Bacterial Diseases of Ornamentals: Identification, Prevention & Management

Bacteria are microscopic, single-celled organisms with a cell wall but no true nucleus—their circular DNA floats freely in the cytoplasm. While most environmental bacteria are beneficial, several species cause devastating diseases in ornamentals: leaf spots, stem rots, root rots, galls, wilts, blights, and cankers.

How Bacteria Operate

Plant-pathogenic bacteria survive in infected plants, plant debris, and occasionally infested soil. Most require a wound or natural opening (stomata, hydathodes) to enter, plus warm, moist conditions. Once inside, they multiply in intercellular spaces or vascular tissue, releasing enzymes that dissolve cell walls, toxins that rupture membranes, growth regulators that distort development, and polysaccharides that plug xylem vessels. Photosynthesis and respiration are severely disrupted.

Bacteria reproduce explosively. They splash easily via overhead irrigation, move on workers' hands and tools, and spread in contaminated soil. The single most effective prevention? Purchase culture-indexed plants proven free of bacterial pathogens.

Key Bacterial Pathogens in Ornamentals

Erwinia chrysanthemi & Erwinia carotovora

Survive in undecomposed plant debris, on infected and asymptomatic greenhouse plants, and sometimes in soil. Broad host range. Cause mushy, foul-smelling soft rots and leaf spots.

Pseudomonas cichorii

Leaf spots and blights on chrysanthemum, geranium, impatiens, and many others. Spots water-soaked, dark brown to black, often with yellow halos.

Xanthomonas species

• X. campestris pv. pelargoni: bacterial blight/wilt of geranium
• Other species attack Dieffenbachia, Philodendron, Syngonium, Aglaonema, and foliage plants

Rhodococcus fascians (formerly Corynebacterium)

Causes abnormal branching and stem distortion (fasciation) near base of geraniums and others. Carried on cuttings; enters propagation media.

Ralstonia solanacearum (formerly Pseudomonas solanacearum)

Vascular wilt of many herbaceous ornamentals, including geraniums. Mimics Xanthomonas blight but does not cause leaf spots. Unlike most bacteria, survives well in soil. Once established in a greenhouse, extremely difficult to eradicate. Symptoms: wilting, vascular discoloration, yellowing, plant death.

Sanitation: The Core Defense

• Destroy infected plants immediately
• Clean and disinfest tools, benches, flats, pots between uses
• Treat potting soil to kill pathogens; discard or thoroughly treat soil from infected crops
• Train workers: never handle soil/debris then plants without washing hands
• Separate plant-handling and soil/debris operations completely

Cultural Practices That Suppress Bacteria

• Irrigate to keep foliage dry—no overhead in susceptible crops
• If overhead necessary: water early so foliage dries fast
• Maximize air circulation: force air under benches up through canopy; horizontal airflow with rows parallel to air movement
• Trickle, capillary mat, or drip irrigation preferred
• Caution: Some bacteria spread in ebb-and-flow systems—filter debris, treat recirculated water

Chemical Control: Limited Efficacy

Once disease is visible, chemicals rarely achieve >50% control commercially (vs. 80–90% in trials). Prevention is everything.

Five-Point Management Summary

1. Purchase culture-indexed, pathogen-free plants
2. Discard infected plants immediately
3. Eliminate overhead irrigation
4. Pasteurize propagation beds and media between crops
5. Never handle soil/debris then plants without hand washing

Prepared by Gary W. Moorman, Professor of Plant Pathology