DWARFING PLUM ROOTSTOCKS

1. Brompton is the most vigorous rootstock. Trees on St Julien A, Marianna 2624 and Tonneboer are roughly of the same size.
2. An extreme variation in tree size was obtained when P. s p i n o s a seedlings were used as rootstocks. Some trees were very weak and had no suckers, whereas other trees were very vigorous with many suckers. Root cuttings were taken from the weak trees. However, the plants obtained were very thorny and therefore discarded.
3. P. pumila is easy to propagate by cuttings. The bud-take of plum varieties is usually good, but the union of the maiden trees is weak. This may lead to breakage. Orchard performance of Czar, Opal and Reine Claude d'Althan on P. pumila was rather poor, but that of Victoria was better. Trees of Warwickshire Drooper on P. pumila were surprisingly good. Tree size of Victoria on P. pumila was ¼ of that of trees on Brompton.
4. A number of P. besseyi selections were studied. At the nursery stage results were comparable with those of P. pumila. Only trees of Victoria on selections no. 1 and no. 6 had better unions. Unfortunately no. 1 showed delayed incompatibility two years after planting. Trees on no. 6 are still healthy after six years. Tree size of Victoria on P. besseyi no. 6 is about 1/3–¼ of that on St Julien A.
5. Pixy is difficult to propagate by layering but easy by softwood cuttings. Trees are much weaker than on St Julien A.
6. Trees on P. pumila, P. besseyi and Pixy have smaller fruits than those on more vigorous rootstocks. Trees also seem to be more drought susceptible because of the smaller root system.
7. A very remarkable observation is the lack of silverleaf (Chonderostereum purpureum) symptoms on almost all the trees on P. pumila and P. besseyi. Silverleaf was very common on trees on other rootstocks in the same trials.

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