Early Blight vs. Late Blight on Tomatoes: How to Tell

Early Blight vs. Late Blight: The Short Answer

Early blight makes brown spots with target-like rings on the oldest, lowest leaves of a tomato plant and climbs slowly, so you can prune, mulch and keep harvesting. Late blight makes brown-black, water-soaked, oily-looking patches without rings, often with white-gray fuzz, and it can kill the plant in 7 to 10 days of cool, wet weather. A plant with late blight should be pulled, bagged and put in the trash. The names have little to do with the calendar. They describe two different organisms, and the right response to one is the wrong response to the other.

	Early blight	Late blight
Cause	The fungus Alternaria linariae, long known as A. solani	The water mold Phytophthora infestans
Where it starts	Lower, older leaves	Often younger leaves; any part of the plant
Leaf symptom	Brownish-black spots, 1/4 to 1/2 inch, with concentric rings	Irregular water-soaked patches that turn brown-black and oily
Around the spot	Yellow tissue, then the whole leaf yellows	Sometimes a lighter halo; no rings
Fuzzy growth	None	White, cottony growth under the leaf in humid weather
Speed	Works up the plant over the season	Plant can collapse in 7 to 10 days
Weather	Warm, above 80°F, with moderate to heavy rain	Nights in the 50s and 60s°F with rain, fog or heavy dew
Where it comes from	Crop debris in your own soil	Windblown spores, infected plants or potato tubers
What to do	Prune, mulch, rotate	Remove and trash the plant

Signs of Early Blight on Tomato Plants

Early blight is caused by Alternaria linariae, a fungus many references still list under its older name, Alternaria solani. North Carolina State Extension describes the leaf symptom as brownish-black spots that expand to about 1/4 to 1/2 inch across with characteristic concentric rings. Those rings give each spot a target or bull’s-eye look, and they are the single most useful feature for identification.

Look first at the lowest leaves, because lesions develop there before anywhere else. The tissue around each spot may turn yellow, and as the disease progresses the whole leaf yellows and falls. Later in the season spots can appear higher up. Heavy leaf loss at the bottom of the plant leaves fruit exposed to sunscald, which is often the real cost of early blight in a home garden.

Stems and fruit can be infected too. Fruit lesions are sunken, leathery and dark brown to black, with the same concentric rings, and they can expand to cover the whole tomato. Seedlings may develop collar rot at the base of the stem, which is one reason to inspect transplants before you buy them.

If your plants have yellow lower leaves but no ringed spots, the cause may be something else entirely, and the tomato yellow leaf diagnosis guide covers the other common suspects.

Signs of Late Blight on Tomato Plants

Late blight is caused by Phytophthora infestans, the same water mold behind the Irish potato famine of the 1840s. University of Wisconsin Extension describes the first leaf symptoms as pale-green or olive-green areas that quickly enlarge to become brown-black, water-soaked and oily-looking. The patches are irregular in shape and sometimes carry a lighter halo. They have no target rings.

Three details separate it from early blight. Late blight often shows up on younger leaves first, so it is not confined to the bottom of the plant. Stems develop dark-brown to black areas. And in humid conditions, the underside of an infected leaf grows a white, cottony fuzz where the organism is producing spores.

Fruit symptoms are distinctive as well. Infected tomatoes develop large, firm, often sunken spots that are golden to chocolate brown. Potatoes are hosts too, and diseased tubers show a reddish-brown discoloration under the skin.

A Look-Alike: Septoria Leaf Spot

Many gardeners who search for blight are looking at Septoria leaf spot. It also begins on older leaves, but it produces numerous small, circular spots with dark borders around a beige center, and the spots lack the bold rings of early blight. Clemson Extension notes the fungus is most active when temperatures range from 68 to 77°F. Clemson’s advice for it is rotation, removal of crop debris and no overhead watering, close to the routine for early blight, so confusing those two costs you little. The identification that matters is separating either of them from late blight.

How Fast Each One Moves

Speed is the tiebreaker when the spots themselves are ambiguous. Early blight is a slow climb from the bottom of the plant, starting on the lowest leaves and reaching higher ones as the season goes on.

