What are the earliest signs of tomato blight?

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Published: October 23, 2025

Updated: October 23, 2025

Recognizing the characteristic initial symptoms as they appear on leaves is necessary to be able to combat tomato blight at its incipient stage. Early blight shows on the lower leaves as small brown focus-shaped rings surrounded by yellow. In late blight, there is an appearance of olive-green, wet spots that feel moist. In Septoria, numerous small grayish eyes with dark outer edges are produced. All of these appear with the greatest rapidity at a humidity of over 60 per cent, and with a temperature exceeding 60°F. (15°C.).

Early Blight Symptom Comparison
Blight Type	First Visible Signs	Location Pattern	Speed of Spread
Early Blight	0.25-inch brown target rings with yellow halos	Starts on oldest lower leaves	Moderate (3-5 days between leaves)
Late Blight	Dime-sized olive wet-looking patches	Often begins on new upper growth	Rapid (entire plant in 48-72 hours)
Septoria Leaf Spot	Pinhead-sized gray spots with dark borders	Lower leaves near soil surface	Fast (defoliation in 7-10 days)

All types cause yellowing and leaf drop in humid conditions above 60°F (15°C)

Early Blight Symptom Comparison

Blight Type

Early Blight

First Visible Signs0.25-inch brown target rings with yellow halosLocation PatternStarts on oldest lower leavesSpeed of SpreadModerate (3-5 days between leaves)

Blight Type

Late Blight

First Visible SignsDime-sized olive wet-looking patchesLocation PatternOften begins on new upper growthSpeed of SpreadRapid (entire plant in 48-72 hours)

Blight Type

Septoria Leaf Spot

First Visible SignsPinhead-sized gray spots with dark bordersLocation PatternLower leaves near soil surfaceSpeed of SpreadFast (defoliation in 7-10 days)

All types cause yellowing and leaf drop in humid conditions above 60°F (15°C)

The evolution of symptoms develops in predictable ways that merit watching. Early blight and Septoria blight move up the plant from the base. The trend of late blight is downward and occurs quickly after it appears on new growth. I carry a 10x magnifier to check the minuscule black fungal structures of Septoria. Notice lesions feel differently: early blight spots remain firm while late blight areas become mushy quickly.

Identification Tools

Use magnifying glass to see Septoria's pycnidia (black dots)
Press affected leaves against white paper for better contrast
Compare lesions to university extension disease photos

Immediate Actions

Isolate plant immediately upon suspicion
Take timestamped photos with ruler for scale
Remove one affected leaf in sealed bag for testing

Common Misidentifications

Sunscald shows white papery patches without yellow halos
Bacterial spot creates smaller black raised dots
Magnesium deficiency causes yellowing between veins

Symptoms are primarily a function of the environment. Blight spreads most rapidly when the average humidity is over 80% for over 48 hours.Temperatures of 65 -80ºF (18 -27ºC) hasten the development of all types. After the rain has stopped, check the plants daily as the spores begin to germinate within hours. I check the local weather warnings and attempt to aid in predicting periods of great danger.

Fruit symptoms help confirm the diagnosis of the leaf. The young leaves with early blight, caused by Alternaria solani, show leathery, black patches at the stem end. The leaves with late blight exhibit shiny, bronze areas that quickly decay. Septoria rarely causes any trouble on the fruit, but may cause small sunken spots. If in doubt, send samples to the diagnostic laboratories, as a mistake in identifying the blights will lead to entirely incorrect and different treatment.

Read the full article: Tomato Blight Treatment Guide: Control & Prevention

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