Blossom end rot is a physiological disorder, not a disease or infection, caused by a localized calcium deficiency in developing fruit. It appears as a dark, sunken, leathery patch on the bottom (blossom end) of tomatoes, peppers, and other garden vegetables. The good news: no pathogen is involved, and it’s largely preventable once you understand what’s actually going on inside the plant.
What It Looks Like
The first signs are small, water-soaked spots that look like bruises on the bottom of green or immature fruit. These spots expand and merge, eventually covering anywhere from an eighth to half the fruit’s surface. As the affected tissue dries out, it shrinks and turns leathery, changing color from a bleached yellow to dark brown or black.
Most of the time the damage shows up on the blossom end, which is the bottom of the fruit opposite the stem. Occasionally it can appear on the side, and sometimes it produces an internal black lesion with no visible damage on the outside. That’s a frustrating surprise when you cut into what looks like a perfectly good tomato.
Which Vegetables Are Affected
Tomatoes get the most attention, but blossom end rot also hits peppers, eggplant, squash, pumpkins, and watermelon. Among tomatoes, large plum and paste varieties are the most susceptible. Cherry tomatoes rarely develop it. Long pepper fruits are also more prone than blocky types.
Why Calcium Doesn’t Reach the Fruit
The underlying problem is almost never a lack of calcium in the soil. Most garden soils have plenty. The real issue is that calcium can’t travel through the plant’s phloem (the sugar-transporting system). It depends entirely on water moving through the xylem, pulled upward by transpiration as moisture evaporates from leaves. That makes calcium delivery completely dependent on a steady flow of water from the roots.
Leaves are powerful transpiration engines. They pull water, and the calcium dissolved in it, toward themselves very effectively. Developing fruit, by comparison, transpires far less, so it loses the tug-of-war for calcium when supply is limited. The blossom end of a tomato is the tissue farthest from the stem’s vascular connection, and it’s also the part expanding most rapidly. That fast growth can actually obstruct or break the tiny xylem vessels inside the fruit, making the calcium shortage even worse in the exact spot where it matters most.
When calcium drops below a critical threshold in those cells, the cell walls and membranes lose their structural integrity. The tissue breaks down, and what you see on the outside is that characteristic dark, sunken patch.
Common Triggers
Three main factors disrupt calcium transport to fruit:
- Inconsistent watering. Periods of drought followed by heavy watering create a stop-and-start calcium supply. During dry spells, transpiration slows and calcium delivery stalls. This is the single most common cause.
- Excess nitrogen fertilization. Too much nitrogen, especially in ammonium form, does two things: it interferes directly with calcium absorption at the root level, and it drives rapid leafy growth that diverts even more calcium away from fruit. Heavy fertilizing early in the season is a classic setup for blossom end rot later.
- Root damage. Aggressive cultivation around plants or compacted soil reduces root function. Fewer working roots means less water and nutrient uptake overall, and calcium is the first casualty because of its dependence on water flow.
How to Prevent It
Since the problem is almost always about calcium transport rather than calcium supply, prevention focuses on keeping water moving steadily through the plant.
Consistent moisture is the priority. Monitor the root zone to make sure it stays evenly moist but not waterlogged. You can check by pushing a finger several inches into the soil or using a moisture meter. The goal is to avoid the cycle of bone-dry soil followed by a flood of irrigation. Deep, regular watering on a consistent schedule works far better than frequent light sprinkles.
Mulch is one of the most effective tools you have. A layer of organic mulch around your plants buffers soil moisture between waterings, slowing evaporation and keeping conditions more stable. Keep the mulch a few inches away from the base of the stem to avoid rot.
Go easy on fertilizer, particularly early in the season. Use a balanced formulation with moderate nitrogen, and avoid high-ammonium fertilizers. The lush green canopy that heavy nitrogen produces looks impressive but comes at the expense of calcium reaching your fruit.
Soil pH matters for calcium availability. Tomatoes grow best in soil with a pH between 6.0 and 7.0. If your pH is already in that range, adding lime (which raises pH) can do more harm than good. A soil test is the only reliable way to know where you stand. Your local cooperative extension office typically offers affordable testing.
Why Eggshells and Epsom Salts Don’t Work
Crushed eggshells are one of the most popular home remedies for blossom end rot, and they’re essentially useless for this purpose. Eggshells decompose far too slowly to release meaningful calcium during a single growing season. By the time they break down, the problem has long since resolved or worsened on its own.
Epsom salts are even more counterproductive. They supply magnesium, not calcium. Adding extra magnesium to soil can actually block calcium uptake at the root level, making blossom end rot worse. Unless a soil test confirms a magnesium deficiency, Epsom salts have no place in your blossom end rot strategy.
The reason these remedies persist is that blossom end rot often corrects itself as plants mature and establish deeper root systems. People credit whatever they sprinkled around the base, when the real fix was the plant settling in and water uptake stabilizing.
Can You Save Affected Fruit?
Fruit with minor blossom end rot is still edible. You can cut away the damaged portion and use the rest. The leathery patch isn’t caused by a fungus or bacteria, so there’s nothing infectious about it. However, the damaged area can become an entry point for secondary mold or decay, so inspect the fruit carefully and use it promptly.
Severely affected fruit won’t improve on the vine. Picking it off actually helps the plant redirect resources to healthy developing fruit. The condition typically shows up on the first clusters of the season, when the root system is still relatively shallow and water uptake is most uneven. Later fruit often develops normally without any intervention, especially if you’ve stabilized your watering routine.