Late blight moves on a different clock. After infection, lesions can appear on leaves within 3 to 5 days. Wisconsin Extension warns that tomato plants can collapse and die in 7 to 10 days when conditions are cool and wet, and Clemson says complete defoliation can occur within 14 days of the first symptoms. If you are unsure, photograph the same leaves on three mornings in a row. New oily patches every day point to late blight.

The reach is different too. Early blight spores travel by wind, air currents and water splash, mostly from debris and soil nearby. Late blight spores can be blown into neighboring plantings 5 to 10 miles away or more, which is why one neglected plant matters to everyone growing tomatoes or potatoes in the area.

The Weather Behind Each Disease

Recent weather is a useful clue. Early blight favors warm temperatures above 80°F combined with moderate to heavy rainfall. It tends to appear as the canopy closes in and the dense foliage keeps humidity high and leaves wet for longer.

Late blight is tied to moisture and cool nights. North Carolina State Extension describes ideal infection weather as nights in the 50s and 60s°F and days in the 80s°F, accompanied by rain, fog or heavy dew. A sudden outbreak after a week of foggy mornings and cool nights fits late blight. Spots that build gradually through a hot, rainy stretch fit early blight.

What to Do About Early Blight

Early blight is a disease you manage, and the steps below slow it down.

1. Prune the bottom-most leaves as the plant grows, so the foliage sits clear of the soil.
2. Mulch the soil. Wisconsin Extension suggests about 1 inch of a good mulch to keep spores from splashing up from the ground.
3. Trellis or stake the plants and give them room, so air moves through and leaves dry quickly. Our guide to tomato cages and stakes compares the supports.
4. Water at the base of the plant and keep the foliage dry. The routine in how to water tomatoes fits this well.
5. At the end of the season, remove the vines and debris. Next year, rotate tomatoes to another bed, and keep them out of the old one for 2 to 3 years.

Fungicides containing chlorothalonil or copper can help, but only when they go on very early, ideally before symptoms develop, and are repeated on the label schedule through the season. If you prefer an organic program, see the comparison of organic fungicides for home gardens.

What to Do About Late Blight

Late blight is a disease you remove. Pull affected plants, roots and all, and place them in plastic bags. Wisconsin Extension advises leaving the sealed bags in the sun for a few days and then putting them out with the trash. Do not compost the plants.

Healthy-looking fruit from an affected plant is safe to eat or preserve, so pick what is clean before you bag the vine. Any tomato or potato that already shows symptoms should not be eaten or canned.

Wisconsin Extension says the organism is unlikely to survive winter in Wisconsin soils. It carries over on infected plant material that stays alive through the winter, such as potato tubers left in the ground after harvest. In spring, dig out and dispose of volunteer tomato and potato plants, along with nightshade weeds, and buy certified seed potatoes each year instead of replanting last year’s tubers.

Protective fungicides with chlorothalonil or copper work only if they are applied before the disease arrives. Once a plant is infected, removal is the answer.

Blight-Resistant Tomato Varieties

If blight visits your garden most years, start with the variety. Cornell’s vegetable pathology program reports excellent late blight resistance in varieties carrying both the Ph2 and Ph3 resistance genes, and names ‘Defiant PhR’, ‘Iron Lady’, ‘Mountain Merit’ and ‘Mountain Magic’. ‘Plum Regal’, which has the Ph3 gene, showed good resistance. Wisconsin Extension’s list adds the cherry types ‘Jasper’ and ‘Matt’s Wild Cherry’.

For early blight, North Carolina State Extension says no variety is fully resistant and lists ‘Mountain Magic’, ‘Mountain Fresh Plus’ and ‘Juliet’ among those that are more tolerant. Cornell’s variety table lists both early blight and late blight for ‘Iron Lady’ and ‘Mountain Merit’, which makes them a sensible starting point for a garden that has seen each disease. The spacing, mulching and dry-foliage habits still apply to any variety.

Quick Recap

• Early blight: ringed brown spots 1/4 to 1/2 inch across on the lowest leaves, slow spread, warm rainy weather above 80°F.
• Late blight: oily brown-black patches on any leaf, white fuzz underneath, plant collapse in 7 to 10 days, cool wet nights in the 50s and 60s°F.
• Prune, mulch about 1 inch deep, and rotate for 2 to 3 years against early blight.
• Bag and trash late blight plants, never compost them, and eat only fruit that looks healthy.