Determining the chill requirement of important apple rootstocks available to the South African fruit industry

We endeavoured to determine the dormancy status of various apple rootstocks by assessing their Progression of Dormancy and Chill Requirements

Thirty six Dormancy Progression Curves were quantified using 10 rootstocks grown in two locations over two years. In 2013 Chill Requirements of all the rootstocks available at both locations were calculated. In 2014 Chill Requirements of M793 and G222 were calculated 5 times during the season using shoots at various depths of dormancy.

Some samples fermented and died during forcing. Deaths were more prevalent in 2014 and certain rootstocks more susceptible.

Three distinct dormancy patterns were evident from the dormancy progression curves. GENEVA rootstocks entered dormancy rapidly, reaching maximum dormancy early in the season and exiting dormancy slowly. Malling and Merton rootstocks entered dormancy more slowly than the GEVEVA rootstocks. M7 and M9 reached maximum dormancy later than M793, M109 and M25 which seemed to enter and exit dormancy at similar rates. All rootstocks exited dormancy simultaneously. M7 and M9 exhibited shallow depth of dormancy. All the GENEVA and remaining MM rootstocks varied in depth of deepest dormancy. Some rootstocks went dormant before fruit harvest and apple scions, the impact of which is unknown. Chill Requirement calculations were higher in the colder location and varied according to the depth of dormancy of the shoots when collected. The Chill Requirement of G222 varied between 2013 and 2014.

Under conditions of inadequate chill, the method widely used to determine Chill Requirement as absolute values is neither accurate nor reliable. The dormancy of rootstocks must rather be assessed generally using both Dormancy Progression and Chill Requirement data over numerous locations and seasons before rootstocks can be ranked from low to high chill.