Pennsylvania Flower Growers #268
Bacterial blight of Pelargonium species caused by the bacterium, Xanthomonas pelargonii, is recognized as one of the most important diseases of florist's geranium, Pelargonium X hortorum, in the USA. This wilt pathogen gets into the water-conducting tissues of the plant and is spread primarily in cuttings during vegetative propagation. As symptom expres^^ sion is suppressed under the cool conditions prevailing at propagation, many symptomless infected plants may be produced and sold. In warm weather, symptom expression occurs and many of these apparently healthy plants may suddenly wilt and die. Knauss and Tammen found a wide range of resistance to Xanthomonas pelargonii among 22 species and four hybrids of Pelargonium. Stem-inoculated plants fell into three distinct groups: highly resistant, intermediate resistance or susceptibility, and highly susceptible. Examples of each group are P. X domesticum, highly resistant; P. graveolens, intermediate resistance or susceptibility; and P. peltatum highly susceptible. The pathogen was found to persist in root-inoculated, symptomless plants of P. X domesti cum maintained for 66 days under conditions optimum for disease development. Although highly resistant, this species cannot be considered im mune as reported earlier by Munnecke. Thus several Pelargonium spe cies possess significant resistance to ^, X. pelargonii, but none of the cultivars of the highly desirable P. X hortorum are resistant. All attempts to incorporate disease resistance into P. X hortorum have failed. This report summarizes the results of a project designed to determine whether anatomical differences exist in resistance and susceptible Pelargonium species, and to study the differences between the reaction of resistant and susceptible Pelargonium species to infection by X. pelargonii.
Keywords: Susceptibility Xanthomonas pelargonii Resistance Water-conducting tissues Wilt pathogen