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. INTRODUCTION Bacterial angular leafspot disease of strawberry, (Fragaria ×ananassa Duch.) has become a chronic and at times serious disease of strawberries. The disease and the pathogen, Xanthomonas fragariae were originally described from Minnesota (Kennedy and King, 1962), but are now known to have a much wider distribution in North America as well as in Europe, South America and Australia (Maas et al., 1995). The primary symptom of the disease is translucent leafspots that are delimited by the veins of the leaf. Pale, white bacterial exudate is commonly observed on the underside of symptomatic leaves under conditions of high humidity, and these bacteria promote secondary spread of the disease in contaminated fields. Although the pathogen is not known to infect strawberry fruit, it is able to invade the plant systemically through the vascular system, and colonize the petioles, crown, stolons and roots (Milholland et al., 1996). Young plants may be entirely free of symptoms, and the pathogen can be disseminated with commercial transplants, which succumb to the disease in the field. All commercial cultivars of strawberry are apparently susceptible to the pathogen, and prior to the initiation of this research, there were no sources of resistance to X. fragariae available to strawberry breeders. In order to identify germplasm resistant to the disease, we assembled a diverse collection of Fragariae germplasm (Maas et al., 2000) as well as a collection of strains of X. fragariae isolated from North America and Europe (Pooler et al., 1996). We characterized the population structure of the pathogen and developed an advance polymerase chain reaction (PCR)-based assay for the pathogen in plant tissue (Hartung and Pooler, 1997; Pooler et al., 1996). In the course of this work, we identified four genotypes of the pathogen, and have used these genotypes to screen our Proc. XXVI IHC – Berry Crop Breeding Eds. P. Hicklenton and J. Maas Acta Hort. 626, ISHS 2003 Publication supported by Can. Int. Dev. Agency (CIDA) 156 collection of Fragaria germplasm (Maas et al., 2000). We have identified two octoploid strawberry genotypes, US 4808 (a wild F. virginiana from Minnesota) and US 4809 (derived from a cross between F. virginiana SG26 and F. ×ananassa ‘Earliglow’) (Maas et al., 2002), that are very resistant to infection by X. fragariae after inoculation by leaf infiltration. We now report the intitial results of experiments to characterize the nature of the resistance to X. fragariae, which is heritable, observed in these octoploid Fragaria