Blog / Crop Health
Early Blight vs Late Blight: How to Tell Them Apart and What to Do
Both diseases hit tomatoes and potatoes, but they behave very differently. Learn the symptoms, the weather that favours each, and the steps that actually protect your crop.
RootRise Team · · 9 min read
If you grow tomatoes or potatoes, you will meet blight sooner or later. Farmers often talk about "blight" as if it were one disease. It is two, caused by different organisms, that spread differently and call for different urgency. One slowly weakens a crop over weeks. The other can wipe out a field in days.
Telling them apart early is one of the most valuable skills a tomato or potato grower can have. This guide explains how.
A note before we start: this is general educational information. Product choices, doses and legal restrictions on chemicals vary by country and change over time, so always check with your local agricultural extension officer or a trusted agro-dealer, and follow the label on any product you use.
The short version
| Early blight | Late blight | |
|---|---|---|
| Cause | A fungus (Alternaria solani) | An oomycete, or water mould (Phytophthora infestans) |
| Which leaves first | Older, lower leaves | Any leaves, often young growth and stems as well |
| What the spots look like | Brown spots with concentric rings, like a target | Large, water-soaked patches that turn dark brown or black, often with a pale green edge |
| Under the leaf | Spots with no fuzzy growth | In humid weather, a white, downy growth at the edge of the lesion |
| Speed | Slow: weeks | Very fast: days in cool, wet weather |
| Weather that favours it | Warm, humid, alternating wet and dry | Cool, wet, humid and overcast |
| Fruit or tubers | Dark, sunken spots near the stem end of the fruit | Greasy brown patches on fruit; rot in the tubers |
If you remember one thing: rings mean early, water-soaked and white fuzz mean late, and late is an emergency.
Early blight
What it looks like
The first signs usually appear on the oldest leaves, lowest on the plant. You will see small brown spots that grow into larger patches. Look closely and you will see a pattern of concentric rings inside each spot, a bit like an archery target. The tissue around a spot often turns yellow. As the disease advances, leaves yellow, wither and fall, working up the plant.
On tomato fruit, early blight shows as dark, leathery, sunken spots near the stem, again with rings. On stems it can cause dark lesions near the soil line on young seedlings.
Why it matters
Early blight rarely kills a plant outright. The damage comes from lost leaves. A plant with few leaves cannot feed its fruit or tubers, so yields fall and tomatoes are exposed to sunscald. The disease is most serious on plants that are already stressed: short of water or nutrients, heavily loaded with fruit, or old.
What encourages it
- Warm weather with humidity, dew or frequent rain
- Plants that are stressed or under-fed
- Leaves that stay wet for long periods
- Spores that survive on old crop debris and in the soil from the last tomato or potato crop, so it returns year after year in the same ground
Late blight
What it looks like
Late blight is the disease behind the Irish potato famine, and it deserves respect. It starts with pale green or dark, water-soaked spots, often at the leaf edges or tips. In cool, wet conditions these spots grow quickly into large, dark brown or black patches. On the underside of the leaf, at the edge of the lesion, you may see a white, fuzzy growth in the morning when the air is humid. That growth is the organism producing spores. Stems can show dark brown lesions, and the whole plant can collapse.
On tomato fruit, late blight causes firm, greasy, brown patches that spread and rot. On potatoes, tubers develop a dry, reddish-brown rot beneath a slightly sunken skin, and they often rot further in storage when secondary bacteria move in.
Why it is an emergency
Late blight spreads by spores carried on the wind and in rain splash, and a few infected plants can seed an entire neighbourhood. In the right weather (cool nights, wet or foggy days) it can turn a healthy field into a rotting one within a week. It is also a disease that does not stay with one farmer: your neighbour's infected plants are a threat to you, and yours to them.
What encourages it
- Cool temperatures and high humidity, especially with rain, mist or heavy dew
- Long periods of leaf wetness
- Infected seed potatoes, volunteer plants or discarded tubers that carry the disease from one season to the next
- Dense planting with poor air movement
In many tropical areas, late blight is mainly a problem in cooler, higher-altitude places or in the cooler, wetter part of the year, such as the rainy season on the Jos Plateau. In hot, dry conditions it usually stalls.
