Blossom end rot
Blossom end rot is not a disease. Calcium travels to fruit in water, so when watering is irregular the far end of the fruit runs short, its cells collapse, and a dark sunken patch forms. Steady watering protects the fruit that follows.
Pollination & fruiting
What it looks like
It always sits at the flower end, never at the stalk.
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Water-soaked spot A pale, bruised-looking patch appears at the tip of young fruit. -
Dark, sunken patch It darkens to brown or black and sinks, leathery and dry. -
Flattened end The fruit looks flat or dented at the base. -
Secondary mould Later, mould grows on the dead patch.
How to confirm it
Its position is unmistakable.
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Opposite the stalk
The patch is on the end that held the flower.
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First fruits worst
The first trusses, formed during a dry spell, are hit hardest.
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Dry, not soft
The patch is firm and leathery at first, not a spreading soft rot.
What to do
The fruit will not recover; protect the next ones.
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Pick affected fruit
Remove spoiled fruit so the plant puts energy into the rest.
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Water steadily
Keep the compost moist all the time; in heat, water pots twice a day rather than once with double.
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Feed only moist compost
Fertiliser on dry compost stops water uptake and makes it worse.
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Mulch
Mulch plants in the ground to hold moisture evenly.
Prevent it next time
- Grow in the largest containers possible.
- Set up regular or automatic watering for grow bags.
- Avoid high-ammonium feeds, which compete with calcium.
- Adding calcium rarely helps: most soils have plenty; the water is the fix.
What not to do
Do not add lime or calcium supplements as a first step. Water supply, not soil calcium, is almost always the cause.
In the growing guides
Where the guides mention it, with plant-specific advice.
Sources
- Blossom end rot, Royal Horticultural Society
- Blossom end rot on vegetables, University of Maryland Extension