Dark Spots on Tomatoes Are a Cry for Calcium
Question: My tomatoes are turning black on the bottom. What is the cause, and how can I prevent it?
Answer: Blossom-end rot causes tomatoes to turn a brown-black color on the bottom (blossom end) of both green and ripe fruit. The area often appears sunken and leathery.
Blossom-end rot results from a low level of calcium in the fruit. This deficiency may be caused by any of several conditions that interfere with the plant’s ability to take up nutrients from the soil. Such conditions include water stress, excessive salinity and root damage from excess water or disease.
The condition affects some varieties more than others; in general, varieties with elongated fruits are more susceptible.
Blossom-end rot is more prevalent with staked or pruned plants, is aggravated by high soil salt content or low soil moisture and is more common in sandier soils. It usually appears when plants are growing rapidly, a sudden hot dry spell occurs and moisture stress taxes the plant.
Prevention of blossom-end rot involves the following:
* Regular and thorough watering. Attempt to saturate the soil several feet all around the plant so the entire root ball takes up moisture. Water deeply but not too frequently. Many garden experts recommend watering every 10 to 14 days. If growing in containers, let the top inch of soil dry out between waterings.
* Maintain even soil moisture. Soil rich in humus will help conserve moisture, as will application of a mulch. Avoid cultivating deeper than an inch within a foot of plants.
* Avoid high-nitrogen fertilizers and be sure to follow the recommended rates for tomatoes. Over-fertilizing can cause salt buildup, which inhibits calcium uptake, or rapid vine growth, which consumes calcium that should be available to the fruit.
Consider applying a calcium supplement such as gypsum (calcium sulfate), fossilized kelp or a foliar form of calcium (such as calcium nitrate, although this works better on greenhouse-grown tomatoes).
Consult a California Certified Nursery professional for guidance in selection.
Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners are here to help. These trained and certified horticultural volunteers are dedicated to extending research-based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or send e-mail to ucmastergardeners@yahoo.com. Calls and e-mail are picked up daily and are generally returned within two to three days.
