Blossom end rot

What it is

Blossom end rot is a physiological disorder, not a disease, so it cannot spread from plant to plant. It is especially common on tomatoes and peppers grown in containers, where compost dries out quickly in hot weather. Affected fruit is disappointing, but the plant itself is healthy and will go on producing.

Symptoms

A small, water-soaked patch appears at the base of the fruit, opposite the stalk, where the flower once was. It darkens to brown or black, becomes leathery and sinks into a flattened area. Green fruit is affected first, often several on the same truss. Mould may later move into the damaged patch. Late blight also browns tomato fruit, but its patches are firmer, bronzy and can be anywhere, with leaf damage too.

Cause

Calcium travels to the fruit in the water the plant takes up. When the compost dries out, even for a short time, the rapidly growing fruit does not get enough, and cells at the far end die. Most garden soils and composts contain plenty of calcium; the shortfall comes from uneven watering, root damage or very high feeding, which can interfere with uptake.

Control

There is no fix for fruit already affected. Pick it off so the plant puts energy into healthy fruit, and remove the damaged part if you want to eat the rest. Then focus on steady watering, so the compost never swings between bone dry and soaked. In hot spells, pots and growbags may need watering morning and evening. Calcium sprays are not usually needed in gardens.

Prevention

Grow tomatoes in larger pots or in the ground where possible, since bigger volumes of compost hold water more evenly. Use a ring culture pot or an extra pot sunk into growbags to help water soak in, mulch the surface, and consider a simple drip or self-watering system if you are often away. Feed at the strength given on the label rather than more.

Plants often affected

• Tomatoes
• Courgettes

Could it be something else?

These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.

• Grey mould
• Potato and tomato blight
• Box blight
• Box tree caterpillar

Months to keep an eye out: June, July, August and September.

Sources

Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.

• Wikipedia — Blossom-end rot
• University of Minnesota Extension — Blossom end rot