Identification of sources of resistance to bacterial angular leafspot disease of strawberry

Bacterial angular leafspot disease (BALD) of cultivated strawberry, caused by the bacterium Xanthomonas fragariae, has become an increasingly serious disease problem. It is of particular concern because it is readily transmitted through asymptomatic nursery plants. Until now, there have been no sources of resistance to this pathogen identified in either commercial varieties or germplasm. We have used four genetically distinct strains of the pathogen, Xanthomonas fragariae, to screen 81 Fragaria genotypes, including both diploid and octoploid accessions, for resistance to this pathogen. Two genotypes, a native F. virginiana from Minnesota and a hybrid between a F. virginiana from Georgia and F. ×ananassa 'Earliglow', were found to be resistant to all four genotypes of this pathogen after leaf infiltration assays. Following infiltration of these genotypes, symptoms of the disease, including localized necrosis, leaf collapse, bacterial ooze production or systemic spread of the pathogen, were not observed. Plants of 'Sweet Charlie', used as the susceptible standard, showed all of these symptoms. The two resistant genotypes, designated US 4808 and US 4809 have been made available to the public as germplasm releases. Controlled crosses were made between the susceptible variety 'Sweet Charlie' and the two resistant genotypes. Resistance to X. fragariae was transmitted to 8-12% of the progeny of the US 4808 cross and to 4-18% of the progeny of the US 4809 cross. Data from these experiments are being analyzed to establish the mode of inheritance. Our research may lead to sustainable control of this disease.