Tomato: dark sunken patch at the bottom of a fruit

What it is likely to be on tomato

1. Blossom end rot

   Candidate 1very common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Blossom end rot
   • Home and Garden Information Center — Clemson Cooperative Extension: Blossom end rot of tomato and pepper

   Checked 2026-08-13

   A dark, sunken, leathery patch at the flower end of a fruit. It is not a disease and nothing is attacking the plant: the fruit could not move enough calcium into its fastest-growing tissue while it was forming, almost always because water supply was irregular.

   Points towards it

   • · it is overwhelmingly a container and growbag problem, because the whole root system is in a few litres of compost that dries and rewets sharply
   • · the fruit cannot get calcium when the plant cannot move water, so a pot that has been allowed to dry out between waterings is the mechanism
   • · a hot spell increases demand faster than a small root zone can supply, which is why the first fruit of a heatwave is the one affected
   • · it affects particular fruit — usually the earliest and the largest — rather than everything on the plant, because each fruit takes its calcium during its own swelling

   Argues against it

   • · a plant standing in water has a root problem rather than a supply problem, though the two can end up looking similar
   • · grey fuzz on the patch means a fungus has taken it, which is a different first cause even where rot followed

   Check this next

   Press the patch with a fingertip and then cut the fruit in half. Blossom end rot is a flat or sunken patch at the blossom end — the bottom, away from the stalk — that is leathery, dry and firm rather than soft, and inside the flesh beneath it is brown and dry through to the middle. It has no mould on it unless something else has moved in afterwards. Then look at the rest of the truss: the affected fruit are usually the earliest and biggest, and later ones on the same plant come out perfect once the watering is steady.

   What to do now

   Water steadily rather than more — this is a transport problem rather than a shortage, so calcium feeds and eggshells do nothing. Keep the compost evenly damp and never let a growbag or pot dry right out between waterings; a self-watering tray or a daily routine in hot weather is worth more than anything applied. Pick off the affected fruit so the plant stops carrying them. Larger containers and mulched compost surfaces both help, and the fruit that set after the watering steadies will be sound.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Blossom end rot — control
   • Home and Garden Information Center — Clemson Cooperative Extension: Blossom end rot — management

   Checked 2026-08-13

2. Late blight

   Candidate 2common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Potato and tomato blight
   • Agriculture and Horticulture Development Board — AHDB: Potatoes — late blight

   Checked 2026-08-13

   A water mould that destroys potato and outdoor tomato foliage in warm, wet spells, and then rots the tubers or fruit. It moves in days rather than weeks, which is why prevention is the only strategy that works.

   Points towards it

   • · blight spores need prolonged leaf wetness to germinate, so an outdoor tomato is at far greater risk than one under cover
   • · warm humid weather with wet foliage for two days together is the condition that triggers an outbreak
   • · the leaves and stems are almost always affected at the same time as the fruit, which is what separates it from anything that only touches the fruit
   • · it moves through the whole plant within days once it starts

   Argues against it

   • · a plant that has never been rained on and is nowhere near an outdoor crop is very unlikely to have it
   • · hot dry weather stops blight, and a fruit failing in a drought is failing for another reason

   Check this next

   Look at the leaves and the stems before you look at the fruit, because blight almost never takes fruit alone. It leaves dark brown-black patches on the leaves, often with a paler ring and a fine white bloom on the underside in humid weather, and dark streaks on the stems — and it moves through the plant in days rather than weeks. The fruit patches are firm, brown and marbled and can appear on green fruit anywhere on the surface rather than sitting neatly at the blossom end. A plant with clean foliage and marked fruit does not have blight.

   What to do now

   Remove and bin the whole plant — bagged for household waste, not composted and not left on the plot — as soon as blight is confirmed, because it spreads to every tomato and potato within reach and nothing available to a home grower stops it once it is established. Pick any sound green fruit first and ripen it indoors; fruit that is already marked will rot. Next year, grow tomatoes under cover where the foliage stays dry, choose a blight-resistant cultivar, and keep them well away from the potato bed.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Tomato blight — control
   • Agriculture and Horticulture Development Board — AHDB: Late blight — management

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on tomato — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general dark sunken patch at the bottom of a fruit page lists causes across all plants, and the Tomato profile lists the problems it is prone to.