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ISHS Acta Horticulturae 704: X International Workshop on Fire Blight

THE NATURE OF RESISTANCE OF THE ´B.9´ APPLE ROOTSTOCK TO FIRE BLIGHT

Authors:   N. LoGiudice, H.S. Aldwinckle, T.L. Robinson, G. Fazio
Keywords:   Malus x domestica, rootstock blight, ┐B.9┐
DOI:   10.17660/ActaHortic.2006.704.81
Abstract:
Rising production costs, associated with replacement of high-density plantings, along with lack of efficient control, especially where streptomycin resistant Erwinia amylovora strains have developed, make the identification of resistant apple rootstocks a high priority in the battle to control fire blight. B.9 rootstocks have exhibited a high level of fire blight resistance (comparable to the resistance of the Geneva« rootstocks), in several field trials. This finding contrasts with the susceptible phenotype that ungrafted B.9 rootstock liners have displayed in previous greenhouse tests. Irregularities in resistance may result from genotypic variation between nursery sources of B.9 rootstock in Europe and the United States. Using apple specific microsatellite markers, B.9 DNA from both sources was examined for variation and compared against parental cultivars. At this time, results have failed to identify any genetic variation between the two sources that would lead to a resistant phenotype. Field and greenhouse trials were conducted to determine if resistance was a product of rootstock growth conditions, or an effect of grafting. By focusing on progression of bacteria through asymptomatic tissue, we hoped to elucidate the mechanism by which grafting influences bacterial entry and colonization of the rootstock. Four rootstock clones, B.9 (US), B.9 (EUR), M.9, and G.16«, as ungrafted liners and grafted with four scion cultivars (Gala, Jonagold, Gingergold, and Red Yorking) were planted in the greenhouse and the field. Trees were inoculated, and bark sections were analyzed using PCR for presence of E. amylovora. Ungrafted B.9 liners from European and US sources displayed similar levels of susceptibility in greenhouse and field settings, indicating their origin does not play a role in resistance. In grafted trees, bacteria were isolated from above and below the union in all rootstocks tested. These results indicate resistance probably results from a suppressive effect on bacterial multiplication by the grafted B.9 rootstock.
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