What is that black leathery patch on the bottom of my tomatoes?
Blossom end rot is a calcium deficiency at the fruit level, almost always caused by irregular watering rather than a lack of calcium in the soil.
Blossom end rot starts as a small, water-soaked spot on the bottom of the fruit (the blossom end, opposite the stem). Over a few days it darkens, flattens, and turns into a leathery black or dark brown patch. It looks like a disease, but it isn't — no pathogen is involved. It's a physiological disorder: the rapidly growing fruit tissue at the blossom end can't get enough calcium, so cells die.
The calcium is almost certainly in your soil. What's happening is that calcium can only move through the plant in the stream of water pulled up from the roots — it doesn't redistribute through the plant after it's placed. When soil moisture is inconsistent (dry spells followed by heavy watering or rain), the plant's water uptake is disrupted and the most rapidly growing tissues don't receive enough calcium. The fruit at the bottom is the first place to show this deficit.
The fix is watering consistency, not calcium amendments. A 2–3 inch mulch layer helps enormously by moderating soil moisture swings. Drip irrigation or a consistent deep-watering schedule is more effective than calcium sprays. Adding lime or calcium to the soil doesn't help in the short term if the underlying water-delivery problem isn't addressed — the calcium transport mechanism still breaks down with inconsistent moisture.
The first fruits on a plant are most often affected. Once you establish consistent watering, later fruits typically come in clean. Remove the affected fruit — it won't recover — and keep watering evenly. Some varieties are more prone to blossom end rot than others; large paste types and large beefsteaks tend to be more susceptible than cherry types.
- AnthracnoseSunken, dark circular lesions on ripening fruit, sometimes with salmon-colored spores in the center.
- Bird DamageBerries pecked or missing, seeds scratched from beds, and seedlings dislodged — birds feeding on ripe fruit, seeds, or soil grubs.
- Black RotV-shaped yellow lesions at brassica leaf margins with blackened veins inside — a bacterial disease that moves through the vascular system.
- Blossom DropFlowers fall before setting fruit, often during temperature extremes or after weather stress.
- Blossom End RotDark, sunken, leathery patch on the blossom end of tomato or pepper fruit — a calcium deficiency disorder.
- Why are my tomatoes cracking and splitting?Tomato skin cracks when the fruit expands rapidly after a period of drought — inconsistent watering is almost always the cause, though some varieties are simply crack-prone.
- We're in a drought — how do I keep my garden going?Mulch, deep infrequent watering, and cutting back on what you're growing are the three adjustments that make the biggest difference during drought conditions.
- When is the best time to add compost to the garden?Fall is often the best time to add compost to garden beds — it has all winter to incorporate and break down further, and beds are ready to plant without delay in spring.