Geranium Stem Rot: Diagnosing Pythium and Xanthomonas in Mature Plants

Five-year-old geraniums with soft stem centers likely suffer from vascular disease—either Pythium root/stem rot (fungal) or Xanthomonas bacterial blight. Both clog the plant's "interstate highway" (xylem/phloem), causing the soft, dying tissue you see.

Disease Profiles

Pythium ("Black Leg")

• Common in young cuttings, but can persist
• Spreads via contaminated tools, water splash, soil
• Causes blackened, soft stem bases

Xanthomonas (Bacterial Blight)

• Attacks mature plants—your 5-year-olds are prime targets
• Spreads on tools, water, insects
• Can remain latent for years before symptoms appear
• Clogs vascular tissue, causing wilting and stem softening

Action Plan

1. Do not reuse pots or soil without sterilization
2. Wash hands thoroughly after handling infected plants
3. Source clean replacements from a local geranium society—they often have verified disease-free stock in your preferred colors
4. Disinfect all tools with 10% bleach solution

Hard Truth

These diseases are systemic and incurable. The kindest—and smartest—move is replacing with clean plants. Your pale pink variety likely exists in disease-free form through specialty growers.