Bacterial Blight of Geranium: A History of Diagnostic Challenges.

Geraniums (Pelargonium spp.) have been an important part of greenhouse potted plant and bedding plant production for almost a century. Cultivars produced by vegetative propagation and by true seeds are grown and sold worldwide. In 1996, the wholesale value of geraniums exceeded $205 million in the United States alone (2). Pelargonium species are affected by a number of fungal diseases, including rust (Puccinia pelargonii-zonalis), gray mold (Botrytis cinerea), and root rot (Pythium spp.). Several bacteria and numerous viruses also cause diseases on geranium. Indeed, the most destructive disease of geraniums is bacterial blight (Xanthomonas campestris pv. pelargonii). Although hybrid geraniums grown from seed are susceptible to bacterial blight, it is the vegetatively propagated cultivars of florists’ geranium (P. × hortorum) and ivy geranium (P. peltatum) that are most commonly affected, because the causal organism inhabits the vascular tissue of infected plants and is carried in the cuttings. Munnecke (22) and Nichols (24) estimated 10 to 15% annual losses due to bacterial blight in the 1950s and 1960s, and such losses continue today (22,24). These loss figures are somewhat misleading because they are based on the industry-wide production of geraniums. In individual greenhouse operations, entire geranium crops have been destroyed by this disease. The reputations of several specialty propagators of Pelargonium have been ruined or severely damaged when these growers unknowingly sold infected cuttings and distributed them throughout the bedding plant industry. Despite efforts to eliminate this disease from geranium production systems, it continues to occur annually.