Blossom end rot

What you are looking at

A pale, water-soaked patch appears at the bottom of a developing tomato, on the end opposite the stalk. Over a few days it darkens through brown to black, goes flat or slightly sunken, and dries to something like leather. The edge is sharp and the rest of the fruit ripens as though nothing had happened.

Peppers do the same thing, usually with the patch a little paler and papery. Courgettes and marrows rot from the flower end, though there the more likely cause is a fruit that was never pollinated.

The position settles the diagnosis. Damage at the blossom end and nowhere else is this, and marks on the shoulders, around the stalk or scattered across the skin are something different.

Why the calcium advice is wrong

Search for this and you will be told your soil is short of calcium, and offered crushed eggshells, garden lime and a calcium spray. British soils are, with rare exceptions, well supplied with calcium, and so is every bag of tomato compost sold. The plant is not short of it.

Calcium moves through a plant only in the xylem, carried along by the flow of water that transpiration pulls up from the roots. It cannot be redistributed through the phloem the way nitrogen or potassium can. Where the water goes, the calcium goes.

Leaves transpire hard. A fruit, with its waxy skin and few stomata, transpires very little, so it always receives a small share. The cells at the far end of the fruit, furthest from the stalk, are last in that queue and are also the ones expanding fastest during the first fortnight after set.

Interrupt the water supply at that moment, by letting a growbag dry out, by a hot bright day, or by damaging the roots, and those cells do not get enough calcium to build their membranes. The membranes fail, the cells collapse, and the tissue dies. The black patch appears a week or two later, when the fruit is large enough to show it.

That is why the fix is a watering can rather than a bag of lime.

Why the first fruit and not the rest

The plant is smallest and its root system least developed when the first truss is swelling, so the buffer against a dry afternoon is at its thinnest. By August the same plant has roots through the whole bag and a canopy that shades its own compost, and fruit set then often comes through clean while the early trusses are ruined.

Anyone who has had a disastrous first truss and a fine crop afterwards has watched this happen. It is not the treatment working. It is the plant growing.

What to do about it

Water more evenly and give the roots more to work with. Three plants in a growbag in a July greenhouse can need watering twice a day, and the difference between a crop with blossom end rot and one without is usually that second can. Standing bottomless pots on the bag, or using containers of at least twenty litres each, doubles the volume of compost holding water.

Feed at the rate printed on the bottle. Potassium, magnesium and ammonium nitrogen all compete with calcium at the root surface, so a plant given a high-potassium tomato feed at double strength, plus a dose of Epsom salts because the leaves looked pale, is being made more susceptible rather than less.

Shade and ventilate a greenhouse through a heatwave and damp the floor down. The point is to lower the peak rate at which the leaves are pulling water, since it is the peak that starves the fruit.

Pick off anything already marked. It will not recover and it is drawing on the plant.

Where this happens every year, change cultivar. Plum and long-fruited tomatoes are far more prone than round or cherry types, and the swap costs nothing.

What it gets mistaken for

Tomato blight produces dark brown patches that are firm at first and then spread across the whole fruit, including the shoulders and the stalk end, and the foliage shows brown blotches with pale margins at the same time. Blossom end rot never touches the leaves.

Grey mould enters through a wound or a dead flower and grows a fuzzy grey coat that puffs spores when knocked. Blossom end rot is dry and leathery, and any mould on it arrived afterwards.

Splitting and cracking come from the same underlying cause, a swing between dry and wet, but they show as rings around the shoulders or radial splits from the stalk. The two often appear on the same plant in the same week.

Sun scald leaves a pale, papery, flattened patch on the side of the fruit that faced the sun, usually after leaves were stripped off. Position again tells them apart.