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The quest for greener Grannies

Consumers want uniformly dark green Granny Smith apples without blemishes. What light environment in the canopy will produce this result?

Granny Smith is among the top three apple cultivars planted in South Africa, accounting for 13% of the total area under apple cultivation in 2020. However, the number of hectares of Grannies has been shrinking, and was down to 3 378 in 2020, compared to 4 186 in 2015.

One reason for this decline is that producers are turning to more lucrative blushed cultivars. Another is the challenge of meeting market demands for perfectly green Grannies — export-quality fruit have to be dark green with no red blush or sunburn.

A study by Jacques Fouché and co-workers based at the Department of Horticultural Science at Stellenbosch University looked at the relationship between canopy light environment and peel characteristics in Granny Smith apples. Their results provide useful guidance for growers struggling to get more of their Grannies into Class 1.

Orientation and location

Trials were carried out for two seasons in commercial orchards in Grabouw. During the first year of the study, data were collected from an orchard in which the tree rows were orientated north-south. During the second year, data were collected from an orchard which had an east-west row orientation. Positions in the outer, intermediate, and inner canopy were compared in both years.

The researchers quantified light exposure and fruit surface temperature in different positions in the canopy, and related these measurements to green colour intensity, chlorophyll concentration, lightness values, and the occurrence of red blush and sunburn in fruit sampled from those positions. Both chlorophyll concentration and lightness values were determined on the darkest as well as the lightest green side of each fruit.

Light exposure and fruit surface temperatures decreased from the outer to the inner canopy — no surprise. The occurrence of sunburn and red blush followed the same pattern. However, the best values for green colour intensity were obtained in the intermediate canopy. The worst values were in the outer canopy.

A trial was also conducted in a commercial orchard in Somerset West. Fruit from the outer canopy were enclosed in 40% shade cloth from 14–56 days after full bloom, and compared to unshaded outer-canopy fruit at 56 and 160 days after full bloom. The researchers measured the chlorophyll concentration and lightness values of the fruit, and recorded the occurrence of red blush and sunburn.

In this trial, shading during early development resulted in significantly lighter fruit both at 56 days after full bloom and at harvest.

The bottom line

Obtaining optimal green colour in Granny Smith is a balancing act. Fruit require an open canopy and good light exposure during the first half of their development so that they can make as much chlorophyll as possible. Shading during the second half of fruit development helps reduce the breakdown of chlorophyll while also protecting the fruit against sunburn and red blush. This is why draped and fixed netting may increase the green colour of Granny Smith apples. Although not assessed in this research, other green cultivars such as Golden Delicious may respond similarly to Granny Smith.

Reference

Fouché JR, Roberts SC, Midgley SJE and Steyn WJ. 2010. Peel color and blemishes in ‘Granny Smith’ apples in relation to canopy light environment. HortScience 45(6):899–905.

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