Late Blight vs Early Blight: Which One Is On Your Tomatoes

The 30 second answer

Walk to the plant and check the lowest leaves first, then the stems and green fruit. Two questions settle it.

Where did it start? Early blight begins on the oldest, lowest leaves near the soil and climbs. Late blight can appear anywhere, including up high, on stems, and on green fruit.

What does the spot look like? Early blight makes tidy brown spots with dark rings inside, like a small archery target. Late blight makes greasy, water-soaked gray-green or blackish blotches with no rings, and they spread while you watch, day over day.

• Bullseye rings, low on the plant, barely changed in a week: early blight.
• Greasy gray-green blotches, anywhere on the plant including stems and fruit, bigger every day: late blight.
• Still unsure? Flip a spotted leaf over in humid weather. A fine white fuzz ringing the edge of the blotch points to late blight.

Early blight: what it looks like

Early blight is caused by an Alternaria fungus, which is why you will also see it called Alternaria leaf spot or target spot. It is the blight almost every tomato grower gets at some point.

The spots are brown with dark concentric rings inside them, like the growth rings on a cut stump, usually with a yellow halo around the edge. It starts on the oldest, lowest leaves and moves up. Infected leaves yellow, then drop, leaving bare lower stems while the top of the plant still looks green and keeps setting fruit.

The fungus overwinters in soil and old plant debris, then splashes up onto the bottom leaves with rain and watering. That is why mulch helps so much. It is driven by warm humid weather and typically shows up from June through September, a few weeks earlier across the warm, humid South.

Early blight rarely kills a plant outright. The real cost is losing enough leaves to shrink the harvest and expose fruit to sunscald. The fruit itself is still fine to eat.

Late blight: what it looks like, and why it is urgent

Late blight is caused by Phytophthora infestans, the same pathogen behind the Irish potato famine. It is the serious one, and it is not subtle once you know the look.

The lesions are greasy and water-soaked, dark brown to blackish or gray-green, often edged with a pale yellow-green halo. They are not tidy round spots. They look like a spreading bruise. In cool humid weather, the underside of a spotted leaf may show a fine white fuzzy growth ringing the edge of the blotch. On fruit it appears as firm, greasy brown patches that rot the whole tomato or potato.

It can show up anywhere on the plant, not just the bottom. Stems and green fruit are fair game, which is a strong tell on its own, because early blight starts low and stays on leaves.

Speed is the difference that matters. Spores travel on wind and rain and germinate on wet leaves in cool damp weather, roughly nights of 50 to 60 F with long dew or fog. It can take a healthy plant to dead in under a week and races through a planting in days. It also blows to gardens down the road, so this is not only your problem. Treat it as a same-day emergency.

Side by side

Five things separate them. If three or more point the same direction, you have your answer.

• Where it starts: early blight on the oldest lowest leaves and climbing. Late blight anywhere, including stems and green fruit.
• Pattern: early blight makes brown spots with dark target rings and a yellow halo. Late blight makes greasy, water-soaked gray-green blotches with no rings.
• Speed: early blight builds over weeks. Late blight can collapse a plant in under a week.
• Weather: early blight likes warm humid spells with wet leaves. Late blight likes cool damp weather, nights around 50 to 60 F with long dew or fog.
• Threat level: early blight shrinks the harvest and is managed. Late blight kills plants and spreads to other gardens, so it is an emergency.

What to do about each

Early blight is managed, not cured. Mulch under the plants to stop rain splash from the soil, strip the affected lower leaves into the trash rather than the compost, and stake or space plants so leaves dry fast. Copper is a last step, and only if the disease keeps climbing. The full ladder with the cautions is in the early blight guide.

Late blight is different. Pull the infected plants the same day, seal them in a bag, and put them in the trash. Do not compost them, because the spores survive and blow to the rest of your garden and your neighbors'. Copper protects healthy tissue from new infection but will not heal a blighted leaf, so it is a preventive step, not a rescue. The late blight guide walks through the whole response.

• Early blight: mulch, strip the bottom leaves, open up airflow. Full steps at the early blight guide.
• Late blight: pull and bag the same day, trash it, never compost it. Full steps at the late blight guide.

How to lower the odds next year

Both diseases need wet leaves to get started, so most of the prevention is the same work. It is cheap, and it is done at planting time rather than in a panic in August.

Rotation matters most for early blight, which overwinters in the soil and in old plant debris. Move tomatoes and potatoes to a new bed on a 2 to 3 year cycle so last season's spores have no host waiting for them. For late blight, the equivalent habit is digging out every leftover potato tuber and volunteer potato plant, because that is where it overwinters.

• Space tomatoes about 24 inches apart and stake or cage them so foliage dries fast.
• Mulch under the plants at transplant to block rain splash from the soil.
• Water at the base in the morning, never over the leaves, so nothing stays wet overnight.
• Rotate tomatoes and potatoes on a 2 to 3 year cycle.
• Clear and trash all plant debris at the end of the season.
• In cool wet regions, buy varieties listed as blight resistant. The catalog or seed packet will name the resistance.