Agronomic principles in apple production

Soil and Water Requirements

Apples trees can grow in a wide range of soils from medium textured clays to gravelly sands. However, poor soils will produce poor results and the best crops are found on fertile sandy soils and loams.

Soils should be well drained. Wet soils lead to poor aeration and increased incidence of crown rot in apples (Phytophthora cactorum). Generally, rooting tends to be shallow, and wet soils will restrict development, resulting in poor anchorage of the tree and a reduced area of soil from which nutrients can be extracted.

Soils with high organic matter contents are normally better structured and allow good rooting.

Irrigation is necessary on dry soils, particularly when establishing and growing young orchards. Trickle irrigation and fertigation are increasingly used. In young orchards fertigation helps increase early tree growth and brings trees into bearing earlier. Sprinkler irrigation can be used to protect the tree buds and fruitlets against frost damage.

Sowing of a grass mulch between the tree rows is common practice, which together with any clippings, helps to increase water holding capacity, infiltration rate, soil aggregation and recycling of nutrients.

Apples prefer a slightly acidic to neutral soil (pH between 5.8 and 7.0). Extreme soil pH values result in nutrient tie-up or toxicity and poor tree and fruit development. It is important to amend the pH in acidic soils by incorporating lime before planting.

Rootstock

Optimum planting density depends upon cultivar, rootstock and likely pruning strategy. Choice of rootstock governs tree size and the efficiency of nutrient uptake.

There are four general categories; vigorous, semi-vigorous, semi-dwarf and dwarf.

Choice of rootstock should suit the soil (pH, structure, humidity) and other local conditions (frost hardiness, pest and disease resistance, etc).

For apples, the dominant rootstocks used are the Malling types (M. and MM. series).

Orchard Systems

A wide range of planting systems are used for apples. All aim to produce high, early yielding, top quality crops. Ease of harvesting and management are also key factors taken into account.

Modern systems use higher densities than older established orchards. Today, normal densities range from 400-2,500 trees/ac, whereas 50 years ago, 28-41 tress/ac would have been more commonplace. In higher yielding, fertile soils and sites, growers can plant up to 4,000 trees/ac and expect 26t/ac yields.

This increased density has been made possible following the introduction of dwarf rootstocks which produce higher yields in the first 10 years of production than were previously possible.

Tree canopies are manipulated to produce four basic shapes of tree – spherical canopies, conical canopies, flat fan shapes, or Y, A or V shapes.

Spherical shapes were most commonly used in traditional European and North American orchards and largely allow the natural shape of the tree to develop.

Conical shapes are now more common. They allow good light penetration by limiting the width of the top of the tree. They require minimal branch and leader manipulation.

Flat fan systems are increasingly common in high density orchards to bring forward bearing, increase yield and make harvesting easier.

V-shaped, or angled canopies on the Tatura or other types of trellis, again allow good light penetration by most effective alignment of the tree. They produce very high yields at maturity and allow the producer to more effectively crop alleyways.

Tree Manipulation

Newly planted trees invariably need to be pruned. The exact pruning depends on the desired shape of the tree.

During full production, all trees need pruning and thinning of fruits to ensure an optimum leaf to flower/fruit ratio and to allow for air circulation through the tree and light penetration to improve fruit quality and size.

Pruning also helps to ensure that water and nutrients are available to an optimum number of well-positioned fruits.

Major pruning is normally carried out while the trees are still dormant in late winter. Pruning in summer is done to remove weak-bearing water sprouts and to allow light into thick canopies.

Pruning in late summer is not recommended as it can delay dormancy and predispose trees to more winter injury.

As a general rule it is better to prune little and often rather than to severely cut back the tree in one pruning session. Such severe pruning generates vigorous vegetative growth acting as a strong sink for nutrients and water, and thereby affecting fruit quality and bud differentiation.

It is desirable in many years to reduce the crop load that results from a heavy bloom and a good “fruit set”. Allowing too many fruits to remain on the tree will reduce fruit size and tree vigor and can cause the tree to bear biennially (every other year).

Five to six flowers bloom on each bud of an apple tree. If all of these were allowed to mature, fruit size would be very small and the tree would become under-nourished and not flower the next year. The crop is thus thinned, leaving about one fruit for each 3 to 5 buds.

Crop Protection

Top quality fruit production requires good disease, pest and weed control.

Many disease-causing pathogens (fungi, bacteria, viruses, and nematodes) attack apple trees.

Diseases may damage the fruit directly, making it unattractive or inedible, but they can also weaken the tree by injuring or invading the leaves, trunk and branches.

Damage to the tree reduces productivity and increases susceptibility to winter injury or attack by additional pests.

Over 50 types of insects attack apple trees, foliage, or fruit. The most serious are those insect pests that directly damage the fruit.

These include apple maggot (railroad worm), various types of caterpillars such as leafrollers, fruitworms, and codling moth, plum curculio, and rosy apple aphid.

Insects and mites, e.g. aphids, caterpillars, scale insects, spider mites, and leafminers feed on foliage or branches, weakening trees and restricting growth, and bloom and fruit set.

Grass or other vegetation, which competes for nutrients and moisture, should not be allowed to grow within at least 15-20 inches of the trunk of the tree.

Where there is a risk of hail, netting is used to protect the developing fruit.

Harvesting

Fruit should be harvested before it is fully ripe, but after it has had time to mature. This varies according to cultivar and different fruit varieties will often ripen at different times over a three-month period.

As the fruit mature, the starch changes to sugar and the aroma and flavor develops. Sugars are the major soluble solid in fruit juice and therefore soluble solids are often used as an estimate of sugar content (referred to as °Brix).

Immature fruit has a starchy taste, an undeveloped aroma and is very hard and crisp when cut. Mature fruits are firm but not hard.

Since there are variations in a wide number of quality parameters each year, considerable practical experience is needed when determining best harvest date.

Storage

Storage conditions are vital to long-term quality and shelf life. Fruit needs to be harvested with minimal bruising, cooled quickly and kept in controlled atmosphere conditions, so as to avoid any further physiological changes to the fruit.

High relative humidities (90-95%) should be maintained in order to minimize moisture loss from fruit. Controlled atmosphere conditions with lower oxygen and higher carbon dioxide levels decrease metabolism and fruit breakdown.

Practices that leave the picked fruit in high temperatures for long periods will result in a rapid deterioration in quality.

Apples that are earlier maturing (summer and autumn apples), produce higher levels of ethylene in storage and are more prone to breakdown than later maturing cultivars (winter apples). They thus have reduced storage potential.