Early Blight vs Late Blight: How to Tell & Treat

Two diseases wreck more tomato and potato crops than almost anything else, and gardeners constantly confuse them. Early blight (Alternaria solani) and late blight (Phytophthora infestans) both blacken foliage and rot fruit, but they are biologically unrelated, behave on completely different timelines, and demand different responses. Misidentify them and you waste fungicide, lose plants you could have saved, or — worse — let late blight torch your whole bed and your neighbor’s in under two weeks. This guide shows you exactly how to tell them apart by the lesions, the conditions that trigger each, and how to treat and prevent both.

The fastest way to triage is to ask two questions: where on the plant did it start, and how fast is it moving. Early blight creeps up from the bottom over weeks. Late blight explodes across the canopy in days, regardless of leaf age. The table below summarizes the differences before we go deep on each.

Feature	Early Blight (Alternaria)	Late Blight (Phytophthora)
Where it starts	Oldest/lowest leaves first	Anywhere, often upper canopy
Lesion look	Brown spots with concentric target rings, yellow halo	Greasy gray-green to brown-black irregular blotches
Underside sign	None distinctive	White downy fuzz in humid weather
Speed	Slow, weeks; chronic	Explosive, 7-10 days
Weather trigger	Warm 75-85°F, humid, plant stress	Cool 60-70°F, wet, foggy nights
Fruit symptom	Dark sunken spot at stem end, leathery	Greasy brown firm rot, can cover whole fruit
Stems	Dark target-ringed lesions (“collar rot” at base)	Brown-black greasy streaks, often near growing tips
Threat level	Manageable, rarely kills	Emergency; can kill in days and spread for miles

Early Blight: How to Identify It

Early blight is the more common of the two and the one most home gardeners eventually meet. The name is misleading — it does not necessarily strike early in the season. It is called “early” because, historically, it tended to show before late blight in the field. In practice you usually see it from midsummer onward, once plants are carrying fruit and showing some stress.

Look at the bottom of the plant first. The earliest lesions appear on the oldest, lowest leaves as small dark-brown to black spots, often only an eighth of an inch across. As they expand, the diagnostic feature develops: concentric rings inside the spot, like the rings of a tiny archery target or a tree stump. A yellow halo usually surrounds each lesion. Spots merge, the leaf yellows entirely, then browns and drops. Defoliation marches steadily up the plant, exposing fruit to sunscald.

On stems, early blight produces dark, slightly sunken lesions that also show faint concentric rings. When it attacks young transplants at the soil line it causes “collar rot,” girdling the stem. On fruit, it strikes at the stem end (the calyx), producing a dark, leathery, sunken spot that may show those same target rings and can have a velvety black look as it sporulates. Early blight favors warm temperatures (75-85°F), high humidity, heavy dew, and any condition that stresses the plant — drought, poor nutrition, or a heavy fruit load all invite it.

How to Treat Early Blight

Start with sanitation the moment you spot it. Remove and bag affected lower leaves — do not compost them — and clear all debris from the soil surface. Improving airflow by pruning the bottom 6-12 inches of foliage slows the spread because spores need leaf wetness to germinate. Mulch heavily with straw or shredded leaves to stop soil-borne spores from splashing up onto leaves during rain or watering.

For active infections, fungicides help if applied early and on a 7-10 day schedule. Copper-based fungicides and biofungicides containing Bacillus subtilis are common organic options; chlorothalonil and mancozeb are conventional protectants. These are protectant products — they shield healthy tissue rather than cure infected tissue, so coverage and timing matter more than dose. Spray in the early morning so leaves dry fast, and reapply after heavy rain. Keep plants well fed and evenly watered; a vigorous plant outruns early blight far better than a stressed one.

Late Blight: How to Identify It

Late blight is the disease that caused the Irish Potato Famine, and it deserves that reputation. It is not a true fungus but a water mold (oomycete), and it moves with frightening speed. If early blight is a slow chronic illness, late blight is a wildfire.

Lesions can appear anywhere on the plant, not just the bottom, and they often start in the upper, younger canopy. The first sign is a greasy or water-soaked gray-green blotch, irregular in shape and with no target rings. Within a day or two it turns brown to almost black and the tissue collapses. In humid weather — especially after a cool, foggy night — look at the leaf underside: a fine white downy mold rings the edge of the lesion. That white fuzz is the spore-bearing structure and is the single most reliable late-blight tell. Stems develop dark brown-black greasy streaks, and entire shoot tips can blacken and flop over.

On fruit, late blight causes large, firm, greasy brown lesions, often on the shoulders or sides of the tomato rather than just the stem end, and the rot can engulf the whole fruit. On potatoes it produces brown, dry, granular rot in the tubers. Crucially, late blight needs cool and wet conditions: temperatures of 60-70°F with prolonged leaf wetness, fog, or rain. Stretches of cool drizzly weather in mid-to-late summer are the classic trigger. The pathogen produces millions of windborne spores that can travel miles, which is why one infected garden can seed an entire region.

How to Treat Late Blight

Treat confirmed late blight as an emergency, because once it takes hold there is essentially no curing it on an infected plant. If only a leaf or two is affected and weather is turning warm and dry, you can try removing infected tissue and applying a protectant fungicide (copper, chlorothalonil, or mancozeb) to the rest of the planting immediately, repeating every 5-7 days. But if the infection is spreading or the forecast is cool and wet, the responsible move is to pull and destroy the entire plant. Bag it and trash it, or burn or deep-bury it — never compost late-blight material, and never leave culled plants lying in the garden, as they keep producing spores.

Because late blight overwinters in living tissue, the biggest source of next year’s outbreak is volunteer potatoes and culled tubers left in the ground. Dig up every potato, and never plant grocery-store potatoes or saved tubers from an infected crop. If you grow both crops, scout daily during cool wet spells — early detection is the only real leverage you have. For more on keeping the broader bed healthy, see our guides on plant and flower care and seasonal disease prevention.

Prevention for Both Blights

Most of the work happens before either disease appears. Choose resistant varieties: look for tomatoes labeled with EB (early blight) or LB (late blight) resistance — ‘Mountain Magic’, ‘Iron Lady’, and ‘Defiant PhR’ offer strong late-blight resistance, while ‘Mountain Fresh’ and ‘Juliet’ tolerate early blight well. Rotate tomatoes and potatoes (and their relatives — peppers, eggplant) out of the same bed for 3-4 years to break the early-blight cycle in the soil.

Space plants generously — 24-36 inches apart — and stake or cage them so air moves through the canopy and leaves dry quickly. Water at the soil line with drip or a soaker hose, never overhead, and water in the morning. Mulch to block soil splash. Keep tools and hands clean when moving between plants. These habits cut the leaf-wetness window both pathogens depend on. If you build raised beds or containers for better drainage and airflow, our notes on small-space and raised-bed growing cover spacing and soil setup in detail.

The Verdict

The two diseases call for two mindsets. Early blight is a chronic condition you manage — expect it most summers, keep plants vigorous, prune the lower canopy, mulch, and spray protectants on a schedule, and you will still get a full harvest. Late blight is an acute emergency you contain — scout obsessively in cool wet weather, learn the white-fuzz-and-greasy-blotch signature, and be willing to destroy plants fast to protect the rest of your garden and your neighbors’. When in doubt, the deciding clues are the concentric target rings and bottom-up march of early blight versus the explosive, anywhere-on-the-plant, white-undersided collapse of late blight. Identify correctly and you will treat correctly.