Effect of rootstock clones on fire blight susceptibility in scion apple cultivars

This study was performed on 24 clonal rootstocks and 4 scion apple cultivars to evaluate rootstock/cultivar combinations for susceptibility to fire blight (Erwinia amylovora). For this aim, the clonal rootstocks were planted in plastic greenhouse in 1992 and grafted with 4 cultivars (Golden Delicious, Dulcit, Domino and Dione) in 1998. A total of 96 rootstock/cultivar combinations were tested during 1998-2001. Each year, 10 to 30 actively growing shoots per one combination were inoculated with the pathogen. The shoots were inoculated by cutting through first unfolding leaves with scissors dipped into inoculum (109 cfu/ml). The trees were misted to create a high relative humidity. The susceptibility of the test roostock/cultivar combinations was evaluated by calculating the length of necrosis to the total shoot length 40 days after inoculation. The degree of susceptibility of cultivar tested varied depending on the rootstocks and years. In order of decreasing to increasing levels of fire blight susceptibility of four cultivars tested, the pattern observed was Dulcit, Golden Delicious, Domino (moderately susceptible) and Dione (very susceptible). The lowest susceptibility of four cultivars tested was observed on TE 52 rootstock (on average, the cultivars were evaluated as intermediate), the highest susceptibility was on JTE-E and M9 rootstocks (the cultivars were very susceptible). The influence of 21 remaining rootstocks on susceptibility in scion cultivars was not marked. Thus some rootstocks can render the scion cultivars more fire blight susceptible or more resistant.