The stability of productivity and fruit quality traits of 'Fuji' apples on different rootstocks

The economic success of an apple orchard depends on the selected rootstock. The standard rootstocks used in South Africa, including MM.109, M.793 and M.7 EMLA are excessively vigorous and lack precocity. The desirable characteristics of Geneva rootstocks have sparked a growing interest in South African orchards. The impact of the genotype-by-environment interaction (GEI) on apple rootstock selection needs to be considered in the selection of stable and adaptable rootstock genotypes. The stability and adaptability of rootstock genotypes were evaluated to select genotypes with high performance and stability.

Rootstock genotypes were planted in two separate but adjacent plantings, i.e., vigorous rootstocks and dwarfing rootstocks. These genotypes were evaluated in three different environments across South Africa – Oak Valley Estate and Breëvlei, both in Grabouw, Western Cape, and Helderwater, in Eastern Cape. The vigorous plantings consisted of MM.109, M.793, M.7 EMLA (control), G.778, G.228 and G.202. The dwarfing plantings comprised M.9 EMLA, M.9 Nic29, M.7 EMLA (control), G.222, G.778/G.222, G.778/M.9 EMLA, MM.109/G.222, MM.109/M.9 EMLA, M.793/G.222 and M.793/M.9 EMLA. The influence of rootstocks on tree vigour, productivity (yield, yield efficiency, cumulative yield, cumulative yield efficiency) and fruit quality traits (mass, diameter, ground colour, red foreground colour, firmness, total soluble solids (TSS) and starch breakdown) was determined. Rootstock productivity may be influenced by the GEI. The environment includes soil factors, climate factors and management factors. The influence of GEI on tree vigour and productivity was investigated. The presence of GEI complicates selection and requires that the stability and adaptability of genotypes be assessed to select the most stable and adaptable rootstock genotypes.

Vigorous: The evaluation of the GEI showed a significant GEI for cumulative yield and cumulative yield efficiency. The stability models selected G.778 as the most stable, high-yielding, and adaptable rootstock genotype for both cumulative yield and cumulative yield efficiency. Cumulative yield efficiency was found to be highly heritable and influenced greatly by genetic factors rather than environmental factors.  Dwarfing: The evaluation of the GEI obtained for cumulative yield and cumulative yield efficiency selected G.778/M.9 EMLA, G.778/G.222 and MM.109/M.9 EMLA as stable, adaptable, and high-yielding genotypes. The rootstock genotypes all displayed high responsiveness for cumulative yield and cumulative yield efficiency to environmental conditions across the sites, however, G.778/M.9 EMLA and G.778/G.222 were the least responsive. The industry standards were ranked lowly for stability and productivity, viz. cumulative yield efficiency and cumulative yield.

The most suitable rootstock genotypes for South African orchards, which displayed wide adaptability, stability, and superior productivity as well as fruit quality were G.778, G.778/M.9 EMLA and G.778/G.222. Genotype G.778 may possibly replace MM.109 as it was more productive, stable, and displayed good postharvest fruit quality traits. The industry standard M.793, may be replaced by G.228, which showed better productivity and fruit quality traits, however, G.228 displayed narrow adaptability for cumulative yield. Possessing both vigour control and high productivity, the adaptability of G.202 may be limited due to the high responsiveness this genotype displayed for cumulative yield efficiency. Subsequently, G.228 and G.202 may only be recommended for environments with favourable conditions, such as minimal trunk renewal. Additionally, for the dwarfing sites, MM.109/G.222 and M.9 Nic 29 showed good cumulative yields, cumulative yield efficiency and postharvest performance, however, these genotypes are recommended for environments with favourable conditions, to improve cumulative yield and cumulative yield efficiency.

The best rootstock genotypes for South African orchards were G.778, G.778/M.9 EMLA, and G.778/G.222, which demonstrated wide adaptability, stability, and improved yield and fruit quality. Genotype G.778 may replace MM.109. The G.228 and G.202 rootstocks are suitable only for environments with favourable conditions, such as minimal trunk renewal.