mistake these common issues for blight. Note these key visual differences:

• Septoria leaf spot - Smaller spots (1/16 inch) with dark border and tan center; affects lower leaves first but doesn't create large dead areas
• Fusarium wilt - Yellowing starts on one side of plant; brown discoloration inside stem when cut open; no leaf spots
• Verticillium wilt - V-shaped yellowing from leaf edges inward; symptoms appear during hottest part of day
• Nutrient deficiency - Uniform yellowing between veins (magnesium) or purple undersides (phosphorus); no distinct spots

Immediate Action Steps After Identification

Once you've confirmed blight through visual inspection, take these steps within 24 hours:

1. Remove and destroy affected leaves (do not compost)
2. Water only at soil level to keep foliage dry
3. Improve air circulation by pruning excess growth
4. Apply appropriate fungicide if caught early
5. Disinfect tools after working with infected plants

Remember that late blight requires immediate, aggressive treatment as it can destroy an entire crop within 7-10 days under favorable conditions. Early blight progresses more slowly but still requires prompt action to prevent significant yield loss.

Preventing Future Outbreaks: Visual Monitoring Schedule

Prevention starts with regular visual inspections. Create this monitoring routine:

• Weekly - Check lower leaves for early spots, especially after rain
• Daily during wet periods - Examine leaf undersides for white growth
• Morning inspection - Best time to spot moisture-related symptoms
• Record keeping - Photograph suspicious spots for comparison

Implementing this visual monitoring protocol reduced blight incidence by 47% in home gardens according to a 2023 study published in the Journal of Sustainable Agriculture.

How quickly does tomato blight spread once visible?

Late blight spreads extremely rapidly—visible symptoms can defoliate plants within 5-7 days under favorable cool, wet conditions. Early blight progresses more slowly, typically taking 2-3 weeks to severely damage plants. The speed of spread depends on temperature, humidity, and leaf wetness duration.

Can tomato blight be mistaken for sunscald?

Yes, sunscald can be confused with blight. Sunscald appears as pale, blistered areas on fruit exposed to intense sun, while blight creates distinct dark, expanding spots with defined margins. Sunscald doesn't show the concentric rings of early blight or white fuzzy growth of late blight on leaf undersides.

Do tomato blight spots have a distinctive smell?

Late blight often produces a distinctive musty, moldy odor when leaves are wet and actively infected. Early blight typically doesn't have a noticeable smell. The presence of this odor, combined with visual symptoms, can help confirm late blight identification.

Can tomato blight affect other plants in my garden?

Late blight affects both tomatoes and potatoes, and can spread between them. Early blight primarily affects tomatoes but can also infect eggplants and peppers. Neither type affects unrelated plants like beans or cucumbers. Keeping potatoes and tomatoes separated by at least 20 feet reduces cross-contamination risk.