Resistance of apple rootstocks to fire blight caused by Erwinia amylovora

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 shoot 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.