Southern bacterial wilt is a widespread and destructive disease affecting multiple crops, including solanaceous crops and a large range of ornamental plants. Some strains of the pathogen are highly aggressive to food crops and are highly regulated in the United States. Detection of the pathogen is critical to minimize its spread throughout North America. This factsheet will focus on southern bacterial wilt of ornamental plants.

Southern bacterial wilt is caused by the soilborne bacterium Ralstonia solanacearum species complex, which includes R. solanacearum, R. psuedosolanacearum, and R. syzygii. The taxonomical classification of the pathogen is constantly evolving, but it is important to know that there are numerous strains of the pathogen that have variable levels of aggressiveness. R. solanacearum “race 1” is endemic in the United States and is moderately aggressive towards numerous hosts. Ralstonia solanacearum “race 3, biovar 2” is a strain that is highly aggressive and classified as a select agent by the USDA. It has been introduced to the US on infected geraniums on several occasions, but has not become established, to our knowledge.

The pathogen has recently been split into three species: R. solanacearum, R. psuedosolanacearum, and R. syzygii based on genetic and phenotypic evidence. Formerly, the pathogen was divided into 5 races based on the host range and further divided into biovars (biological varieties). Even though the race-biovar structure does not align with genetic evidence of species separation, the race-biovar structure is still used by the United States Department of Agriculture’s Animal and Plant Health Inspection Service (USDA-APHIS). Race 1 is the only race endemic in the United States, while the other 4 races do not occur in North America unless introduced. Race 3 biovar 2 is a highly virulent strain of the pathogen, and has been introduced to the US on infected geraniums (Pelargonium) on several occasions. This strain is classified as a select agent by the USDA and is considered to be a major threat to agriculture in the US.

Fig 1. Bacterial wilt symptoms on geranium (Jean L. Williams-Woodward, University of Georgia, Bugwood.org).

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Fig 1. Bacterial wilt symptoms on geranium (Jean L. Williams-Woodward, University of Georgia, Bugwood.org).

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Different species and strains have different host ranges, but broadly speaking, the pathogen has a wide host range and affects almost 450 plant species in 54 plant families (Prameela and Suseela 2020). “Race 1” can be a destructive pathogen of tobacco, tomato, potato, peppers, eggplant, and also affects many ornamental plants, including geraniums, annual vinca (Catharanthus roseus), impatiens, Ageratum, Chrysanthemum, , Salvia, petunia, daisy, Nasturtium, Verbena, marigold, zinnia, and others. Both races 1 and 3 infect geranium, potato, eggplant, and tomato, and may infect ornamental Brassica spp., Nasturtium, and nettles (USDA-APHIS).

Symptoms may vary depending on the plant that is affected. Latent infections may show no symptoms at all. Later, foliage may wilt slightly in the early stages of the disease, but a characteristic symptom is rapid wilting, often while the foliage is still green. Plants may appear to recover overnight only to wilt again when temperatures rise during the day. Infected plants may become stunted, and foliage may become chlorotic (yellow) or gray green in color. Often the vascular system in the main stem or petioles will have dark streaks.

In late stages of the disease, stems and roots may have brown discoloration and become necrotic, and collapse of the stem can also occur. Diseased plants often die within 7-14 days after the first symptoms are observed.

Lookalike diseases. There are many other soilborne diseases that can occur on the plants listed above and wilting is a common symptom of these diseases. Examples include root and crown rots and Fusarium wilt. It can be difficult to diagnose these diseases in the greenhouse. Therefore, it is critical to get a diagnosis by a plant clinic.

Fig 2. Vascular tissue with brown discoloration caused by infection with R. solanacearum (Clemson University- USDA Cooperative Extension Slide Series, Bugwood.org).

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Fig 2. Vascular tissue with brown discoloration caused by infection with R. solanacearum (Clemson University- USDA Cooperative Extension Slide Series, Bugwood.org).

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Fig 3. Annual geranium showing chlorotic leaf symptoms from infection with R. solanacearum (Jean L. Williams-Woodward, University of Georgia, Bugwood.org).

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Fig 3. Annual geranium showing chlorotic leaf symptoms from infection with R. solanacearum (Jean L. Williams-Woodward, University of Georgia, Bugwood.org).

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Disease development is favored by warm temperatures (>85°F) and wet soils. The bacterium multiples in susceptible crops and weeds and can survive for several years in the soil. The pathogen can spread via the movement of water, contaminated soil, infected plants, and on the surface of infested tools or equipment. The pathogen gains entry into a plant through microscopic wounds (often caused by insects, cultivation, or transplanting) of susceptible plants. Once inside plant tissue, the bacterium clogs the vascular tissue in the stem, preventing the transport of water throughout the plant and eventually leading to plant death.

Southern bacterial wilt is a difficult disease to control. There are few effective chemicals or biologicals that can be used to manage this disease, so cultural controls are key.

• Sanitation: All potting media, pots, tools, and trays used in propagation should be kept clean. Avoid contact with native soil, as the pathogen may be present. Water used for irrigation should be sanitized and/or filtered prior to use to eliminate pathogens.
• Source of plant material: The best way to prevent disease is to avoid its introduction. This involves obtaining plant materials from reputable sources and growing and propagating pathogen-free plant material in pathogen-free potting media or soil. Purchasing culture-indexed geranium stock is recommended. New plant material should be kept separate from all other plants upon arrival at a production facility, and thorough scouting should be conducted. If you suspect that you have disease caused by Ralstonia solanacearum, get a diagnosis as soon as possible. Race 3 biovar 2 is a federal select agent, meaning that its introduction is tightly regulated by the USDA’s Animal and Plant Health Inspection Service (USDA-APHIS).

• Sanitation: The best way to prevent disease it to avoid its introduction. This involves obtaining plants from reputable sources and growing and propagating pathogen-free plant material in pathogen-free potting media or soil. If you suspect that your plants are infected with the bacterial wilt pathogen, contact your local Extension agent or the NC State University Plant Disease and Insect Clinic for help getting the problem diagnosed.
• Choice of host: If an endemic strain of R. solanacearum is present in a landscape, there’s no way to eliminate it from the soil. The only management strategy is to grow species that are not hosts of the pathogen, such as grasses. With tomatoes, grafting onto certain rootstocks has been successful, but this is not an option currently available for ornamental plants. Information regarding the resistance of ornamental bedding plants to this disease can be hard to find. If it is necessary to grow a susceptible variety in an area where the disease has been a problem in the past, plant the susceptible variety in another part of the landscape, or plant into new soil or a soil-less media in planter boxes. Avoid moving soil from infested beds into clean areas on equipment, tools, and boots.

Prameela TP; Suseela Bhai R. 2020. Bacterial wilt of ginger (Zingiber officinale Rosc.) incited by Ralstonia pseudosolanacearum - A review based on pathogen diversity, diagnostics and management. Journal of Plant Pathology 102(3), 709–719.

USDA-APHIS. 2023. "Ralstonia."

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