Blossom End Rot on Tomatoes and Peppers: How to Save Fruit from Black Spots

In the peak of the summer season, when long-awaited first fruits appear on the bushes, gardeners frequently notice a frustrating issue: the bottom part (the apex) of tomatoes or sweet peppers begins to darken, becomes flat, dry, and gets covered by a large black patch. This is blossom end rot. Many panic, considering it a dangerous fungus, but in reality, it is a physiological disorder triggered by an acute deficiency of one vital element.

1. The Real Cause of Blossom End Rot Appearance

The main culprit behind this issue is a calcium (Ca) deficiency inside the cellular structure of the fruit. Calcium is responsible for the strength of cell walls. However, the most interesting fact is that your soil might be full of calcium, but the plant simply cannot absorb it. Unlike elements found in mineral NPK fertilizers, calcium travels through the plant strictly via water movement during transpiration. If summer heatwaves hit and the soil dries hard, water movement stops, and fruits begin to starve.

2. Prevention: Consistent Irrigation and Thermal Balancing

The best way to protect your bushes is to maintain consistent soil moisture without sharp jumps from drought to a swampy state. Make sure to use a thick layer of mulch (straw) so the ground doesn't convert into a dry crust. If your plants grow inside a greenhouse, utilize shading nets covered in our guide on protecting the garden from heatwaves.

3. Table of Quick Relief for Blossom End Rot (Good)

Treatment Remedy	How to Prepare and Apply Correctly Under the Bush?	Why Does It Work the Fastest?
Calcium Nitrate (Foliar)	Dilute 15-20 grams of calcium nitrate in 10 liters of water. Spray bushes over foliage and young fruit ovaries during evening hours.	Through leaf stomata, calcium enters fruit tissues directly within a few hours, halting the disorder.
Calcium Chloride (Ampoule)	1 ampoule (10 ml) of 10% calcium chloride solution per 10 liters of water. Urgent foliar spraying over tomatoes and sweet peppers.	An easily bioavailable form of pure calcium that instantly resuscitates plant cells during droughts.
Deep Root Irrigation	Regular, deep watering with warm water in the morning (3-5 liters under every mature tomato bush).	Restores capillary juice movement from the root base up to the highest fruit clusters.

4. Major Errors That Worsen Fruit Rot (Bad mistakes)

• Feeding with manure or poultry droppings during an outbreak: Organics are rich in nitrogen and potassium. An excess of potassium and magnesium in the soil works as a chemical antagonist to calcium — they will completely block its trace remnants in the ground, causing fruits to blacken even harder.
• Leaving infected fruits on the bush: A black dry patch will not heal. Do not force the bush to waste energy on a ruined tomato — pluck it off so the plant can direct all calcium supplies to new upper fruit ovaries.

AgroPlanner Tip: On acidic soils, blossom end rot manifests three times more frequently. In autumn, make sure to apply lime or chalk to beds where solanaceous crops are planned for next year. Control soil balance in our planner tool!