Bacterial wilt of Pelargonium: Development of a screening method for resistance

Pelargonium is one of the most important ornamental plants for home and garden. Cultivars are propagated vegetatively. Two bacterial pathogens (Xanthomomas hortorum pv. pelargonii and Ralstonia solanacearum) cause bacterial wilt and blight on Pelargonium resulting in high economic losses. The first symptom of both diseases is the characteristic wilting of single leaves. After the invasion of the bacteria, infected stems become brown or black and the whole plant is dying. Based on symptoms, these two bacterial diseases can not be distinguished, but a microbiological differentiation is possible. As the inoculation pathway of these species is different, the aim of this project was to develop reliable inoculation methods for both pathogens. These were developed on plant material provided by Elsner pac^® Dresden using bacterial strains of the collection of the Institute for Resistance Research and Stress Tolerance and of the German Collection of Microorganisms and Cell Cultures (DSMZ). For the inoculation with X. hortorum pv. pelargonii contaminated scissors were used because the transmission of the disease in practical Pelargonium production occurs by cutting plant parts during the propagation process. As the natural inoculation by R. solanacearum is via roots, the bacteria suspension was filled into pots and the roots of respective plants were cut (wounded) with a knife to foster the bacteria invasion of bacteria. By using these methods, 6 out of 91 genotypes of Pelargonium turned out to be resistant against Xanthomonas and 4 out of 116 against Ralstonia. Plants were classified as resistant if neither symptoms nor bacteria were detected. As the most important result, 3 genotypes were identified as resistant to both pathogens. The resistant genotypes will be used in future breeding for resistance.