RESISTANCE OF 'GENEVA' APPLE ROOTSTOCKS TO ERWINIA AMYLOVORA WHEN GROWN AS POTTED PLANTS AND ORCHARD TREES
The objective of the USDA / Cornell apple rootstock breeding project is to develop pomologically excellent rootstocks with resistance to biotic and abiotic stresses. Progeny of controlled crosses were selected for fire blight resistance following direct inoculation of seedling shoots with Erwinia. amylovora. Recently 3 rootstocks, Geneva (G.) 11, G.30, and G.16 were released for commercial sales and several other selections are in the final stages of evaluation. To compare their resistance with that of other apple rootstocks, liners of 48 different rootstocks were grown in the greenhouse, inoculated with differentially virulent strain E4001a and other highly aggressive E. amylovora strains, and the percent of the current season's shoot length blighted (SLB) calculated. The rootstocks most susceptible to direct inoculation were Budagovsky (Bud.) 9 (88% SLB), Ottawa 3 (87%), M.9 (86%), M.26 (80%), and Supporter 4 (69%), while M.7 (28%), G.30 (22%), G.16 (18%), G.11 (7%), and 13 other Geneva selections (CG.) (8-30%) were the most resistant. To evaluate the resistance of the rootstocks under orchard conditions, 3-yr-old Royal Gala trees on 18 Geneva rootstocks and 6 check rootstocks were spray-inoculated with strain E4001a twice during bloom in May 1999. All trees developed severe infections in the scion. Presence of ooze on rootstocks was first observed on 16 June 1999. Greatest incidence of tree death by 15 Oct 1999 occurred on M.26 (92% and 60% on two clones), and M.9 (83%). No trees on MM.111, Marubakaido, or Bud.9 died. Of the commercialized Geneva rootstocks, G.16 and G.30 had no tree death, and G.11 had 25%. Nine other advanced rootstock selections from Geneva had no tree death. In 2000, a natural blossom blight epidemic occurred in a duplicate planting of the Geneva rootstocks at another location. The resistance observed agreed closely with that previously observed in the inoculated block.
Norelli, J.L., Aldwinckle, H.S., Holleran, H.T., Robinson, T.L. and Johnson, W.C. (2002). RESISTANCE OF 'GENEVA' APPLE ROOTSTOCKS TO ERWINIA AMYLOVORA WHEN GROWN AS POTTED PLANTS AND ORCHARD TREES. Acta Hortic. 590, 359-362
Malus x domestica, fire blight, disease resistance, disease susceptibility