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Title:   Author  item
                                            Hartung, Johnitem
                                            Paul, Cristinaitem
                                            Lewers, Kimberlyitem
                                            HOKENSON, STAN - UNIV MINNESOTAitem
                                            Maas, John
  Submitted to: International Society for Horticultural Science Meeting
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 7/1/2001
                                    Publication  Date: 8/2/2001
                                    Citation: Hartung, J.S., Gouin, C.C., Lewers, K.S., Hokenson, S., Maas, J.L. Identification of sources of resistence to bacterial angular leafspot disease of strawberry. International Society for Horticultural Science Meeting.Interpretive Summary: Bacterial angular leafspot disease of strawberries has been increasing in importance to strawberry producers in recent years, because it is spread by infected but asymptomatic strawberry plantlets used in annual row culture systems.  There are no control methods for the disease and no resistant varieties.  We have identified four genetically distinct strains of the pathogen, Xanthomonas fragariae, and have used these strains to screen a collection of 81 strawberry accessions for resistance to the pathogen.  We have identified two strawberry genotypes that are very resistant to this disease.  These genotypes, US 4808 and US 4809 have been released to the public.  We have used these genotypes as parents in controlled crosses and shown that these genotypes pass on the resistant phenotype to between 4-18% of their progeny.  Data from the crosses is being analyzed to establish the genetic basis of the resistance.  The physiological basis of the resistance is interesting.  Hypersensitive resistance, commonly observed in plants against bacterial pathogens, was not observed.  Instead, in the resistant plants the bacteria fail to multiply and cause necrosis, although they survive for weeks at the site of inoculation. Our work will be of interest to both strawberry breeders and producers as well as to plant pathologists interested in plant disease mechanisms.  Use of this germplasm may contribute to more sustainable and profitable strawberry culture.
                                Technical Abstract: 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 characterized the population structure of Xanthomonas fragariae, and have used this knowledge to screen  81 Fragariae genotypes, including both diploid and octoploid accessions, for resistance to this pathogen.  Two genotypes, a native F. virginiana from Minnesota and a cultivated F. virginiana X F. ananassa hybrid from Georgia 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 also made between US 4808 and US4809 and the susceptible variety 'Sweet Charlie'.  Resistance to X. fragariae was transmitted to 8-12% of the progeny of the US4808 cross and to 4-18% of the progeny of the US4809 cross.  Data from these experiments is being analyzed to establish the mode of inheritance.  Hypersensitive resistance, characterized by rapid collapse of the tissue at the inoculation site followed by death of the bacterium (single gene dominant) was not observed.  Further experiments are planned to better understand the nature of the observed disease resistance.


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