Ralstonia solanacearum is the name of a cluster of related bacterial pathogens. One strain of R. solanacearum native to the southern U.S. causes wilt diseases of garden flowers and vegetables. But now that ornamentals production is a truly global industry, we are increasingly exposed to new R. solanacearum strains that cause disease in protected greenhouse culture. Although their quarantine status varies, R. solanacearum strains all cause systemic, often lethal, wilt diseases. The bacteria are easily moved internationally in plant material, and locally in soil or water, or on tools.
The geranium story
Want to strike fear into the heart of a geranium grower? Just whisper “Ralstonia” — that’s the geranium world nickname for Race 3, biovar 2 (R3b2), an R. solanacearum subgroup that is a quarantine pest in Europe and North America now designated a U.S. Select Agent subject to the strictest quarantine regulation. R3b2 is a Select Agent because it’s considered a threat to the American potato industry: These bacteria cause potato brown rot disease, which wilts plants and turns potato flesh to a soft, brown goo. R3b2 is present in many countries, but is not established in Canada or the U.S. And we want to keep it that way.
But geraniums have an ugly history with Ralstonia R3b2: In 2003, infected cuttings were unknowingly shipped to the U.S. from a Kenyan facility. The infection may have come from upstream diseased potato fields because Ralstonia can spread via water. This “find” of a Select Agent in the U.S. led to quarantining of many businesses. After the Kenya debacle, geranium production shifted to Guatemala, but once again the pathogen slipped into the system, and infected cuttings were shipped to North America. More businesses were quarantined, and the disease caused economic disaster to propagators and finishers. This dreadful series of events eventually led to a happy ending when the geranium industry worked with USDA-APHIS to improve clean stock standards for geraniums. Today a detailed and (so far) effective program backs the cleanliness of geranium cuttings received from offshore.
The mandevilla story
Dave Norman’s lab at the University of Florida-Apopka has been collecting R. solanacearum strains from Florida nurseries since 2012. Although these bacteria are not the Select Agent, they do cause disease on mandevilla occasionally. Mandevilla cuttings are frequently imported from places that may not follow good sanitation practices, so Ralstonia has been introduced. It has taken time for the mandevilla growers to clean up their operations. Traditionally, stock plants are grown in ground beds. Once these have become infested with Ralstonia, eradication is nearly impossible. Also, growers in Florida are pressured to conserve water and thus frequently recycle untreated water. Moving mandevilla stock plants to pots on raised benches and using only well water has greatly reduced bacterial wilt incidence.
The osteospermum story
In 2014, Ralstonia reappeared from an unexpected source — in osteospermums shipped into the U.S. from Guatemala; many recalls followed. This caused significant dollar losses for both the propagation company and growers. Osteospermum was not known at the time to be susceptible to R. solanacearum. Experiments in the Allen lab at U. Wisconsin showed that this Ralstonia was not the R3b2 Select Agent, which was a big relief for all concerned. This different strain, which we typed as “phylotype 1, sequevar 14” (“I-14” for short) could cause symptoms in geranium, osteospermum and mandevilla as well as tomato. Worryingly, the strain could also symptomlessly infect a number of ornamentals, including dahlia, New Guinea impatiens, verbena, vinca, lobelia, bacopa, petunia and calibrachoa. These all developed populations of more than 1,000 bacterial cells per gram of stem after inoculation, meaning that even though they appeared healthy, they contained bacteria that could infect other plants. Ralstonia can create these botanical “Typhoid Mary” plants — sources of continuing disease and quarantine problems.
The rose story
And now a new foe: We were horrified to learn in 2015 of an outbreak of a Ralstonia disease in hybrid tea roses in greenhouses in the Netherlands. Despite vigorous eradication efforts, occasional outbreaks continue in northern European rose production. Symptoms on roses include stunting, leaf wilting and yellowing, and whole plant death as well as browning of vascular tissue in the lower stem. Unfortunately, this rose strain also forms symptomless infections. This may explain why it has been difficult to eradicate. Typing in the Allen lab indicated that the infected roses all contain a phylotype I sequevar 18 (I-18) strain, not the dreaded R3b2 Select Agent. Nothing is known of the effect of this bacterium on shrub roses such as KnockOut, but we do know that it is aggressive on blueberry.
This organism isn’t in the U.S. now, but because of the proven ability of Ralstonia to form latent infections, it might inadvertently be shipped into the US at any time.
What should be done
Being aware of the possibility of this disease is important. Even imported cut roses could contaminate waterways in the U.S. if diseased flowers were discarded near a creek or river. Within production facilities, good phytosanitation remains the best way to exclude the pathogen or prevent it from spreading. Tools must be disinfested between plants and surface water used for irrigation should be disinfested. Studies in the Allen lab found that ornamental Ralstonia strains are killed by 10 minutes at 55° C, by exposure to 150 mM chlorine or 50 ppm stabilized hydrogen peroxide product for 10 minutes, or by treatment with in-line UV radiation.
We need to improve the speed and accuracy of identifying different Ralstonia strains to allow appropriate regulatory actions. Biological studies of this cluster of bacteria are required to better understand and block the multiple types of Ralstonia solanacearum that threaten so many ornamentals. Most importantly, we need research to develop methods to control Ralstonia diseases, because currently management is brutal and costly: plant destruction. There are no registered products to protect ornamentals from Ralstonia. Along with its many benefits, globalization now brings more of our ornamental crops into contact with Ralstonia. We need to reverse this trend of surprise outbreaks on new crops by improvements in knowledge about diagnostics, sanitation, monitoring and protecting crops against Ralstonia diseases.