Blossom end rot is a physiological disorder, not an infection. The fruit does not get enough calcium while it is growing, and the most common trigger is uneven watering. Fix the moisture pattern and later fruit usually comes through clean, but the damaged fruit will not recover.

It belongs in the wider family of Australian garden pests and problems, yet it behaves differently from most of them: nothing is attacking the plant, so sprays aimed at pests or disease are the wrong tool.

What does blossom end rot look like?

It shows on the end of the fruit furthest from the stalk. The Pacific Pests fact sheet describes a clear to light brown, water-soaked patch that darkens to black or dark brown, then turns dry, sunken and leathery. It lists tomato as the main host and includes capsicum.

Key points:

• It usually starts when fruit is roughly one-third to one-half full size and still green.
• The first fruit of the season is often hit hardest. Iowa State Extension notes the same pattern.
• Grey or fuzzy mould on the patch is a later arrival feeding on dead tissue, not the cause.
• Capsicums are affected in the same way as tomatoes, so use the same checks and fixes.

Why does uneven watering cause it?

Calcium travels from the roots to the fruit in the water the plant draws up. It moves slowly and does not shift easily between plant parts. When soil swings between dry and soggy, or hot dry wind makes leaves pull water faster than roots can supply it, the fruit at the tip of the flow misses out. The UF/IFAS Extension explanation describes calcium as carried up from the roots in water, with cell walls breaking down where the supply falls short.

So the soil can contain plenty of calcium and the fruit can still show symptoms. The problem is delivery, not stock.

Other factors named in the sources:

• Excess nitrogen, which drives leafy growth that competes with fruit for calcium. UF/IFAS also notes ammonium-form nitrogen can interfere with calcium uptake.
• Root damage, for example from deep digging or hoeing close to the plant.
• Hot, dry or windy weather that increases water loss from leaves.
• Low soil pH or sandy soil, which the Pacific Pests sheet lists as contributors.

How do you tell it apart from other fruit problems?

Look at where the damage is and what else is happening on the plant. This table is editorial guidance for a first check, not a definitive diagnosis.

If the pattern does not fit, do not assume. Local extension officers can usually diagnose from a clear photo or a fruit sample.

What actually fixes it?

Treat the cause, which is the moisture and root environment. Steps in rough order of impact:

1. Water on a steady rhythm. Aim for even moisture through flowering and fruit set rather than big soaks after dry spells. Both too little and too much water are linked to the disorder.
2. Mulch thickly. Mulch slows evaporation and smooths out moisture swings. The Pacific Pests sheet suggests at least 10 cm of dried grass or similar after transplanting. Keep it clear of the stem itself.
3. Check drainage. Waterlogged soil is as risky as dry soil. Heavy or compacted beds benefit from added organic matter or raised beds.
4. Go easy on nitrogen. Avoid heavy feeding that pushes leaves at the expense of fruit.
5. Protect the roots. Avoid digging or hoeing deeply near plants.
6. Add shade or wind protection during hot, dry, windy weather if you can.
7. Pick off affected fruit. UF/IFAS states an affected fruit cannot be fixed. Removing it lets the plant put energy into the rest.

Container plants are the most exposed because pots dry out fast and then get over-corrected. A larger pot, mulch on top and a consistent routine make a noticeable difference.

Do calcium sprays and additives work?

Mostly not. Both US extension sources checked here are blunt about it. Iowa State says adding calcium to the soil is generally ineffective. UF/IFAS says soil calcium helps only where it is genuinely lacking, and that foliar calcium sprays are generally ineffective because calcium does not move from leaves into fruit.

A soil test is the sensible way to decide. It shows whether calcium and pH are an issue or whether you are chasing the wrong problem. If the test does show a calcium or pH problem, correct it according to the lab’s advice and the product label.

Fungicide is no help either. A Western Australian Department of Agriculture journal article states that because the disorder is not parasitic, fungicidal or bactericidal sprays cannot control it. This is an older publication, but the principle is consistent with the newer sources.

Does the right timing vary across Australia?

The mechanism is the same everywhere; the pressure points differ with climate. This is editorial guidance and local conditions matter.

• Tropical and subtropical (Qld coast, northern NSW, Top End): Heat, wet-season downpours and dry-season dry spells create swings. Mulch and drainage matter most. See the guide to growing tomatoes through Queensland’s seasons for timing.
• Temperate: Watch for early-season fruit after a cool spring, then hot dry spells in summer when watering lapses.
• Cool and highland areas: Shorter seasons mean fewer fruit to lose, but a sudden warm dry spell can still trigger it.
• Arid and inland: Hot, dry winds and fast evaporation make steady moisture hard. Drip irrigation and deep mulch are the practical answer.

Common mistakes

• Spraying fungicide or a calcium product instead of correcting the watering.
• Watering lightly every day on top of a mulch that is too thin to hold moisture, then soaking after a dry spell.
• Heavy nitrogen feeding early in the season.
• Digging around established plants and cutting roots.
• Assuming the whole crop is ruined. Later fruit often comes through clean once conditions settle.

Early fruit is the most likely to be affected, so a few lost tomatoes at the start of the season does not mean the plant is failing. Keep watering steady, keep the mulch topped up, and judge the plant by the fruit that sets next.