Avocados.
Hass variety
Inset: Avocado flowers.

Avocado, also called Alligator Pear

Persea americana

Origin:

Native to Central America, with carbon dating evidence showing they were used as a food there more than 9,000 years ago. There are 3 races, West Indian, Guatamalan and Mexican. These inter-cross readily so there are many natural hybrids in addition to those produced through breeding.

Climate:

It is a very adaptable sub-tropical tropical species that grows within 30 degrees of the Equator. Most varieties are sensitive to water stress and frosts, with very little tolerance of poor drainage. Generally, well-distributed rainfall of 1250-1750mm pa is optimal. Most growing areas have dry periods when irrigation is essential. West Indian cvs are the least cold sensitive and Mexican the most, with Guatamalan intermediate.

Plant Description:

Trees can be tall, 15-18m, or spreading with height controlled by pruning. It appears evergreen, but some lose their leaves at flowering with rapid replacement. Leaves are spirally arranged, dark green and elliptic-ovate, 20-25cm long and 10-13cm wide. Growth occurs as flushes on a 30-60 day cycle with roots alternating with shoots. The species evolved in regions with copious water supply and consequently it does not have an efficient root system for water uptake. Avocados have few or no root hairs.

Relatives:

Lauraceae Family. Relatives are bay trees, cinnamon and sassafras

Soils:

Provided there is good drainage, a wide range is tolerated such as sandy loams or volcanic and calcareous soils, with pH 5-7 acceptable. Avocados perform poorly where there is saline water or soils.

Propagation:

Grafting or budding is widely used, as cuttings and air layering have been problematic. Rootstocks for grafting have traditionally been seedling-grown, but efforts to reduce root rot problems have led to special cvs and inter-stocks being used in high risk areas.

Cultivars:

The Mexican and Guatamalan varieties are most suitable for south west WA. The harvesting period amongst these can cover several months of the year. Some that are readily sourced and given in the order of harvest time, include Shephard, Bacon, Fuerte, Sharwill, Wurtz (dwarf), Hass (the main cv worldwide) and Reed. Rootstock breeding for root rot resistance has so far been only partially successful.

Flowering and Pollination:

The small yellow-green flowers are borne on multi-branched axillary panicles, terminating in a vegetative shoot bud. Each tree can produce 1-2 million flowers but fruit set is only a small proportion of this. Flowers are perfect with 9 stamens and a single pistil with one ovule. Nectaries attract bees and other insects for pollination, but self- or wind-pollination also occurs.

Flowering exhibits protogynous, diurnally synchronous dichogamy, with flowers belonging to 2 types called A and B. The As function as females on one morning and then males the following afternoon, while the Bs operate as females in the afternoon and males the next morning. This behaviour encourages cross-pollination, but molecular studies have shown considerable selfing; this is enhanced by cool weather during flowering, meaning that in their appropriate environments most cvs are reasonably self-fertile. Nevertheless, the conventional practice is to endeavour to have a mix of As and Bs. There is usually considerable fruit drop and alternate bearing is common.

Cultivation:

Young plants should preferably be protected with shade screens for the first 1-2 years. Fertilizer use will depend on soil fertility. Provision of N is the main nutrient that has shown predictable effects on yield provided the others are sufficient. In alkaline soils Fe, Zn and other micronutrient deficiencies may occur and will need correcting, often with foliar spraying. Too much N predisposes to vegetative rather than reproductive growth.

Wind Tolerance

This is poor and needs to be addressed with windbreaks and pruning.

Pruning:

To control size and ease of harvesting and spraying, young plants should be trained to an open vase form to enhance light penetration throughout the canopy. Lateral branches are more fruitful than verticals. In late Autumn, pruning is best done at branch bud rings to encourage multiple shoots and flowering. The trunk and branches are very sensitive to sun burn, so if protective canopy cover is impaired, the tree should be shaded or the bark painted with a white water-based paint.

The Fruit:

It is a large single-seeded berry, 200g-1.5kg. The skin of some cvs is thin and in others several mm thick. Skin colour can be green through to red and various shades of dark purple. The yellow-green to bright yellow flesh has a buttery consistency. Similarly to the olive, it is unusual in storing most of its energy reserves as lipids (7-40% of fresh weight) rather than carbohydrates. Like the olive, the majority of these are unsaturated and very healthy. Avocados also have good levels of Ca and vitamin A. The flesh in most cvs will slowly brown (oxidise) once cut open.

Fruit Production and Harvesting:

Seedlings may take 5-15 years to start cropping and fruit quality is variable, while for grafted plants this period may be only 3-4 years. Yield will steadily increase over the next several years.

Fruit should be clipped from the tree and not simply pulled off. Although hard, they still bruise easily. Avocados can be stored on the tree and will ripen indoors in 1-2 weeks if not picked pre-maturely. However if this practice is overdone it delays replenishment of tree reserves for the following season and will trigger the alternate bearing habit. Once activated it is difficult to rectify, but the standard practice is to thin fruit in ‘on’ years.

With green-skinned cvs, it is not easy to assess when they are mature and ready for picking, but often the skin loses its shine and the stem develops a yellowish colour. Mature but not soft-ripe fruit can be stored in a refrigerator for up to a month and ripe fruit much less. The ideal storage temperature is 5-7°C as below this may cause chill injury.

Fruit Uses:

Most people eat them by spooning out the flesh and also as slices in a fruit salad or sandwiches, but they’re also used to make guacamole spread. Oxidation can be slowed by drizzling with lemon or lime juice. Avocados should not be cooked or heated excessively, but they can be used for uncooked dressings, drinks and desserts such as ice cream and cheesecake.

Pests and Diseases:

The major disease problem for avocado worldwide is root rot caused by Phytophthora; it can kill whole orchards of mature trees. No one method of control is sufficient, but purchased stock should be disease-free, care taken to avoid use of any infected grafting material, trees should be planted in well-drained soil, good levels of competitive soil microbes with high organic levels maintained and periodic use of phosphite sprays or trunk injection may be necessary. Stem end rot can degrade fruit that has not been cut from the tree. Scale and thrips may sometimes be found, but generally pests are not a major problem in the south west of WA.

Comments:

Mature trees grow well here with our porous soil, which minimises root rot problems that can be much more prevalent elsewhere. Avocados are luxury fruits that are expensive, but if you manage a tree appropriately you can enjoy many dozens of fruit per year when fully grown.