Bacterial Wilt (Ralstonia solanacearum): A Global Threat to Ornamentals & Crops

Ralstonia solanacearum attacks nearly 200 plant species across 33 families—one of the broadest host ranges of any plant-pathogenic bacterium. Known as bacterial wilt, Granville wilt (tobacco), Moko disease (banana), or southern wilt, it thrives outdoors between 45°N and 45°S where rainfall exceeds 100 cm/year, growing seasons exceed 6 months, winters stay above 10°C (50°F), and summers above 21°C (70°F). But it moves globally via vegetatively propagated plants, contaminating greenhouses worldwide.

Taxonomy & Strains

Formerly Bacillus solanacearum, then Pseudomonas solanacearum. Non-spore-forming, Gram-negative, rod-shaped (0.5–1.5 µm), single polar flagellum. Divided into races/biovars by host range, biochemistry, phage susceptibility, serology:

• Race 1: U.S. distribution; attacks geraniums (Pelargonium), Catharanthus, Impatiens, Ageratum, Chrysanthemum, Gerbera, Tagetes, Zinnia, Salvia, Capsicum, Lycopersicon, Nicotiana, Petunia, Solanum melongena, Tropaeolum, Verbena, and more
• Race 3 biovar 2 (R3B2): Tropical; not native to North America. Introduced inadvertently on geranium cuttings from Central America/Africa. Major agricultural threat—causes brown rot of potato. Also infects Pelargonium, tomato, pepper, eggplant, bean, beet, and weeds (nightshades, purslane, mustards, lambsquarters, bittergourd)

Infects through roots and fresh wounds. Loses pathogenicity quickly in artificial culture—difficult lab organism.

Symptoms

• Above-ground: Wilting of 1–2 leaves on young plants during heat; recovery at night. On large-leaved plants (e.g., Nicotiana), one side of leaf wilts. Leaves yellow, interveinal tissue browns. Main stem stays upright.
• Internal: Light tan to yellow-brown vascular discoloration; dark brown/black streaking in advanced stages; pith and cortex darken
• Geraniums: Very similar to Xanthomonas blight, but Ralstonia does not cause leaf spots

Diagnostic Signs

• Slimy, sticky ooze forms tan-white to brownish beads on cut vascular tissue
• Cross-cut stem: hold cut ends together briefly; slow separation reveals thin ooze thread
• Suspend cut end in clear water: bacterial ooze streams into water

Management: Exclusion Is Everything

No chemicals or biologicals adequately control Ralstonia. Infected plants must be discarded immediately.

Propagation-Phase Prevention

• Pathogen-free potting media
• Stock plants tested and confirmed free
• Worker training: prevent media contamination, protect stock plants
• Maintain system through entire propagation cycle

Receiving Cuttings/Pre-Finished Plants

Isolate all incoming plants as if health status unknown—even if certified. Keep each source group together in a defined greenhouse area. This allows group monitoring, early detection, and targeted containment/quarantine without compromising other sources. Critical for 'stop-sale' pathogens like Ralstonia.

References

1. Jones, R.K. 1993. Southern bacterial wilt. In Geraniums IV, ed. J.W. White. Ball Publishing.
2. Kim, S.H. et al. 2003. Ralstonia solanacearum race 3 biovar 2 in geraniums in PA, DE, CT. Plant Disease 87:450.
3. Lucas, G.B. 1975. Diseases of Tobacco, 3rd ed. Biological Consulting Associates.