Screening of apple rootstocks for resistance to woolly apple aphid

The objective of the project was to evaluate the potential rootstocks available to the SA apple industry for their comparative susceptibility, resistance or tolerance to woolly apple aphid infestation and to develop a screening protocol for use on new rootstocks as they become available in future.

Methodology entails the bulking up of plants via in vitro technique from mother plants, rooting of genotypes, establishment and hardening off under greenhouse conditions, transplanting into larger bags and transferring to shade-house to reach sufficient size for inoculation with WAA samples. Trees of the required minimum height of 40 cm were inoculated with root pieces of trees infested with WAA. Trees were examined and scored. for symptoms of WAA infestation.

Of the originally selected 23 genotypes, 21 were successfully micro-propagated and established within a reasonable (but extended) time-frame for this trial. Inoculation and subsequent re-inoculation resulted in symptoms which could be scored and some resistant genotypes could be identified. These were CG 5890, G228, G778, G202 CG6969 and CG11. All our current main commercial rootstocks (M793, MM109, M7, M9) were found to be highly susceptible.

The project took longer than expected because of difficulty in micropropagation of some of the rootstocks. Eventually some recalcitrant and contaminated rootstocks had to be  eliminated from the current trial. Trial trees had to be older and infestation period longer than originally proposed in order to obtain reliable results. The protocol developed was successful in identifying resistant and susceptible rootstocks, but it may take longer orchard trials to identify those rootstocks with a degree of tolerance to WAA.