Blossom end rot is the most misunderstood problem in the vegetable garden. A tomato or pepper looks perfect, then a dark, sunken, leathery patch appears at the blossom end — the end opposite the stem — and spreads, and the fruit rots. The first instinct is to reach for a spray, and the second is to blame a pest or a disease. Both are wrong. Blossom end rot is not a pest, and it is not an infectious disease. It is a calcium-uptake and watering issue: the fruit is not getting enough calcium at the moment it is growing fastest, usually because the watering has been uneven, and the tissue at the blossom end collapses.
This guide explains what blossom end rot is, why it happens, and how to prevent it — with steady watering, mulch, soil care and plant choice. There is no spray that cures it, and there is no pest to kill. If the fruit on your tomatoes does not match, the plant disease checker can help you compare, and our early blight on tomatoes guide covers the disease that is most often confused with it.
Symptoms
Blossom end rot starts as a small, water-soaked spot at the blossom end of the fruit — the end opposite the stem — on tomatoes, peppers, squash and eggplant. The spot grows, turns dark brown or black, and becomes sunken and leathery. The affected tissue may be invaded by fungi, which makes it look like a rot, but the fungi are secondary — they arrive after the tissue has already collapsed. The rest of the fruit may look normal, and the plant itself usually looks healthy, which is the clue that this is not an infectious disease.
The first fruit of the season is usually the worst affected, and the problem often eases as the season goes on and the plant's root system develops. It is most common on large-fruited tomato varieties, on plants grown in containers, and on plants that have had uneven watering. Peppers show the same dark, sunken patch at the blossom end, and squash and zucchini can show it too.
Why it happens: calcium uptake and watering
Calcium is needed for cell walls, and the fruit needs a steady supply of it while it is growing. Calcium moves through the plant in the water the roots take up, so if the water supply is interrupted — by drought, by overwatering, by a cold snap, or by root damage — the calcium supply to the fruit is interrupted too, and the tissue at the blossom end, which is furthest from the stem and growing fastest, collapses first.
This is the key point: blossom end rot is usually not caused by a lack of calcium in the soil. Most garden soils have enough calcium already, and adding more rarely helps. It is caused by the plant's inability to take the calcium up and move it to the fruit, and the most common reason for that is uneven watering. A plant that is dry for a few days, then soaked, then dry again, will show blossom end rot even in soil that is rich in calcium. Root damage from hoeing, transplant shock, and a small root system in a container all make it worse, because they reduce the plant's ability to take up water.
Conditions that favour it
Blossom end rot is favoured by uneven watering, drought, overwatering, cold soil, root damage and heavy nitrogen feeding. It is worst on the first fruit of the season, on large-fruited varieties, and on plants in containers, where the soil dries out fastest. It is also worse after a cold snap, which slows root activity, and after heavy rain following a dry spell, which floods the roots and then leaves them dry again. Acidic soil can make calcium less available, but in most gardens the watering is the problem, not the soil.
It is not spread from plant to plant, and it is not carried on seed or on tools. A plant with blossom end rot does not infect its neighbours, and the fruit is not poisonous — the affected part can be cut away, though the fruit is best used promptly rather than stored.
Cultural prevention
Cultural prevention is the only reliable control, and it starts with water. Water tomatoes and peppers deeply and evenly, keeping the soil consistently moist but not waterlogged, and avoid letting the soil dry out completely between waterings. Water at soil level, in the morning, and mulch thickly — 5–8 cm of straw, compost or wood chips — to keep the soil moisture even and the soil temperature steady. Mulch is the single most effective step, because it smooths out the wet-dry cycles that cause the problem.
Avoid heavy nitrogen feeding, which produces fast, soft growth that outruns the calcium supply, and feed steadily with a balanced fertiliser instead. Avoid hoeing close to the plant, since root damage reduces water uptake, and handle transplants gently so the roots are not damaged. Grow tomatoes in the ground where possible, rather than in small containers, since containers dry out fastest — and if you do grow in containers, water them daily in hot weather and use a large pot. Choose varieties that are less prone to blossom end rot where they are available — small-fruited and cherry varieties are usually less affected than large beefsteak types — and your local extension service or seed supplier can advise on what suits your area. Check the planting calendar for zone 5 in April for transplant timing, since planting out into cold soil can damage the roots and make the problem worse.
What to remove, and what not to compost
Remove affected fruit as soon as you see it, and put it in the bin or the compost — it is not infectious, so composting it is safe, unlike diseased material. Removing the fruit lets the plant put its energy into the next fruit, which may be healthy if the watering is steadied. Do not remove the leaves or the plant itself — the plant is healthy, and it will usually produce good fruit once the watering is even. Do not compost the plant at the end of the season if it shows signs of disease, but blossom end rot alone is not a reason to keep it out of the compost.
Treatment, in order
1. Steady the watering. This is the treatment. Water deeply and evenly, mulch thickly, and avoid letting the soil dry out. This alone will usually stop the problem on the next fruit, even if the fruit already affected cannot be saved.
2. Check the soil. If your soil is very acidic, a soil test will tell you, and liming may help — but do not lime without a test, and do not expect it to fix the problem on its own. Check your local extension service for soil testing in your area. Adding calcium to the soil rarely helps if the watering is uneven, and adding it to the leaves — as a foliar spray — is of limited value, since the calcium does not move well from the leaves to the fruit.
3. Do not spray for a pest or a disease. Blossom end rot is not a pest and not an infectious disease, so insecticides and fungicides will not help. Save the sprays for the problems they can actually solve, such as early blight on tomatoes and late blight on tomatoes.
4. Be patient. The first fruit is often the worst, and the problem usually eases as the season goes on, the roots develop, and the watering settles into a routine. A plant that shows blossom end rot on its first fruit will often produce perfectly good fruit for the rest of the season.
Frequently asked questions
Is blossom end rot a disease? No. It is a physiological disorder — a calcium-uptake and watering issue, not a pest and not an infectious disease. It does not spread from plant to plant, and it cannot be cured with a spray.
Can I eat tomatoes with blossom end rot? Yes, once the affected part is cut away. The rest of the fruit is safe to eat, though it is best used promptly rather than stored, since the damaged tissue can rot.
Will adding eggshells or calcium fix it? Probably not, on its own. Most soils already have enough calcium, and the problem is usually the plant's inability to take it up because of uneven watering. Steady the watering and mulch first, and only add calcium if a soil test shows it is needed — check your local extension service for testing.
Why is only the first fruit affected? The first fruit grows when the plant's root system is still small and the weather is often unsettled, so the water supply is most uneven then. As the roots develop and the watering settles, the later fruit is usually fine.
Does blossom end rot affect peppers too? Yes. Peppers, squash, eggplant and melons can all show the same dark, sunken patch at the blossom end, for the same reason — uneven watering and calcium uptake. The prevention is the same: steady water, thick mulch and gentle handling of the roots.
Blossom end rot is a problem where the answer is water, not spray. Keep the soil evenly moist, mulch thickly, feed steadily, and handle the roots gently — and the fruit that follows will usually be sound. For the tomato diseases it is often confused with, see our early blight on tomatoes and late blight on tomatoes guides, and for general help use the plant disease checker.