Tissue-culture G.778 rootstocks excel on replant site. By Anna Mouton
The Cripps Red orchard was established with feathered 1.8 – 2.0 m tall trees in late September 2020. Farm manager Dirk Meyer related how nothing was pruned at planting – no topping or feather removal – but bending started in earnest in December 2020.
Winter 2021 saw the removal of overly thick or upright branches. No growth regulators have been applied but rest-breaking and chemical thinning were used in the second leaf.Read More
Horticulturist Nigel Cook thinks that the trees would have filled out better had more aggressive rest-breaking treatment been possible in spring – soggy conditions limited orchard access. He plans at least two rest-breaking sprays this year. The first spray targets the top half of the tree, and a second spray one to two weeks later covers the entire tree.
Cook said that he would also encourage bud break with additional measures such as growth regulators and notching if necessary.
Despite their skinny tops, the trees produced an impressive first harvest of 54 tonnes/ha with an average fruit size of 155 g in 2022. Even the Mahana Red cross-pollinators produced 37 tonnes/ha. Meyer shared that the orchard is already on a full-bearing fertilisation programme.
Cook is bullish – given no unpleasant surprises – about reaching a cumulative 100 tonnes in the third leaf.
Step on the fruit gas, not the growth gas
The trees have some blind wood and about 20% of the orchard canopy has sparse branching. Poor branching at the top of trees is typical for apples in the Elgin-Grabouw-Vyeboom-Villiersdorp area due to basal dominance resulting from insufficient winter chill.
Fruit set and vegetative growth in colder areas are determined by apical dominance and relative position – they are predictable. But in areas with insufficient winter chill, it becomes harder to know which buds are dominant and where fruit will set.
This is why pruning systems in Europe tend to have a natural progression from north to south. Renewal pruning is favoured in the colder northern regions whereas solaxe pruning is preferred in the warmer southern regions. Trees need more potential-bearing positions in warmer areas – hence the adoption of non-renewal pruning systems in the south.
Cook is opposed to trunk renewal in the relatively warm Elgin valley as a method for improving branching – he describes trunk renewal as stepping on the growth gas instead of the fruit gas.
The amount of biomass that a given combination of rootstock and scion will produce in a season is largely determined by site and climatic conditions in the previous season. Soil nutrients and plant reserves are already in place when growth kicks off. The question is how the plant will allocate the biomass that it produces.
Growers want their trees to allocate 70% of biomass to fruit. They thwart energy allocation to vegetative growth by pruning and by applying growth retardants. Trees respond by sending their excess energy underground to make roots. These roots power vigorous growth in late summer or the following spring. This is why Cook warns against pruning hard.
He also cautioned against growth regulators, and believes that spraying gibberellins to elongate the leader can increase chilling requirements and worsen the problem of blind wood.
Cook intends to train this orchard in a familiar solaxe style with an end goal of 20 branches per 3.2 m tree. Bud quality will be managed on the branches and pruning will be aimed at optimising light interception and distribution. Light interception drives fruit yields and light distribution determines fruit quality and also influences yield.
Start clean with tissue-cultured rootstocks
Soil preparation for the orchard included deep ploughing to remove old roots, followed by fumigation to reduce the risk of replant disease and white root rot. The orchard was established with healthy nursery trees grown from tissue-cultured rootstocks – Cook considers clean trees to be the biggest reason for the outstanding orchard performance.
Cook emphasised that nursery trees should be made with tissue-cultured rootstocks and grown in virgin soils or sterile media to ensure freedom from woolly apple aphids, nematodes, wood-rot fungi, viruses, and replant-disease pathogens.
He explained that these organisms build up in stool beds and nursery soils over time where they thrive on the continuous supply of apple-tree roots. White root rot is likewise spreading due to stool-bed rootstock production. This is why he advocates an industry-wide move toward tissue-cultured rootstocks.
Some growers are put off by experiences of small tissue-cultured trees. Cook pointed out that a healthy tissue-cultured tree would leave much larger trees in the dust if the latter harbour infections. He acknowledged that the size of tissue-cultured trees could improve but considers phytosanitary status as far more important than tree size.
The trees on G.778 have shone so far but Cook is concerned about their vigour. The trees already have too much vegetative growth despite having just cropped 54 tonnes/ha. This growth will have to be removed, which risks further invigorating the trees.
The newer Geneva rootstocks such as G.202 and G.213 are less vigorous and more productive and probably a better option for future plantings. Cook has also seen comparable results from trials with tissue-cultured M.7.
Not many growers can boast yields of 54 tonnes/ha in the second leaf regardless of rootstock. But Cook thinks that similar achievements are possible for anyone open to adopting new technology and able to execute effectively. The cost structure of a mediocre orchard is similar to that of an exceptional one – but the profits are worlds apart.
Thanks to managers Neil Reid and Dirk Meyer for hosting the visit and orchard horticulturist Dr Nigel Cook for leading the discussion.
Featured Image: Attendees listen to the discussion at Elgin Orchards
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