How to check your own plants
A quick routine, once or twice a week, makes a big difference.
- Walk the field in the early morning, when dew is on the leaves and any white growth is visible.
- Turn leaves over. Check the underside of leaves with spots.
- Check older leaves, then the newest growth. Spots only on the lowest leaves with rings point to early blight. Spots high on the plant, on young leaves or on stems, with water-soaked margins, point to late blight.
- Check the stems and fruit.
- Take clear photographs of a leaf, front and back, in daylight, with a coin or your hand for scale. They help you compare week by week and let an extension officer help you, even from a distance.
- Note the weather. Cool and wet should make you more watchful.
Do not rely on a single look. Some other problems resemble blight, including leaf spot diseases, nutrient deficiencies, sunscald, and chemical burn. If you are unsure, ask an extension officer or a nearby experienced grower to look at a sample.
What to do about early blight
Early blight is manageable if you act steadily rather than urgently.
- Rotate your crops. Do not plant tomatoes, potatoes, peppers or garden eggs (all in the same plant family) in the same soil year after year. A break of two to three years away from that family starves the fungus of its host.
- Remove and destroy old crop residue at the end of the season. Do not leave infected plants lying in the field.
- Space your plants so that air can move between them, and stake or trellis tomatoes to keep leaves off the soil.
- Water at the base of the plant, in the morning, and avoid wetting the leaves in the evening. Drip or careful hand watering is better than sprinklers.
- Mulch to stop soil splashing up on the lowest leaves.
- Feed and water the crop properly. A healthy, well-fed plant tolerates early blight far better than a stressed one.
- Remove the lowest affected leaves early, and dispose of them away from the field.
- Choose varieties with some tolerance where they are available.
- Use a registered fungicide only if needed, as recommended by your extension officer, and follow the label exactly: the dose, the timing, and the days required between spraying and harvest.
What to do about late blight
Late blight demands faster action, because it moves faster.
- Act at the first sign. If you see the pattern in cool, wet weather, do not wait to see whether it passes.
- Tell your neighbours and your extension officer. This is a disease that is controlled by communities acting together.
- Remove and destroy infected plants, bagged and taken away from the field, not left in a heap on the edge of it. For potatoes, destroy volunteers and cull piles.
- Protective fungicides work best before infection, not after. In places where late blight is a known risk, extension services often advise a protective spray programme during the wet season. Ask what is recommended and legal where you farm, and follow the label.
- Do not harvest wet and store wet. Let potato foliage die back and the skins set before you dig, and lift tubers on a dry day. Cull any tuber with rot before storing, since one rotten tuber can spoil the heap.
- Use clean seed. Plant certified or properly inspected seed potatoes, never tubers saved from an infected crop.
- Avoid long leaf wetness. Do not irrigate in the evening, and plant where air moves.
Mistakes that make blight worse
- Waiting to see. The first spotted leaves are the cheapest to deal with.
- Spraying the wrong thing. Many products target different diseases. Using an insecticide on a fungal disease wastes money.
- Overhead watering in the evening.
- Leaving infected plants in the field "to rot down".
- Replanting tomatoes or potatoes in the same spot straight away.
- Selling or moving infected plants or tubers, which spreads the disease to other farms.
Keep a record
Blight is easier to manage when you remember what happened last time. Note down when it appeared, what the weather was like, which field, what you did, and what it cost. After a season or two, you will see a pattern: the week it usually arrives, the fields it likes. That knowledge is worth more than any single spray. A simple activity log, such as the disease-control and pest-control records in AgroPulse, keeps the details in one place. We say more about that in Record-Keeping for Smallholder Farmers.
In summary
- Rings on older leaves, slow spread: early blight. Manage it with rotation, clean-up, spacing, careful watering and good feeding.
- Water-soaked patches, white fuzz underneath, fast spread in cool, wet weather: late blight. Treat it as an emergency, act within a day, tell your neighbours, and protect healthy plants.
Check your plants regularly, keep leaves dry, and when in doubt, ask. A photograph and a conversation with an extension officer early in the season will always be cheaper than a lost crop at the end of it.