Container Tomato Blossom-End Rot: Fungus vs. Physiological Disorder

Bottom Rot on Container Tomatoes: Not Just Calcium

Fruit rotting only at the blossom end in Southern California containers suggests blossom-end rot (calcium deficiency), but the description "fungus or bacteria" warrants clarification.

Distinguishing Blossom-End Rot from Fruit Rot

Symptom	Blossom-End Rot	Fungal Fruit Rot
Appearance	Dark, sunken, leathery bottom	Soft, fuzzy, expanding lesion
Progression	Stays at blossom end	Spreads up sides
Timing	On green fruit	Usually on ripening fruit
Cause	Calcium + water fluctuation	Pathogens (Alternaria, Botrytis)

If It's Blossom-End Rot (Most Likely)

1. Remove affected fruit—they won't recover
2. Water consistently: daily in heat, but deep soaks
3. Mulch 2 inches
4. Calcium nitrate foliar spray (1 tsp/gallon) weekly

If It's Fungal Rot

• Apply copper fungicide (Soap Shield) weekly
• Improve air circulation—prune dense foliage
• Reduce humidity: water early morning only
• Remove all diseased fruit and fallen debris

Container-Specific Prevention

Use 15+ gallon pots with 50% compost/50% potting mix. Add 1 tbsp gypsum per gallon of mix at planting for calcium. EarthBox-style reservoirs provide most consistent moisture.

Diagnose correctly first—treatment differs entirely. Blossom-end rot is physiological; fungal rot is infectious.