Tomatoes with brown spots at the bottom: blossom-end rot

What is blossom-end rot?

Blossom-end rot appears as sunken, dark brown patches on the bottom of tomatoes. It starts small and grows with the fruit. Many growers think it is fungal disease, but it is not: calcium deficiency causes this. Calcium is crucial for plant tissue, and with rapid growth and irregular watering, plants cannot transport enough calcium to young fruit. The result: withering at the bottom.

This happens especially with large varieties and potted plants with inconsistent watering. The bad news: damaged fruit does not recover. The good news: this is completely preventable with one simple rule: water regularly.

Water is everything

Calcium in tomatoes is absorbed via soil water. When soil dries then suddenly gets heavy watering, plant stress occurs. Water potential changes rapidly, calcium does not reach growing fruit. You see this worse in dry periods.

Regular watering works much better than occasional heavy watering. Two dry days followed by drenching causes much blossom-end rot. Better: daily light watering in pots, or thorough watering every two to three days. This is essential for potted plants because pots dry faster than garden soil.

A second cause of calcium transport problems is poor soil structure. Soil that does not hold water and does not drain well causes root stress. Too much nitrogen in soil also inhibits calcium uptake. Feeding balance helps too.

Which tomatoes are susceptible?

Not all tomatoes suffer equally. Large, meaty varieties and indeterminate types that keep growing are more susceptible. Potted plants suffer much more than ground tomatoes. 'San Marzano', beefsteaks and large salad types often have problems. Small cherry tomatoes have less trouble.

Container plants on patios dry out much faster than ground tomatoes. On hot days you water twice sometimes. Ground tomatoes have deeper roots and more buffer capacity.

Mulch layer helps stabilize

A mulch layer 5 to 10 centimeters around the plant holds water and minimizes moisture fluctuations. Straw, grass clippings or compost work well. The layer also acts as a buffer against temperature swings and prevents sun heat from drying the soil out completely.

With mulch you water less often but deeper. Water penetrates better, soil dries slower. This is crucial on hot days in July and August, when evaporation peaks. A thick mulch layer also prevents weeds and animals from disturbing the soil.

Make sure the layer does not touch the stem: leave 5 centimeters space, otherwise rot problems and insects occur. Apply mulch to the drip line where roots grow actively. Replace yearly in spring, as mulch breaks down and adds organic matter to your soil.

Set watering schedule

Potted plants: water daily, possibly twice on hot days. Test by pushing finger 2 centimeters into soil: if dry, water. Water deep and slowly so water penetrates well.

Ground tomatoes: one or two times per week, depending on rain and heat. Always water at the base, not on foliage. Early morning is best, then foliage can dry and diseases have less chance.

Common mistakes

A frequent error is thinking calcium supplement fixes the problem directly. This does not help if watering remains irregular. Calcium in granules dissolves slowly and does not reach growing fruit fast enough. A stable water regime works much faster than any feeding.

Second mistake: placing potted plants on saucers. If water sits in the saucer, roots get too much moisture and calcium does not move to fruit properly. Better: place pots on elevated surfaces so water drains away.

Third problem: too much digging around the plant. This damages shallow roots and worsens calcium uptake. Work gently around tomatoes, especially after watering when soil is soft.

Many growers also try fertilizer instead of water. More feeding probably adds nitrogen, which inhibits calcium uptake. Water remains the simplest solution to everything.

Calcium supplement: secondary

Ground eggshell powder (calcium) can help, but works slowly and is secondary. Consistent water is much more effective. Sprinkle eggshell powder in July for next harvest.

Calcium sprays on foliage do not help: they do not deliver calcium to fruit. Better watering is the solution.

Soil type and structure

Heavy, compacted soil usually leads to blossom-end rot because calcium and water do not transport well. Sandy soil does not hold water. The ideal is soil with organic matter: 20 to 30 percent compost or humus mixed into potting soil. This holds water better and still drains well.

Potted plants are extra vulnerable because potting soil holds water less well than garden soil. Always choose potting soil with peat replacement or coco, and mix in compost. Check your potting soil yearly and replace after two years, as structure breaks down.

What does not help

Too much nitrogen worsens the problem, as nitrogen inhibits calcium uptake. Use a balanced fertilizer with an even nitrogen, phosphorus and potassium ratio.

Overwatering stresses roots and worsens calcium transport. Ensure good drainage in pots with bottom holes. A drainage hole is essential, otherwise water collects in the potting soil.

Step-by-step

Step 1: Apply mulch layer

Apply 5 to 10 centimeters of organic material around the plant, at least 5 centimeters from the stem.

Step 2: Do moisture test

Push your finger 2 centimeters into soil. If dry, you water.

Step 3: Make watering schedule

Potted plants daily, ground tomatoes 2 to 3 times weekly, deep and slow.

Step 4: Water early

Water before nine in the morning so foliage can dry.

Step 5: Calcium optional

Sprinkle fine eggshell powder in July for next harvest.

Step 6: Follow new fruit

Fruit after stable watering gets no blossom-end rot.

Frequently asked questions

Can you eat fruit with blossom-end rot?

Yes. Cut off the damaged spot at the bottom, eat the rest normally.

Does the plant recover?

No, damaged fruit does not heal. But new fruit after stable watering is fine.

Does calcium spray help?

No, spray does not reach the fruit. Better watering really helps.

Is it a disease?

No, absolutely not. No pesticide helps. It is purely water management.

Variety susceptibility

Large salad types: very susceptible. Regular water essential.

San Marzano, beefsteaks: susceptible. Mulch helps greatly.

Small tomatoes and cherries: little trouble, but also need regular water.

Plum tomatoes: susceptible in pots, less in ground.

Bottle tomatoes: fairly resistant, but also need regular water.

Summarized

Blossom-end rot is almost always water management, not nutrition. Irregular water plus fast-growing plant plus heat disrupts calcium transport. Mulch layer and regular watering prevent it completely.

This season fruit does not recover, but next growth becomes good with stable water. Plant with mulch, water daily (pots) or every 2 to 3 days (ground), and blossom-end rot does not return.

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See it in your own garden first

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