Seedling Rootstock and Fireblight Resistance

     Seedling apple trees will have variable susceptibilities to the Fireblight bacterium erwinia amylovora simply due to genetic differences in each plant. There are, however, several  factors inherent in the general behavior of most seedling trees. Pear trees may be considered similar in many of these respects, but it should be remembered that most pear rootstocks, seedlings included are considered highly susceptible to blight.

      The factors of resistance in seedling stocks will be dealt with here as a comparison of their behavior as compared with dwarfing stocks. Vigorous clonal stocks will behave similar to seedling in these respects.

–          Seedling rootstock will produce a tree which will delay flowering until such time as a large and strong trunk and branching system has been established. Since fireblight often gains entry via the flowering system, it is advantageous to have a hardier structure in place. Older and more developed limbs and trunk are often less prone to attack or spread.

–          A seedling will produce a very large and crowded branch area, thus it is less likely to produce blooms in the interior nearer the trunk. This puts the susceptible flowers out toward the periphery. Since the protocol in fireblight management is removal of infected wood, it is more easily and completely dealt with. This is the difference between pruning out limb portions versus performing trunk surgery.

–          Most seedling rootstocks are less likely to sucker compared with many dwarfing stocks. Fireblight often attacks the vigorous tender growth of suckers.

–          The layout in the standard tree orchard places large distances between trees. Dwarf tree high density plantings are quite the opposite, often with trees branches comingling. In the event of outbreak of blight, a seedling orchard is far less likely to receive the devastation possible in the high density plantation. 

–          The large canopy of the  seedling tree will buffer the damage due to hail, keeping wounds to tissue at the extremities.

–          It is important to keep in mind that management techniques can affect the susceptibility of trees to blight. The above considerations are based on a system that allows standard trees to grow in a more natural way. Excess pruning, for instance will often result in excessive and vigorous new growth, which is susceptible to blight attack. Succulent growth attracts insects which open wounds for infestation by the bacteria. This is especially true with regard to pear. Growing season pruning will also open tissue to attack.

Some of these points are mentioned in a paper by Cummins and Norton 1974 in the publication Plant Sciences.