Blossom-end rot occurs on tomatoes, peppers, squash, and watermelons from a lack of calcium in the developing fruit. This results from slowed growth and damaged roots caused by any of several factors:
1. Extreme fluctuations in soil moisture, from very wet to very dry.
2. Rapid plant growth early in the season, followed by extended dry weather.
3. Excessive rains that smother root hairs.
4. Excess soil salts.
5. Cultivating too close to the plant.
The first fruit are the most severely affected. The disorder always starts at the blossom end, and may enlarge to affect up to half of the fruit. Moldy growths on the rotted area are from fungi or bacteria that invade the damaged tissue. The rotted area is unsightly but the rest of the fruit is edible.
To prevent future blossom-end rot, follow these guidelines:
1. Maintain uniform soil moisture by mulching (see Mulching), and by following the watering guidelines in Tomatoes.
2. Avoid using high-nitrogen fertilizers or large quantities of fresh manure.
3. Plant in well-drained soil, as outlined in Soil Structure, Groundwater.
4. If your soil or water is salty, provide more water at each watering to help leach salts through the soil.
5. Do not cultivate deeper than 1 inch within 1 foot of the plant.
Plant Care for Tomatoes