What Causes Blossom End Rot?

What You Need to Know

Blossom end rot strikes up to 50% of a tomato harvest in bad years, appearing as a dark, sunken, leathery patch on the bottom of tomatoes, peppers, squash, eggplant, and watermelons. Understanding its true cause is essential because the most common advice about adding calcium often misses the real issue.

The direct cause of blossom end rot is insufficient calcium in the developing fruit tissue. Calcium is a critical structural component of cell walls. Without adequate calcium, cell walls collapse during rapid fruit growth, creating the characteristic dark, sunken lesion on the blossom end, which is the part of the fruit farthest from the stem and last to receive nutrients.

However, in the vast majority of cases, the soil contains plenty of calcium. Most garden soils and virtually all soils that have been limed contain adequate calcium for plant growth. The problem is not supply but delivery. Calcium is unique among plant nutrients because it is immobile within the plant. Once deposited in a cell, calcium cannot be redistributed. Plants can move nitrogen, phosphorus, and potassium from older leaves to newer growth, but calcium must be continuously supplied to growing tissue through the transpiration stream.

This means calcium transport depends entirely on water movement through the plant. Any disruption to consistent water uptake will cause calcium delivery failure to the fastest-growing tissues, which are the expanding fruit. The most common causes of disrupted water uptake are inconsistent irrigation, drought stress, root damage, and soil conditions that affect water availability.

Inconsistent watering is the primary trigger. When soil alternates between dry and wet conditions, the plant's water uptake becomes irregular. During dry periods, calcium transport to fruit slows or stops. Even a single day of water stress during the rapid fruit expansion phase can cause blossom end rot that becomes visible a week later. This is why blossom end rot often appears in clusters, corresponding to a dry spell that occurred during a critical growth window.

Over-fertilization, particularly with nitrogen, is a major contributing factor and directly relevant to fertilizer management. Excess nitrogen stimulates rapid vegetative growth, creating more leaf surface area that demands water for transpiration. This leaf growth competes with developing fruit for the limited water and calcium moving through the plant. The vigorous leaves win this competition because they transpire more actively than fruit, diverting calcium away from where it is needed. Ammonium nitrogen is particularly problematic because it directly competes with calcium for uptake at the root level.

Root damage from any cause reduces the plant's ability to absorb water and calcium. Cultivating too close to plants, root rot from overwatering, nematode damage, and salt burn from over-fertilization all compromise root function. This is another way that excess fertilizer contributes to blossom end rot beyond the nitrogen competition mechanism.

To prevent blossom end rot, focus on consistent moisture management. Water deeply and regularly, providing one to two inches per week. Mulch with two to four inches of organic material to moderate soil moisture fluctuations. Drip irrigation or soaker hoses are preferable to overhead watering for maintaining consistent soil moisture.

Avoid excessive nitrogen fertilization, especially once plants begin flowering and fruiting. Switch from high-nitrogen fertilizers to balanced or low-nitrogen formulations after the vegetative growth phase. Use a fertilizer calculator to apply the correct amount rather than guessing.

If blossom end rot has already appeared, remove affected fruit to redirect the plant's energy. Correct the watering schedule immediately. A foliar calcium spray can provide short-term calcium directly to foliage but is not as effective as fixing the underlying water management issue. Calcium sprays applied directly to fruit show limited absorption and are generally not recommended as a primary solution.

Soil calcium amendments like lime or gypsum are only helpful if a soil test confirms actual calcium deficiency, which is uncommon. Adding calcium to soil that already has adequate levels will not resolve blossom end rot caused by water management issues and can disrupt the balance of other soil nutrients.