Submitted to: Acta Horticulture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/5/2004
Publication Date: 10/1/2004
Citation: Barritt, B.H., Konishi, B.S., Dilley, M.A., Pusey, P.L. 2004. Resistance of apple rootstocks to fire blight caused by erwinia amylovora. Acta Horticulture Proceedings #658. pp. 387-390. Interpretive Summary: The susceptibility of apple trees to fire blight (Erwinia amylovora) can result in significant canopy loss, yield reduction and tree death. Fire blight has become a major concern of apple growers because of the widespread acceptance of high density orchards of susceptible scion varieties such as Gala, Pink Lady and Fuji on susceptible dwarfing rootstocks such as M.9 and M.26. Extensive tree losses due to rootstock infection have occurred throughout the United States. Fire blight infection of rootstocks an occur 1) directly through infection of rootstock suckers, rootstock tissue cracks or mechanical injury or 2) indirectly through systemic movement of the bacteria from the infectd scion via the trunk to the rootstock. Resistance to fire blight has been one of the breeding objectives of the Cornell University and USDA-ARS apple rootstock breeding program at the New York Agricultural Experiment Station in Gevena, New York. Young seedlings are inoculated in the greenhouse with only healthy seedlings proceeding to evaluation of horticultural traits such as degree of dwarfing and ease of propagation. The objective of this study was to determine the fire blight susceptibility of newly introduced Cornell-Geneva (CG. or G.) and Vineland (V.) rootstocks and relatively new rootstocks Mark and B.9 in comparison with standard dwarfing rootstocks M.9 and M.26 and the semi-dwarfing M.7 rootstock.
Technical Abstract: Ungrafted rootstocks were screened for susceptibility to fire blight (Erwinia amylovora). Dormant rootstock liners from commercial layer (stool) beds were planted in midwinter in a greenhouse. New growth from each liner was reduced to one shoot. Vigorously growing shoots were inoculated with a composite of races of Erwinia amylovora. Four weeks after inoculation the extent of infection (susceptibility) was recorded as the percentage of shot length infected. Five trials were conducted (one per year) with rootstocks M.9, M.7 and G.30 in each trial. Replication in each trial varied from 6 to 43 rootstock liners per rootstock. Based on the five separate trials, extremely susceptible rootstocks were Ottawa 3, M.9 (several clones) and M.26. Susceptible rootstocks were P.2, B.9 and CG.13. Moderately resistant rootstocks were M.7 and G.11 and extremely resistant rootstocks (showing essentially no disease) were G.30 and G.65.