Heat Treatment to Eradicate Ralstonia solanacearum R3bv2 from Plant Growth Medium

The pathogenic bacterium Ralstonia solanacearum Race 3 biovar 2 (R3bv2) is highly regulated because it could threaten potato production. R3bv2 can also infect geraniums, another high-value crop. Geranium cuttings infected with R3bv2 have been accidentally imported to the U.S. five times since 1980, triggering costly eradications. To prevent further introductions, USDA APHIS regulates offshore geranium production practices. Geraniums are typically grown from cuttings in volcanic scoria rock medium. Re-using this growth medium saves labor and carbon. Growers must currently steam medium at 80°C for 120 min before re-use. This required treatment is both costly and environmentally destructive. We empirically determined the conditions required to inactivate R3bv2 in infected geranium tissue with the goal of conserving resources while maintaining rigorous biosecurity. Roots of R. solanacearum-infected geraniums grown in scoria under typical production conditions were exposed to a range of time and temperatures. Surviving pathogen populations were quantified using a sensitive combination of serial dilution plating and enrichment culture. Over 99.9% of R3bv2 cells in infected roots were inactivated by exposure to 55°C dry heat for 15 minutes. However, these roots sometimes contained a small subpopulation of heat-resistant R3bv2 cells. The heat-resistant subpopulation was no longer detectable after infected roots were held at 75°C for 15 minutes. This treatment also eradicated R3bv2 cells in stems of infected geranium plants and in infested volcanic scoria growth medium. These results demonstrate that R. solanacearum R3bv2 is eradicated from infected geranium tissue and growth medium by cooler and shorter heat treatments than currently required.