Blossom End Rot (BER) is a common affliction affecting squash, peppers, and tomatoes. It is classified as a physiological disorder, stemming from an internal imbalance within the plant’s system rather than an external pest or pathogen. The disorder manifests as a sunken, dark brown or black patch that develops on the end of the fruit farthest from the stem. This lesion often starts when the squash is small and rapidly expands into a leathery patch.
The Immediate Cause: Localized Calcium Deficiency
The direct cause of the rot is a localized deficiency of calcium within the developing fruit tissue. Calcium is a structural nutrient essential for maintaining the strength and integrity of plant cell walls. When the fruit expands rapidly, the cells at the blossom end require a continuous supply of calcium to complete their structure.
If this supply is interrupted, the cell walls collapse and disintegrate, leading to the characteristic dark, necrotic tissue. This problem often occurs even if the soil contains ample calcium. Calcium is immobile once incorporated into plant tissue and must be continuously supplied to new growth. Therefore, the issue is usually a failure of the plant to absorb or transport calcium efficiently to the developing fruit, rather than a lack of calcium in the environment.
Environmental Triggers That Disrupt Nutrient Uptake
Calcium is transported from the roots through the xylem, moving with the water flow generated by transpiration. Any environmental factor that disrupts this water movement inhibits calcium delivery to the fruit.
Inconsistent Moisture
Inconsistent moisture is the most significant factor, as cycles of drought followed by heavy watering prevent the steady uptake of water and dissolved calcium. Fluctuations in soil moisture, whether too dry or waterlogged, damage the fine root hairs responsible for nutrient absorption.
When water flow is restricted, leaves have a much higher rate of transpiration than fruit. This causes them to win the competition for the limited calcium supply, diverting it away from the developing fruit.
Soil Chemistry and Nutrients
Soil chemistry affects calcium availability at the root level. If the soil pH falls too low (typically below 6.0), the acidity locks up the calcium, making it inaccessible to the roots.
Overuse of fertilizers high in nitrogen can also induce a deficiency. Excessive nitrogen promotes fast, leafy vegetative growth, which draws available calcium away from the fruit.
Temperature Stress
Plant stress from cold temperatures reduces the plant’s ability to take up nutrients early in the season. Cold soil or low nighttime temperatures slow down root activity, restricting the initial absorption and movement of calcium. This explains why the first fruits of the season are often the most susceptible to BER.
How to Prevent and Treat Blossom End Rot
Addressing Blossom End Rot requires a strategy focused on immediate stabilization and long-term soil management. Since affected fruit will not recover, remove any squash showing symptoms to prevent the plant from wasting resources. Foliar calcium sprays can be applied as a temporary measure, but they are a stopgap, as calcium does not readily move from the leaves to the fruit.
Moisture Management
The most effective long-term solution is ensuring consistent moisture delivery to the root zone. Water deeply and regularly, rather than shallowly, to stabilize the water supply. Applying a thick layer of organic mulch, such as straw or shredded leaves, maintains an even soil temperature and reduces moisture evaporation from the soil surface.
Soil Correction
For a permanent fix, a soil test provides an accurate measure of calcium levels and soil pH. Squash plants prefer a slightly acidic to neutral pH, ideally between 6.0 and 6.5, for optimal calcium uptake. If the test shows low pH, amendments like agricultural lime can correct the acidity and increase calcium availability. Using a balanced, low-nitrogen fertilizer also helps prevent excessive vegetative growth that competes with the fruit for calcium.