Evaluation of new apple rootstocks representing three vigour classes (Morester and Oak Valley)

Continuous evaluation of newly released rootstocks will stay relevant for a well organised and prosperous apple industry. The aim of this project is to evaluate new apple rootstocks representing three vigour classes in three separate trials.

The trials were conducted using ‘Bigbucks’ as the scion cultivar and were designed using a randomised complete block design. Each treatment was arranged in 10 randomised blocks with 3 trees per plot, across two commercial orchards: ‘Môrester’ in Koue Bokkeveld and ‘Oak Valley’ in Grabouw. During the 2017 growing season, trees were planted on rootstocks representing different vigour classes. At ‘Oak Valley’, vigorous rootstocks (Group A) including G.228, G.778, M.793, and MM.109 were established. At ‘Môrester’, semi-vigorous rootstocks (Group B) such as M.7 EMLA, G.890, G.222, G.757, and Supporter 4, as well as dwarfing rootstocks (Group C) including G.202, M.9 EMLA, and Nic® 29, were planted. Groups B and C were spaced at 3.5 m x 1.25 m, whereas Group A trees were spaced at 4 m x 1.5 m. ‘Granny Smith’ apple trees were included as cross-pollinators within the orchards, and all trees were trained to single leaders to ensure uniform growth and management.

Group A at Oak Valley Estate:

G.778 and G.228 emerged as the top performers for both yield and yield efficiency, with G.778 particularly excelling in these aspects. Although early differences in tree height and scion diameter were observed, they did not lead to long-term disparities in tree height. Fruit quality evaluations showed that while some rootstocks had distinct attributes, such as G.228’s higher sugar-to-acid ratio and MM.109’s better red coverage, overall fruit quality differences were less pronounced. The results suggest that G.778 and G.228 are particularly promising for maximising productivity and maintaining high-quality fruit, whereas MM.109 and M.793 lagged behind in both yield and yield efficiency.

Group B at Môrester:

M.7 EMLA and G.890 emerged as the top performers for tree height and yield, with G.890 demonstrating the highest cumulative yield and M.7 EMLA exhibiting significant variability. G.757 showed superior yield efficiency but with high variability. Fruit quality varied by rootstock, with G.757 having the highest starch breakdown and G.222 the highest TSS and TA post-storage. Overall, while fruit quality differences were not significant across rootstocks for most parameters, G.890 and M.7 EMLA were particularly advantageous for maximising productivity and growth in production systems where more vigour is required, highlighting the critical role of rootstock choice in apple production.

Group C at Môrester:

G.202 initially produced lower yields and poorer yield efficiency compared to M.9 EMLA and M.9 Nic29 but performed similarly in later years. Fruit quality metrics, including firmness, diameter, mass, and various quality parameters, showed no substantial differences among the rootstocks, suggesting that rootstock selection had limited impact on fruit quality. Overall, M.9 EMLA generally achieved better yield efficiency compared to G.202, but the performance of all rootstocks was relatively consistent across the trial period. Additionally, in the 2022/23 season,

many trees (only M.9 EMLA and M.9 Nic29) died suddenly due to pathogen infection (Cytospora) that causes Valsa canker. It is not known why trees on G.202 seemed to be unaffected.

Rootstock selection is crucial for optimising tree growth and yield, with G.778, G.890, and M.9 EMLA emerging as strong performers in the various vigorous categories. The influence of rootstock on fruit quality was less pronounced. While rootstock can significantly affect growth and yield, other factors like initial planting conditions also play a vital role.

The rootstock trials revealed that G.778, G.890, and M.9 EMLA excel in growth and yield across different vigour classes. However, rootstock choice has a limited impact on fruit quality, which remains largely consistent across rootstocks.