Blossom end rot

also called BER

A sunken, leathery brown or black patch at the blossom end of tomatoes, peppers and courgettes, opposite the stalk.

Disorder Disfiguring Doesn't spread

What you'll see

A water-soaked spot at the flower end of the fruit that turns tan, brown then black and leathery
The patch is flattened or sunken, often covering a third of the fruit
First fruits of the season and the ends of grow-bag or container plants worst affected
The rest of the fruit ripens normally and the skin over the patch stays dry, not slimy

Why it happens

It is a calcium-delivery failure, not a lack of calcium in the soil. When the plant cannot move enough calcium to the fast-growing fruit tip — usually because watering is erratic — those cells break down. Adding calcium, eggshells or lime almost never helps, because the soil already holds plenty; the problem is getting it to the fruit.

Brought on by swings between wet and dry at the roots, especially in pots and grow bags during hot, bright spells when the plant is carrying a heavy first truss. High salt levels and over-feeding with nitrogen or potash make it worse by pulling water away from the fruit.

Treating it

Water consistently so the compost never dries out then floods; little and often in hot weather
Remove affected fruit so the plant puts its energy into healthy ones
Ease off high-potash feeds until the watering has steadied
Mulch containers and shade grow bags from fierce midday sun to slow drying

Keeping it away

Keep soil moisture even — a mulch or self-watering system helps far more than any feed
Do not let plants wilt between waterings, particularly on hot days
Avoid overdoing nitrogen and potash, which encourage growth the roots cannot supply
Choose larger pots that dry out more slowly, and never let grow bags run bone dry

Who gets it

Tomatoes are the classic sufferers, along with peppers, chillies, aubergines and courgettes.