Blossom rot is a calcium deficiency in the fruit, not a disease
Blossom rot appears as a dark, sunken spot on the bottom of a tomato — the end opposite the stem. It starts small and spreads, turning the fruit mushy and inedible. The spot itself is dead tissue where the plant failed to move enough calcium to that part of the fruit as it grew. This is not a fungus, bacteria, or virus. It is a nutrient problem.
The plant has calcium in the soil, but something is preventing it from reaching the developing fruit. The most common cause is uneven watering — long dry spells followed by heavy rain or watering. When the soil dries out, the plant cannot pull calcium up through its roots. When water returns, the plant floods with moisture, but the calcium does not catch up fast enough to reach the new fruit growth.
Blossom rot happens most often on the first fruits of the season, when plants are young and their root systems are still small. Once the plant matures and roots go deeper, the problem usually stops on its own — even if you do nothing else.
Key Takeaways
- Blossom rot is caused by calcium not reaching the developing fruit, usually because of inconsistent soil moisture rather than a lack of calcium in the soil itself.
- The most reliable fix is watering deeply and on a regular schedule — daily or every other day depending on heat and rainfall — so the soil stays evenly moist.
- Mulch around the base of the plant to keep soil moisture steady and reduce the impact of rain and temperature swings.
- Fruits already showing blossom rot cannot be saved and should be removed so the plant can direct energy to healthy fruit.
- Adding calcium to the soil (lime, gypsum, or crushed eggshells) may help prevent future rot, but only if watering stays consistent.
How inconsistent watering triggers blossom rot
When soil dries out, the plant's roots cannot absorb water or the nutrients dissolved in it — including calcium. The plant closes off water movement to conserve what it has. Calcium moves through the plant only in water, so when water stops moving, calcium stops moving too.
When you water again after a dry spell, the plant roots take up water quickly, but calcium absorption lags behind. The fruit that is actively growing at that moment does not get the calcium it needs. The damage is done before the calcium catches up, and the affected tissue dies and darkens.
Heat and low humidity make this worse. Hot days pull moisture from the soil faster, and the plant's demand for water increases. If you are watering by hand or relying on rain, the soil can swing from soaked to bone-dry in just a few days during a heat wave.
Watering on a schedule prevents most blossom rot
The single most effective fix is to water deeply and regularly so the soil stays consistently moist — not soggy, but not dry either. In hot weather, this usually means watering every day or every other day. In cooler weather or after rain, you may only need to water twice a week.
Water at the base of the plant, not on the leaves. Wet leaves invite fungal disease and do not help the roots absorb moisture. Soak the soil to a depth of 6 to 8 inches so water reaches the deeper roots. A soaker hose or drip line is more reliable than hand watering because it delivers water at a steady rate and does not wash away.
Check the soil with your finger before watering. Push your finger 2 inches into the soil near the plant. If it feels dry, water. If it still feels moist, wait another day. This straightforward test prevents both overwatering and underwatering.
Mulch keeps soil moisture steady between waterings
A 2- to 3-inch layer of mulch around the base of the plant — straw, shredded leaves, or wood chips — slows water loss from the soil. Mulch also keeps soil temperature more stable, which reduces the plant's stress during hot days and cold nights.
Keep mulch a few inches away from the stem so it does not trap moisture against the plant and invite rot at the base. Refresh the mulch once or twice during the growing season as it breaks down.
Adding calcium to the soil works best with consistent watering
If blossom rot appears on your plants year after year, the soil may genuinely be low in calcium. A soil test from your local extension office will tell you for certain. If calcium is low, you can add it in several forms.
Lime (calcium carbonate) is the cheapest option and raises soil pH at the same time. Gypsum (calcium sulfate) adds calcium without changing pH, which is useful if your soil is already alkaline. Crushed eggshells or bone meal add calcium slowly as they break down. Sprinkle any of these around the base of the plant in spring, work it into the top inch of soil, and water well.
None of these will fix blossom rot if watering is still inconsistent. Calcium in the soil does no good if water is not moving through the plant to carry it to the fruit. Fix the watering first; add calcium only if the problem persists after a full season of regular watering.
Remove affected fruit so the plant can focus on healthy growth
A tomato with blossom rot cannot be saved. The damaged tissue will not heal, and the fruit will rot from the inside out. Pick off any fruit showing the dark, sunken spot on the bottom and throw it away.
Removing damaged fruit redirects the plant's energy to the healthy fruit still growing. This is especially important on young plants with limited resources. The sooner you remove the bad fruit, the sooner the plant can put that energy into fruit that will actually ripen.
Blossom rot usually stops on its own as the plant matures
Most gardeners see blossom rot only on the first few fruits of the season. As the plant grows, its root system expands deeper into the soil where moisture is more stable. Once roots reach that deeper, more consistent moisture, the plant can absorb calcium reliably even if surface watering is uneven.
This does not mean you should ignore watering. Consistent watering still matters for overall plant health and fruit quality. But the blossom rot itself often disappears without any intervention beyond what you would do anyway to keep the plant healthy.
Frequently Asked Questions
Can I eat a tomato with blossom rot?
No. The affected tissue is dead and will taste bitter or mushy. The rot spreads inward as the fruit ripens, so even if the spot looks small, the damage inside is usually larger. Remove the fruit and compost it.
Is blossom rot contagious to other plants?
No. Blossom rot is not caused by a disease organism, so it cannot spread from one plant to another. It is a nutrient and watering problem specific to each plant's growing conditions.
Will blossom rot rot spread to other fruits on the same plant?
Not directly. But if the watering problem that caused the first fruit's rot is not fixed, new fruit developing at the same time will also develop blossom rot. Fix the watering schedule and remove damaged fruit to stop new rot from forming.
What if I water every day but blossom rot still appears?
Check whether water is actually reaching the roots. Soil that is hard or compacted may shed water instead of absorbing it. Break up the soil around the plant, add mulch, and water more slowly so water has time to soak in. Also check for root problems — if roots are damaged or the pot is too small, the plant cannot absorb water even if it is present.
Do I need to add lime or gypsum if I fix the watering?
Not necessarily. If blossom rot stops once you water consistently, the soil probably has enough calcium. Add lime or gypsum only if the problem returns after a full season of regular watering, or if a soil test shows calcium is genuinely low.