Watermelon Blossom End Rot

Blossom end rot (BER) on watermelon is a physiological disorder linked to poor calcium supply in the expanding fruit, not an infection. Because calcium moves in the plant with the transpiration stream and root uptake, any factor that interrupts steady water flow - drought, heavy rain after dry spells, shallow roots, or root injury - raises BER risk during warm-season fruit set.

Watermelon (Citrullus lanatus) is a warm-season crop in the family Cucurbitaceae and sets fruit when soils and air are warm and plants are actively growing. The typical soil window for avoiding BER is pH 6.0-6.8 in a well-draining, fertile bed; that keeps calcium soluble and roots healthy. Growers who favor deeper, less-frequent watering and keep surface mulch in place tend to see fewer BER outbreaks.

BER is strongly associated with moisture swings: a stretch of dry weather followed by heavy irrigation or thunderstorm rain commonly triggers new BER on fruit that were small or just starting to expand. Rapid fruit expansion in hot weather raises demand for calcium and can expose marginal calcium supply issues even when soil tests show adequate total calcium content.