Resistance to Xanthomonas campestris pv. pelargonii in geranium and diagnosis of the bacterial blight using polymerase chain reaction

Geranium bacterial blight is caused by Xanthomonas campestris pv. pelargonii (Xcp). In this study, we tested the susceptibility and resistance of 17 geranium species to infection by Xcp based on both visually scored leaf blight symptoms and bacterial detections with Xcp-specific PCR. The initial symptom observation indicated that most of the tested geraniums are susceptible to Xcp infection with six exceptions of Geranium maderense, G. pratense, G. sanguineum, G. wallichianum, Pelargonium peltatum, and P. regalia. Of these, G. sanguineum showed delayed, mild symptoms and proved tolerant to Xcp infection, whereas the other five species displayed few symptoms and are resistant to the infection. Using PCR, the presence of Xcp bacterium could be detected in G. sanguineum only after the onset of symptoms but was never detected in the five resistant species. In contrast, the presence of the bacterium could be detected from the susceptible Pelargonium plants before the onset of symptoms. To our knowledge, this is the first report describing the use of both symptom-based assay and PCR for screening geranium bacterial blight resistance.