Issue One • June 2018
This checklist of dos and don’ts can increase the green colour of fruit at harvest. By Wiehann Steyn.
- Fruit need good light exposure in the first 40 days after fruit set to get satisfactory green colour at harvest. The chlorophyll (green pigment) level in fruit peel at this point determines the potential green colour at harvest.
- Fruit sitting in deep shade during their entire development tend to have low chlorophyll levels at harvest and will become whitish during storage and shelf life.
- In apples, more so than pears, fruit exposed to full sunlight for their entire time on the tree tend to have higher levels of yellow pigments, and chlorophyll levels can also be lower due to bleaching. These fruit may become yellowish during storage and shelf life. In addition, fully exposed Granny Smith fruit may develop sunburn or an undesirable red blush or bleached white spots close to harvest.
- The greenest fruit on the tree at harvest are those exposed to high sunlight during early development and then become partially shaded for the rest of their development. Recreating of these light conditions over the entire tree would maximise green colour at harvest.
- It follows that shade netting should improve the green colour of fruit.
- Shading by nets will decrease sunburn and red blush, and result in higher levels of chlorophyll.
- However, shade nets typically increase vegetative growth and excessive shading will increase the proportion of “white” fruit with low chlorophyll levels. This will also have various undesirable effects on cropping and fruit quality. Thus, rigorous light management is required under nets—some issues can be addressed by the complementary combination of nets and a more dwarfing rootstock.
- The light absorbance properties of nets need to be considered. A white net combined with a more dwarfing rootstock and a training system with high light exposure of fruit may not yield the desired positive outcome with regard to green fruit colour and reduced sunburn. At the opposite extreme, a black net combined with a semi-vigorous rootstock and poor light distribution in the canopy may have negative effects on cropping, pest control, and green colour in the inner canopy. Combining net, rootstock, and a training system is a decision taken after consulting a knowledgeable, independent (read: not a net manufacturer) technical adviser.
- Overhead netting is a consideration for new plantings and ideally these nets should be retractable (under the suggestions of the Orchard of the Future committee).
- Draped netting has potential for mature orchards, which are often planted to more vigorous rootstocks and where the cost of an overhead net may not be economically justifiable. A positive aspect of draped nets is that they can be used after the fruit set period— which should maximise fruit peel chlorophyll levels and also make fruit set management easier.
- While excessive nitrogen levels have various negative effects, suboptimal nitrogen levels result in less green fruit colour. This might be a particular issue in orchards where Granny Smith is planted as a cross pollinator to blushed cultivars and where nitrogen levels may be managed to the lower end of optimum.
- Deficiencies in various elements playing a role in photosynthesis or forming part of the chlorophyll molecule may result in lower chlorophyll levels. Apart from nitrogen, minerals most often associated with green colour are magnesium (part of the chlorophyll structure), iron and manganese (functions in photosynthesis).
- Water stress may decrease fruit green colour by increasing light stress in the fruit peel.
- Any factor that negatively impacts on root health (such as woolly apple aphid infestation, nematodes etc.) may affect nitrogen uptake but also synthesis of the hormone cytokinin that is positively linked with green colour.
- Peel chlorophyll levels and green fruit colour gradually decreases during fruit growth. Fruit will be greener when harvested at the beginning of the optimal picking window compared to at the end of the optimum picking window or post optimum.
- There seems to be a positive link between tree vigour and peel green colour. This may be due to a greater proportion of fruit borne in partially shaded positions or it may have a hormonal or physiological basis. The link between vigour and green colour requires further research.