Design and manage for fruit quality

Growers need to consider all the preharvest influences on fruit quality when establishing and maintaining apple orchards. By Anna Mouton.

Fruit quality is more important than ever in a time of skyrocketing living costs. This is why Prof. Stefano Musacchi of the Tree Fruit Research and Extension Center at Washington State University believes that growers should design orchards and manage canopies to optimise fruit quality.

Musacchi explained that consumers respond to both external and internal quality. “The first thing the consumer does is buy with their eyes,” he said. “There have been several studies that prove that consumers will pay more for larger and more colourful fruit.”

Research has also shown that consumers are willing to pay more for sweeter and crisper apples. People prefer fruit with higher levels of dry matter — and consumers can detect differences in dry-matter content of as little as 1%. Fruit containing more dry matter also has better sensory characteristics after storage.

The only way growers can increase dry-matter content in their apples is by manipulating the dry-matter allocation by the tree. Dry-matter production is driven by photosynthesis. The tree channels the products of photosynthesis to different structures such as fruit, leaves, wood, and roots.

“The partitioning of dry matter in the orchard changes with age,” explained Musacchi. “In the beginning, there is more dry matter used for building up the structure of the tree and later there is more dry matter going into the fruit.”

Genetic complexities

“There is a plethora of cultivars, and every year we have more,” said Musacchi. “It’s very difficult to determine how to manage the quality of all these varieties — and it becomes even more complicated when you combine scion and rootstock.”

Research on the specific weight of different apple cultivars showed that it depends on genetics rather than season or location. Apples with a high specific weight contain more dry matter than apples with a low specific weight.

“Many of the cultivars that have high specific weights are the ones that have a very good market in the United States,” commented Musacchi. Cultivars with high fruit specific weight include Scifresh, Scilate, Nicoter, and Braeburn. Cultivars with low fruit specific weight include Golden Delicious, Red Delicious, Cripps Pink, and Granny Smith. Gala and Fuji are intermediate.

Rootstocks have a huge impact on dry-matter partitioning — 75% of dry matter is partitioned to fruit in the case of a dwarfing rootstock compared to only 45% for a vigorous rootstock. Musacchi pointed out that 40% of dry matter is partitioned to wood — canopy development — on vigorous rootstocks.

Red colour development and fruit maturity are also affected by the rootstock. In addition, mineral uptake differs for different rootstocks. This can exacerbate losses due to disorders such as bitter pit in sensitive cultivars on rootstocks such as G.41.

Environmental effects

Most temperate deciduous fruit crops are produced between 35° and 50° latitudes in both hemispheres. Areas with lower relative humidity — for example, Washington State — have higher evapotranspiration than more humid areas such as northern Italy.

More sunlight in Washington State theoretically means a high potential for dry-matter accumulation but high temperatures reduce photosynthesis. The effects of direct sunlight can be mitigated by evaporative cooling and shade netting. Netting reduces sunburn but also red colour development.

Sunlight drives photosynthesis and therefore yields, so light interception is highly correlated with yield. But tree performance increasingly depends on the light distribution as light interception exceeds 50%. “This is why light interception and light distribution are key to all the orchard system work we are doing,” said Musacchi.

Musacchi shared four points for maximising fruit yields and quality through improving light interception and distribution.

1. Keep canopies open early in the growing season to maximise light capture by spur leaves as this significantly impacts yields.
2. Keep canopies open until at least 4–6 weeks after full bloom to prevent shade from reducing fruit growth.
3. Maintain light exposure to promote the development of good spur complexes and flower buds.
4. Assist colour development through late summer pruning but keep in mind that this will not undo the damage caused by excessively dense early-season canopies.

Agronomic actions

Crop load affects both external and internal fruit quality. Higher crop loads generally mean smaller fruit. Musacchi presented data showing that increased crop loads were also associated with poorer red colour development in Honeycrisp.

Honeycrisp apples harvested on the same day were more mature and had higher dry-matter content in trees with a lower than trees with a higher crop load.

“Another big effect of crop is bloom return,” said Musacchi. A higher crop load leads to less return bloom. “We want the same amount of fruit every year. This is why we are now working on a big project on precision crop-load management.”

Training systems are central to fruit quality as they determine light interception and distribution. “The planar canopy, in my opinion, is the future,” stated Musacchi. Planar or two-dimensional canopies optimise light efficiency and lend themselves to mechanical pruning and automatic harvesting.

Musacchi favours the bi-axis — known as a double-leader in South Africa. Multiple leaders reduce the vigour of individual leaders. Therefore, bi-axis trees have more but shorter shoots at all heights than spindles. Fruit quality across the canopy is more uniform in bi-axis trees.

Lastly, Musacchi stressed that correct pruning is essential for managing crop load and fruit quality. Click pruning helps to reduce biennial bearing in type 4 cultivars. Mechanical pruning is commonly used in Washington State to control vigour and improve red colour development.

“The leaf-area indices become really low when you do mechanical pruning,” said Musacchi. “We are starting a study to see how many leaves we need to produce good quality fruit.”

Finding ways to produce better-quality fruit is vital for apple industries everywhere. “We really need to be careful when we choose how to optimise quality,” concluded Musacchi, “because so many factors play a huge role in the final quality that we deliver to the consumer.”

Image: Prof. Stefano Musacchi, Tree Fruit Research and Extension Center, Washington State University. Supplied by Prof. Stefano Musacchi.