An In Vitro Screen for Detecting Resistance in Pelargonium Somaclones to Bacterial Blight of Geranium. Kerry B. Dunbar, Graduate Research Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Christine T. Stephens, Associate Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 73:910-912. Accepted for publication 25 May 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0910.

Resistance to Xanthomonas campestris pv. pelargonii was detected among somaclones of Pelargonium species with an assay of plantlet reactivity to inoculum titer, strain, and pathovar. Three weeks after plantlets growing in culture tubes were inoculated with a 10⁷ cfu/ml cell suspension of X. c. pv. pelargonii, seed geranium (P. × hortorum) somaclones from susceptible cultivars had a significantly lower survival rate than did those from the more resistant regal geranium (P. × domesticum). Symptoms did not develop among plantlets treated with X. c. pv. campestris or sterile water. Somaclones of P. scabrum, P. betulinum, P. grandiflorum, P. multicaule, and P. hispidum had significantly higher levels of resistance to X. c. pv. pelargonii than did those of P. × hortorum. This assay holds promise as a means of rapidly identifying bacterial blight-resistant somaclones.