Kromfontein

Bigger is better
Kromfontein experimented with different approaches when establishing the M.9 blocks. They compared bare-rooted nursery trees to trees in either 10-litre plastic bags or 2.5-litre Ellepots. With the Ellepots, they compared removing the wrapper with not removing it before planting. They also assessed planting in May versus October. “We saw small differences in growth initially,” said Erik Conradie, Kromfontein’s horticultural adviser. “But these differences disappeared during the second season.” That doesn’t mean Conradie thinks all these nursery-tree options are equal. The bare-rooted and plastic-bag trees were easier to establish due to their larger root systems. Kromfontein estate manager Arno Marais added that the dense growth medium in the Ellepots presented irrigation challenges. Trying to keep the Ellepot medium moist usually meant excessively wetting the surrounding soil due to the dissimilar water infiltration and retention characteristics of the growth medium and the soil. “The other important consideration with nursery trees is trunk diameter,” said Conradie. “You want length – 2.0–2.2 metres is fine – but I look at diameter. And I think a larger root volume lends itself to thicker trees.” Kromfontein planted strong nursery trees and recorded 1.20 metres of leader growth in the Bigbucks and 1.06 metres in the Lady in Red in the first leaf. The stem diameter increased by 1.78 cm in both cultivars. “The one big factor with M.9 is that you must plant the right tree,” said Conradie. “If you plant a weak M.9 tree, you’ll struggle to come into production. This is true of other rootstocks, but especially of dwarfing rootstocks.” Reynolds agrees. In his experience, most M.9 trees take a year or two longer than M.7 trees to fill their space. “We achieved 70–85 tonnes a hectare with the Kromfontein blocks because we planted at high density – we planted our area full – and we planted 1.8–2.0 metre trees that grew well during their first three to four years.”

Adaptive nutrition
The M.9 development is on a replant site that previously housed pears. Preparation of the medium to medium-high potential soil in-cluded fumigation. The clay soil has 10–15% clay and 1.2–1.9% carbon in the topsoil. Compost was included in the planting holes at 20 m3 per hectare and has been re-applied yearly until now. The orchard is also mulched with about 500 bales of straw per hectare annually. For their first three seasons, both the Bigbucks and the Lady in Red blocks were on a young-tree programme of 240 kg per hectare of nitrogen and zero potassium. Reynolds explained that the programme included a little more nitrogen than usual because of concerns about the growth performance of M.9. Nitrogen was reduced to 180 kg per hectare, and potassium was added at 36 kg per hectare in the fourth leaf for the Bigbucks. The Lady in Red only received 98 kg of nitrogen and 24 kg of potassium in the fourth leaf. During the Hortgro field day, participants noted that the Lady in Red trees were far more vigorous than the Bigbucks trees, sparking a lively discussion about the optimal nutritional strategy for the coming season. Reynolds recommends 238 kg of nitrogen and 36 kg of potassium per hectare, plus compost for the Bigbucks in the fifth leaf. The Lady in Red will only receive 23 kg of nitrogen per hectare. Given their yields, the potassium will be increased to 62 kg per hectare, but their compost will be taken away for now. “We’ve experienced that trees can become too vigorous with too much compost,” said Conradie. “But we do want to move away from chemicals and use more natural fertilisers.” In hindsight (an exact science), Reynolds thinks that the nitrogen and compost could have been reduced earlier in the Lady in Red orchard. “But I don’t see these trees as a problem,” he said. “To calm a vigorous tree is easier than, for example, trying to make the Bigbucks grow more vigorously.”

Vigour management
The contrasting performance of the adjacent Bigbucks and Lady in Red blocks provides an interesting window into the effect of M.9 on the behaviour of different scion cultivars. The Lady in Red trees are more vigorous than the Bigbucks, even though the nursery trees were similar. And the Lady in Red cropped 85 tonnes per hectare compared to the 70 tonnes of Bigbucks in fourth leaf. “In general, it seems as if Lady in Red on M.9 tends to grow more vigorously than Bigbucks,” said Conradie. But Marais noted that, across Kromfontein, Bigbucks is a less vigorous Gala clone regardless of rootstock. One option for future plantings of Bigbucks on M.9 under similar conditions would be spacing trees at 0.75–0.80 metres or narrowing the work row to 3.25–3.30 metres. Or, as Marais prefers, switching to a slightly stronger-growing Gala strain. The more vigorous Lady in Red is currently of greater concern to the Kromfontein team. In addition to reducing nitrogen and increasing crop load, they will explore options such as prohexadione-calcium application, girdling, and possibly root pruning and deficit irrigation. “Fruit are generally too large in Lady in Red, so deficit irrigation can help restrict growth and fruit size,” said Reynolds. Irrigation shouldn’t be reduced during the cell-division phase as this can increase postharvest disorders such as internal browning. Withholding irrigation for two to three weeks from about 40 days after full bloom during the cell-enlargement phase will stop vegetative growth, and reducing irrigation until harvest will decrease fruit size. Reynolds cautioned that deficit irrigation increases the risk of sunburn, so additional water should be applied during heatwaves. Nonetheless, the consensus is that a little too much vigour on M.9 isn’t the worst problem to have. “If you’re afraid that the trees aren’t going to grow, you push them,” said Marais. “We would rather have to put the brakes on in year three than not have a tree. So now we’ve learnt something, but we first had to build a tree.”

BOX:

Acknowledgements
Thanks to Arno Marais and his team for hosting the orchard visit and Louis Reynolds and Johan van Schalkwyk for manning the site.

The orchard at a glance
Cultivar: Bigbucks and Lady in Red
Rootstock: M.9
Size: 10 hectares
Spacing: 3.5 x 1.0 metres
Planted: 2020