Evaluation of apple rootstock cultivars for woolly apple aphid resistance.

Woolly apple aphid (WAA) feeds on the roots of apple trees and causes the formation of woody galls. Root damage weakens the tree by reducing the capacity of roots to transport water and nutrients. This research is focused on a preliminary survey of the susceptibility of rootstocks to WAA. The results of this project will help in the selection of resistant rootstock genotypes to achieve suppression of WAA in orchards.

Various industry-relevant rootstocks were assessed in rootstock evaluation blocks and commercial orchards. Root samples were collected from five trees per rootstock at each location. In the laboratory, roots were washed, screened for WAA using the sugar floatation method, and all aphids counted. Roots were weighed, inspected for galling, and galls were classed using the method of Damavandian & Pringle (2002). Soil samples were sent to Labserve Laboratories for a 5-fraction soil texture analysis.

Overall, WAA numbers were low in most rootstocks evaluated. In the rootstock evaluation blocks, WAA was present in root samples of 7 rootstocks: CG778, G228, G757, M7 Emla, M9 Emla, M9 T337, and MM109. In the commercial orchard samples, WAA was present in M7 Emla and M9 (unspecified). Galling was found to be present on samples from 7 of the 13 rootstocks assessed. G228 and G757 were found to be free of WAA. MM109, G778, and M9 (unspec.) were found to be moderately affected by WAA. M7 Emla, M9 Emla, and M9 T337 were found to be susceptible. No obvious correlations between soil type and WAA infestations could be detected.

WAA numbers were low overall, and galling was absent from several rootstocks, indicating potential resistance. The data were combined into an easy-to-read table. Future work should focus on the impact of WAA galls on apple production and yield. With a larger sample size, a more detailed statistical analysis linking soil composition and WAA damage should be investigated.

Evidence from this study and previous work showed that several rootstocks are resistant to WAA, namely: G.202, G.210, G.222, G.228, G.757, and G.890. Integrated management of woolly apple aphid should incorporate resistant rootstocks and other methods, such as biological control and cultural control.