ISHS Acta Horticulturae 663: XI Eucarpia Symposium on Fruit Breeding and Genetics
RESISTANCE OF APPLE ROOTSTOCKS TO FIRE BLIGHT INFECTION CAUSED BY INTERNAL MOVEMENT OF ERWINIA AMYLOVORA FROM SCION INFECTIONS |
| Authors: | H.S. Aldwinckle, N. LoGiudice, T.L. Robinson, H.T. Holleran, G. Fazio, W.C. Johnson, J.L. Norelli |
| Keywords: | Malus x domestica, rootstock blight, susceptibility |
| Abstract:
Infection of the susceptible rootstock of a grafted apple tree by Erwinia amylovora usually results in girdling, and tree death. Rootstocks can become infected by direct infection through wounds, by infection of suckers, and by internal movement of E. amylovora cells from scion infections to the rootstock. It was uncertain that inoculation of ungrafted liners of rootstocks could reliably predict their susceptibility as rootstocks of grafted bearing trees. It was determined that inoculation of the blossoms of 3-yr-old trees caused a high frequency of infection of susceptible rootstocks. Therefore this technique was used to determine resistance of 24 rootstocks under orchard conditions. All trees, grafted with ‘Royal Gala’, developed severe scion infections following inoculation of blossoms on 19 May 1999. Presence of characteristic bacterial ooze was first observed on rootstocks on 16 June 1999. Greatest incidence of tree death occurred on M.26 (60-92% on two clones), and M.9 (83%). No trees on MM.111, Marubakaido, or B.9 died. GenevaTM (G).16 and G.30 had no tree death, and G.11 had 25%. Very few trees of nine other advanced rootstock selections (including CG3041) from the Geneva rootstock breeding program died. In 2000, a severe natural epidemic of fire blight occurred in another duplicate planting of the same rootstocks. The resistance observed agreed closely with that previously seen in the inoculated block. A second set of rootstocks was evaluated in 2002 by blossom inoculation of 3-yr-old trees grafted to ‘Royal Gala’ and to ‘McIntosh’. Trees on M.9 had 83-100% rootstocks infected; M.26, 30%; M.7, 27-43%; MM.106, 40%; B.9, 0%; and P.14, 100%. G.30, G.65, and CG3041 had 0% rootstock infection. The results indicate that although in most cases the selection for fire blight resistance of seedlings in their first year in the greenhouse, and subsequent retesting as potted liners in the greenhouse, as carried out in the Geneva rootstock breeding program, provide a good indication of the resistance of rootstocks of orchard trees, in some instances there appeared to be a clear difference between resistance as a liner and as a grafted rootstock. | |
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