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Behavior of Ralstonia solanacearum Race 3 Biovar 2 During Latent and Active Infection of Geranium

Southern wilt of geraniums (Pelargonium hortorum), caused by the soilborne bacterium Ralstonia solanacearum race 3 biovar 2 (R3bv2), has inflicted significant economic losses when geranium cuttings latently infected with this quarantine pest were imported into the United States. Little is known about the interaction between R. solanacearum and this ornamental host. Using UW551, a virulent R3bv2 geranium isolate from a Kenyan geranium, we characterized development of Southern wilt disease and R3bv2 latent infection on geranium plants. Following soil inoculation, between 12 and 26% of plants became latently infected, carrying average bacterial populations of 4.8 × 10⁸ CFU/g of crown tissue in the absence of visible symptoms. Such latently infected plants shed an average of 1.3 × 105 CFU/ml in soil run-off water, suggesting a non-destructive means of testing pools of asymptomatic plants. Similarly, symptomatic plants shed 2 × 10⁶ CFU/ml of run-off water. A few hundred R. solanacearum cells introduced directly into geranium stems resulted in death of almost all inoculated plants. However, no disease transmission was detected after contact between wounded leaves. Increasing temperatures to 28°C for 2 weeks did not convert all latently infected plants to active disease, although disease development was temperature dependent. Holding plants at 4°C for 48 h, a routine practice during geranium cutting shipment, did not increase frequency of latent infections. R. solanacearum cells were distributed unevenly in the stems and leaves of both symptomatic and latently infected plants, meaning that random leaf sampling is an unreliable testing method. UW551 also caused potato brown rot and bacterial wilt of tomato, surpassing race 1 strain K60 in virulence on tomato at the relatively cool temperature of 24°C.