The evaluation and management optimisation of apple rootstocks

Scarcity of material because of poor propagation ability of the new Geneva rootstocks limited the initial phase of evaluation to the establishment of a single statistically laid out orchard trial site in 2000, situated on Bo-Radyn farm in Villiersdorp. Treatments comprised Golden Delicious apple trees (Reinders clone), planted on nine different newly imported Cornell Geneva rootstocks, compared to standard rootstocks and managed according to the same standard commercial practices.

The following Geneva rootstocks, listed in order of increasing tree size and decreasing yield efficiency, were found to be significantly more yield efficient than the industry standard, M793, when considering cumulative yield from 2004 to 2010 relative to tree size CG 222 (M9 vigour class), CG 189 (M7 vigour class), CG 007, CG 228 and CG707 (M793 vigour class) and CG 778 (MM109 vigour class). Other factors such as fruit quality, tree support and survival characteristics narrow down these choices. CG 222 is the most yield efficient, significantly more so than all the other rootstocks, except Cepiland, an M9 clone.

Although fruit size is related to current and previous season’s crop load, the rootstock CG 007 promoted larger fruit size together with good yield efficiency, even in a year of drought stress. Both the dwarfing stocks, M9 and CG 222, generally gave smaller, yet acceptable fruit size. More specific thinning procedures related to tree crop load could improve fruit size to a certain extent, however, it was determined that small fruit size is a drought-stress related phenomenon more apparent in the dwarfing rootstocks. More intensive irrigation and soil surface management practices will therefore be imperative for these dwarfing rootstocks in order to maintain fruit size. Rootstock effect on fruit maturity was not consistent. The trend of both bitterpit and russet incidence over the long term appears to be correlated to tree vigour and negatively correlated to yield and yield efficiency. Sunburn incidence was higher on M793, M7 and on the low vigour CG 222 and M9, especially in the early years.

Preliminary pot trials, with a limited range of available rootstocks, indicated that CG 222 was resistant to replant soil conditions as well as to woolly apple aphid infestation. Survival of CG 3007 was significantly poorer in the non-sterilised soil, while survival of CG 4210 was poor in both soils, indicating a poor transplantability. CG 5202 and CG 6210, showed a measure of tolerance to replant disease.