Apple agronomic principles

Rootstocks

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

There are four general categories; vigorous, semi-vigorous, semi-dwarf and dwarf. Most modern cultivars are grown on dwarf or semi-dwarf rootstocks. While yield per tree is lower, smaller trees yield more per hectare, reach maximum production at an earlier stage, and are more easily pruned, sprayed and managed.

Choice of rootstock should also suit the soil (pH, structure, humidity) and other local conditions (frost hardiness, pest and disease resistance, etc). Dwarf trees will bear fruit early, normally in the second year after planting, whereas semi-vigorous and vigorous trees won't bear fruit for 4-7 years.

Various rootstocks are used for pears. In Europe, quince rootstocks (A, Adams, C) have been traditionally used for dwarf or semi-dwarf trees. However, these rootstocks are susceptible to wind-throw, low temperature damage, fire blight and graft compatibility problems. The more recently introduced OH x F clones are winter hardy and tolerant to fire blight. Dependent on the clone used, tree size is dwarf to vigorous.

Orchard systems

A wide range of planting systems are used for pome fruit. 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 1000 - 6000 trees/ha, whereas 50 years ago, 70-100 trees/ha would have been more commonplace.

In higher yielding, fertile soils and sites, growers can plant up to 10,000 trees/ha and expect 60t/ha yields.

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. During full production, pruning and thinning of fruits in needed 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.

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.
Five to six flowers bloom on each bud of an apple tree and there are 7/8 flowers/bud on pears. 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 pome fruit 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.

Most common diseases are rust, scab, different rots and fire blight. Fire blight is the greatest limitation to European pear production. Over 50 types of insects attack pome fruit 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.

In some countries – e.g. China, fruit is bagged on the tree 45 days after petalfall so as to physically minimize pest and disease ingress and subsequent damage. More commonly, pesticides are applied regularly throughout the growing season, as need dictates.

Harvest

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

Each country has developed its own standard procedures to determine the right time to pick apples and pears. Various indices have been devised, but because there are considerable 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.

Apples that are maturing earlier (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.