Pests & diseases
Blossom end rot
A dark, sunken patch at the base of tomatoes, courgettes and peppers, caused by calcium failing to reach the fruit, usually from erratic watering.
Blossom end rot is not a disease, despite the name. It is a disorder caused by a lack of calcium reaching the developing fruit, which leaves a dark, sunken, leathery patch at the blossom end — the bottom of the fruit, furthest from the stalk. It mostly affects tomatoes, but courgettes, peppers, chillies and aubergines get it too. Because nothing is attacking the plant, you cannot catch it from a neighbour or spray it away. The fix lies in how you water and feed.
What causes it
The trouble is almost never a shortage of calcium in the soil or compost — there is usually plenty. The problem is the plant struggling to move that calcium up to the fruit. Calcium travels with water, so anything that disrupts a steady flow of moisture starves the fruit of it. The main triggers in a UK garden are:
- Erratic watering — letting the compost dry out hard, then drenching it. This is by far the most common cause, especially in growbags and pots.
- Hot, dry spells — under glass or on a sunny patio, plants transpire fast and the roots can't keep up.
- A small or pot-bound root system — containers and growbags hold little compost, so they dry out quickly and swing from wet to dry.
- Too much nitrogen — heavy feeding pushes lush leaf growth that competes with the fruit for what calcium is available.
How to spot it
Look at the base of the fruit, not the leaves. You'll see a watery, pale patch first, which turns brown or black, sinks inwards and goes tough and leathery. It often hits the earliest fruits of the season and can look alarming, but the plant itself stays green and healthy. That healthy top growth is the giveaway that this is a watering disorder, not blight or another infection.
How to prevent it
The cure is consistency, not chemicals:
- Water little and often. Keep the compost evenly moist rather than swinging between dry and flooded — this is the single most important fix.
- Don't let pots dry out. In hot weather, growbag and container tomatoes may need watering twice a day. A self-watering tray or reservoir helps hugely.
- Mulch the surface to slow evaporation and even out moisture.
- Go easy on high-nitrogen feed. Switch to a balanced or high-potash tomato feed once fruit sets.
- Don't panic. Pick off affected fruits, sort out the watering, and the next trusses usually come good.
Adding lime or calcium rarely helps, because the calcium is already there — it's the water supply that needs fixing.
In a UK garden
In the UK it shows up most in greenhouse tomatoes and growbag or container plants during a hot, dry spell, when the compost swings between bone-dry and soaked and the roots can't take up calcium steadily.
Example
The first few tomatoes on a growbag plant ripen with a flat, leathery brown-black patch on the bottom, while the leaves and the rest of the plant look perfectly healthy — that's blossom end rot.