The fastest way to settle early blight vs late blight is to look at where the spots sit and how they behave. Early blight starts low on old leaves with neat brown rings and creeps along over weeks. You can live with it. Late blight hits anywhere on the plant, spreads in days, and can flatten a whole row. Treat early blight as a chore and late blight as an emergency.
Location and shape tell you most of the story. Early blight damage shows up on the lowest, oldest leaves first and climbs slowly. The spots are small and round, with crisp edges. Late blight does not care about leaf age. It jumps to stems, upper leaves, and green fruit, and the blotches look wet and greasy. So check the bottom of the plant for one and scan the whole plant for the other. If the trouble is stuck near the soil, you are likely looking at early blight.
Here is what early blight looks like up close. You get round brown spots up to 1/2 inch (1.3 cm) wide, each ringed with darker bands like a tiny archery target. Those target ring leaf spots are the giveaway, and a yellow halo often forms around them. The fungus behind it is Alternaria, and it works at a slow pace. It takes weeks to move up the plant. Your tomatoes keep producing right through most of it, which is why you can shrug off a mild case and keep harvesting.
Late blight is the one that should worry you. The late blight symptoms start as dark, gray-green blotches that look water-soaked, like a bruise on the leaf. The edges spread past the veins instead of stopping at them, so the patches look ragged and ignore the leaf's normal lines. In damp or humid weather you will see white fuzzy growth on the underside of those patches, often along the edge of the dead tissue. Stems turn brown and brittle, and green fruit develops greasy brown patches.
The pathogens explain the difference in danger. Late blight comes from a water mold called Phytophthora infestans. This is the same organism that caused the Irish potato famine. It moves fast in cool, wet spells and can take a plant down in days, not weeks. Worse, its spores ride the wind for miles. Your sick plant becomes a threat to every tomato and potato patch downwind of you. That is why one is a nuisance and the other is a real community problem.
Weather gives you one more clue. Early blight likes warm, humid summer days and builds up gradually. Late blight thrives in cool, soggy stretches, often after several days of rain or heavy fog. If a wet, chilly spell hits and spots appear fast and high on the plant, suspect late blight first. If the season is hot and the damage stays low and slow, early blight is the better bet.
So treat them as two different jobs. With early blight, snap off the worst lower leaves, mulch to block soil splash, and keep growing. A few spotted leaves cost you nothing. With suspected late blight, act fast. Pull the infected plants, bag them, and throw them out or burn them so spores cannot spread. Then tell your neighbors what you found and call your local extension office. They can confirm it and warn other growers nearby before the wind carries it to them.
Read the full article: Tomato Blight: Identify, Treat, and Prevent It
