It’s a heartbreaking sight: dark brown, leathery patches appear on the ends of tomatoes that just a week before seemed to glow with good health. The corruption spreads upward and inward, ruining fruit that was destined for salads, salsas, sauces…
“Blossom-end rot”—which isn’t really a rot at all—has claimed another crop of tomatoes.
After nurturing a throng of tomato plants through their various stages of development—sprouting them on a windowsill in early spring, coaxing them through the stresses of repotting and transplanting, protecting them from late frosts, pruning and staking their tender stems to ensure good air circulation, and finally watching those first bright blossoms set fruit—witnessing the gradual mummification of one’s tomatoes is certainly disappointing.
There’s no choice but to pluck the fruit and toss it into the compost heap. Even when trimmed away, the decay often extends upward into the fruit’s interior, making it inedible.
Some gardeners—mistakenly believing that blossom-end rot is caused by a fungus—wrench their plants from the ground and destroy them.
In reality, blossom-end rot is caused by a nutritional deficiency, coupled with inadequate or uneven watering during fruit formation.
Calcium Deficiency Causes Blossom-End Rot in Tomatoes
Calcium is a mineral that is required for normal cell growth and division. When a rapidly-growing tomato (or eggplant or pepper) is deprived of the calcium it needs—or if the plant’s needs for calcium outstrip the available supply—cellular maturation ceases and the fruit’s tissues begin to degrade.
As the tissues break down, a characteristic sunken, brownish lesion appears at the blossom end of the fruit; the damaged area expands, sometimes covering more than half of the fruit.
Occasionally, the lesion is colonized by bacteria or molds, reinforcing the notion that blossom-end rot is caused by an infectious or parasitic organism.
Causes for Calcium Deficiency
- Low calcium concentrations in soil
- Relatively high concentrations of competitive cations in soil (e.g., ammonium)
- Drought
- Excessive soil moisture fluctuations during fruit development, leading to impaired uptake and transport of calcium through the plant
- Vegetative growth overwhelms available soil calcium concentrations (may occur with excessive nitrogen fertilization, which increases leaf growth without necessarily contributing to fruit growth)
How to Prevent and Cure Tomato Blossom-End Rot
The best way to deal with blossom-end rot is to prevent it. A soil test prior to transplanting tomatoes, peppers, or eggplants into the garden is one way to determine the soil’s viability, but it isn’t essential for growing healthy plants or harvesting blemish-free fruit.
The following measures will help to ensure a healthy harvest:
- Maintain soil pH around 6.5 (a pH of 7.0 is neutral)
- Apply lime, bone meal, or other high-calcium fertilizers to the plot prior to planting tomatoes—ideally during the previous fall—and work in thoroughly with a spade or tiller
- If nitrogen fertilizers are used, avoid those that use ammonium as the nitrogen source. Ammonium ions interfere with a plant’s calcium uptake
- Mulch tomatoes heavily to improve water retention and avoid fluctuations in soil moisture
- Irrigate with 1 – 2 inches of water weekly (more during drought conditions or in sandy soils)
- If blossom-end rot does develop (it is usually more problematic on the season’s first fruits)withhold nitrogen fertilizers and apply side dressings of bone meal, lime, or other calcium-rich fertilizers. Water well. As new fruit sets, it may develop normally.
(From Miller S, Rowe R, Riedel R. Blossom-end rot of tomato, pepper, and eggplant. The Ohio State University Extension Fact Sheet. HYG-3117-96, and personal files)
Blossom-end rot in tomatoes, peppers, and eggplants is a frustrating but preventable—and even treatable—problem that most gardeners have faced at least once in their careers. With no more effort than it usually takes to raise a healthy garden, blossom-end rot can be banished for good.
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