Blossom End Rot
A physiological disorder in tomatoes, peppers, and squash characterized by a dark, sunken, leathery lesion at the blossom end (bottom) of the fruit. Caused by calcium deficiency in developing fruit tissue, typically due to inconsistent watering rather than calcium-poor soil.
Blossom end rot (BER) appears as a dark, sunken, water-soaked lesion on the bottom (blossom end) of tomatoes, peppers, squash, and watermelons. The affected tissue eventually turns brown, black, and leathery. The fruit is not poisonous and the unaffected portion can be eaten, but the fruit is unsellable and often deteriorates quickly as secondary rot fungi colonize the lesion.
BER is not a disease. It’s a physiological disorder - a problem with how the plant is functioning rather than with a pathogen attacking it.
The Cause
The tissue damage results from calcium deficiency in developing fruit cells. Calcium is needed to build cell walls; when developing fruit tissue doesn’t receive adequate calcium, cell walls fail and the cells collapse and die. The result is the characteristic dark, sunken lesion.
The critical nuance: BER usually occurs even when soil calcium is adequate. The problem is calcium delivery to developing fruit, not calcium supply in the soil.
Calcium moves through the plant in the xylem (water-conducting tissue) along with the transpiration stream. It moves from roots upward wherever water moves. The problem: calcium is minimally mobile once in plant tissue. If developing fruit is not receiving adequate water flow through the xylem at the critical period of cell development, calcium doesn’t reach those cells in sufficient quantities.
Inconsistent watering is the most common trigger. A wet-dry-wet cycle creates a period of water stress that interrupts calcium delivery to developing fruit. The fruit cells forming during the dry period are calcium-deficient; the lesion appears a week or two later as that tissue matures.
Root damage from drought, compaction, or mechanical disturbance also reduces water uptake and calcium delivery.
Rapid vegetative growth during conditions of abundant water and nitrogen creates competition: the rapidly growing shoot tips and leaves also demand calcium through the xylem. Growing fruit may be outcompeted during stress periods.
High ammonium nitrogen can interfere with calcium uptake at the root. Soils very high in potassium or magnesium can also inhibit calcium uptake through ion competition.
Management
Consistent watering is the most effective prevention. Even soil moisture - maintained through mulching and regular irrigation on a schedule rather than responding to drought stress - eliminates most BER in most gardens. Tomatoes need roughly 1-2 inches of water per week during fruit development; inconsistency is the enemy.
Mulch the soil surface to reduce moisture fluctuation from surface evaporation.
Avoid excessive nitrogen applications at fruiting time. High nitrogen pushes rapid vegetative growth that competes with fruit for calcium.
Soil calcium: Test soil if BER is severe and recurring. True soil calcium deficiency is uncommon in most established gardens but can occur in sandy soils with low organic matter or acidic soils where calcium leaches. If soil calcium is genuinely low, incorporate ground limestone or gypsum.
Foliar calcium sprays (calcium chloride, calcium nitrate solutions) applied to developing fruit are widely sold and used. The efficacy evidence is mixed. Calcium taken up through leaves has limited mobility and may not reach fruit tissue efficiently. These sprays may help in cases of acute deficiency but are not a substitute for consistent watering.
Lime-based sprays are a folk remedy. Calcium hydroxide (hydrated lime) applied to leaves is too caustic in any effective concentration. Avoid.
The First Fruits Pattern
BER often affects the first fruits of the season more than later fruits. This is partly because early fruits set when plants are smaller and root systems less developed, and partly because plants are calibrating their calcium distribution to fruiting demands they haven’t yet experienced. If BER is limited to the first 3-5 fruits and subsequent fruits are normal, consistent watering has likely corrected the issue.
Paste tomatoes and large-fruited slicing tomatoes are more prone to BER than cherry tomatoes. ‘San Marzano’, ‘Amish Paste’, and ‘Roma’ show BER regularly in inconsistently watered gardens.