Glen Fruin managing director Ross Heyns was initially underwhelmed by the trees he planted in June 2018. The smallest were only about 70 cm high and pencil-thin. Even the strongest trees stood just 1.3 m tall. But they went on to produce 8 tonnes/ha in their second and 47 tonnes/ha in their third leaf.
Growth took off towards the end of the first summer and the trees averaged 1.3 m leader growth in the first leaf – the best put on 2.2 m. Unfortunately, most of their lateral growth was lower down and most of the fruit in the second leaf was against the trunk. This further inhibited branching.
In hindsight, Heyns thinks these trees may have benefitted from trunk renewal in the second leaf. But most have reached a height of 100% of row width and are starting to fill their space.
Pot pros and cons
Heyns could plant in early winter because the trees were in bags. And the trees were in bags because that was how trees on M.9 rootstocks were sold at that stage. He is now planting M.9 trees grown in Ellepots as early as the first week of April.
Prof Karen Theron explained that flexible planting time is a big benefit of containerised trees. Planting in autumn allows the tree to develop its root system over winter for a head start in spring.
Another bonus is better tree health when using tissue-cultured plants grown in a sterile medium.
The biggest challenge with the early planting of containerised trees is keeping the roots moist while the trees are still in leaf. This is exacerbated by the contrasting textures of the fibrous growth medium and the soil. To counter this, trees at Glen Fruin are planted in watering wells that are filled and refilled to help soak the soil.
Care is also needed to ensure that fertiliser reaches the compact root system of containerised plants. Fertigation is best until the roots have grown out sufficiently to access granular fertilisers. Heyns applies dissolved fertiliser with a spray cart for the first three seasons.
Feeding and fruiting in M.9
Heyns has observed that trees on M.9 react rapidly to fertiliser. The M.9 trees usually flush a week earlier than trees on other rootstocks.
Technical manager Angelique Pretorius of Kromco stressed the importance of potassium in feeding precocious trees. Rootstocks such as M.9 allocate carbohydrates to fruit from as early as the second leaf – these trees need more than just nitrogen.
Excessive nitrogen and irrigation in high-density plantings can stimulate vigour at the cost of bearing-unit development. Too much nitrogen applied in the first two to three years can continue causing problems for another two to three years, while it remains in the tree. Meanwhile, fruit suffers from hammering and poor red colour.
Pretorius also noted that trees on M.9 appear to slow their photosynthesis if the crop load is too light – they close their leaves. Crop load is therefore a balancing act between giving the tree what it needs, while maintaining the right ratio to trunk diameter.
She likes to describe M.9 as precocious rather than as dwarfing. This rootstock does best when carrying a crop – but trees have to be managed to ensure sufficient short-bearing units.
More management measures
The trees were sprayed with 750 /ha of 4% Dormex in the third leaf and 1 100 /ha of 3% Dormex in the fourth leaf. Lower volume applications of rest-breaking agents tend to be less effective because full coverage is harder to achieve – rest-breaking agents have to be applied to the point of run-off to ensure direct contact with buds.
Precocious rootstocks such as M.9 and G.202 are susceptible to sunburn that could stunt tree growth. Protecting rootstocks and interstems with paint is recommended.
Production up to the fourth leaf
The trees yielded 47 tonnes with an average fruit mass of 147 g in the third leaf and 60 tonnes with a fruit mass of 150 g in the fourth leaf, of which 52 tonnes were delivered to the packhouse. Sunburn was the biggest culling factor in the past season.
Pack-outs were 82% first and 15% second grade in the third leaf and 84% first and 12% second grade in the fourth leaf. Russet was the biggest second-grade defect in the third leaf, but insignificant in the fourth leaf. No russet treatments were done – russet tends to decrease in older Bigbucks trees in Elgin.
Pretorius ascribes the uniform maturities in these trees to their open canopies. But she cautioned that the risk with Bigbucks is picking too early rather than too late. The intense red colour of the fruit can be misleading because it develops relatively early in the season when starch breakdown is still only 10 – 15%.
Heyns no longer plants on MM.109 and M.793 at Glen Fruin. New orchards on poorer soils are planted on G.778, whereas those on better soils are planted on M.9. Bigger fruit and earlier ripening are bonuses of M.9. He mentioned significantly improved size and colour with Royal Beaut on an M.9 x MM.109 interstem compared with the more vigorous traditional rootstocks.
Heyns has seen more vigorous growth on M.9 Nic 29 than on M.9 EMLA.
For now, he considers M.9 the safer dwarfing option – but Heyns is already looking at what the newer Geneva rootstocks could offer.
Read more about the performance of precocious rootstocks in Brazil, as reported at the recent Hortgro Technical Symposium in the September issue of Fresh Quarterly.
Acknowledgements
Thanks to owner Ross Heyns for hosting the orchard visit. Technical manager Angelique Pretorius of Kromco and tree physiologist Prof Karen Theron were on hand to share their insights. Piet Nieuwoudt is the horticulturist for this orchard.
