Submitted to: Acta Horticulture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/10/2004
Publication Date: 2/19/2006
Citation: Logiudice, N., Aldwinckle, H.S., Robinson, T.L., Fazio, G. 2006. The nature of resistance of the 'b.9' apple rootstock to fire blight. Acta Hort. 704:515-520. Interpretive Summary: The Budagovsky 9 (B.9) apple rootstock is extremely susceptible to fire blight caused by the bacterium Erwinia amylovora when inoculated as an ungrafted liner. However, B.9 rootstocks have exhibited a high level of resistance in the field as grafted finished trees. A series of experiments were conducted with grafted and ungrafted liners of B.9 in the field and in the greenhouse along with resistant and susceptible controls. Results tend to confirm that B.9 ungrafted liners are as susceptible to fire blight as the most susceptible control (M.9). They also confirm the field resistance as grafted trees where trees inoculated with E. amylovora were relatively healthy compared to the susceptible control (M.9). The bacterium was detected in the B.9 rootstock portion of the grafted trees indicating that resistance probably results from a suppressive effect on bacterial multiplication by the graft. Two sources of B.9 (Europe and North American) were tested in this study.
Technical 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.