Establishing an Organic Avocado Orchard

Suitable Races and Cultivars

There are three distinct horticultural races of avocado:

West Indian (var. americana), Guatemalan (var. drymi- folia) and Mexican (var. guatemalensis), plus hybrids between them. Hybrids of the Guatemalan race with the other two races include many of the more important varieties in commerce. Pure West Indian types are generally unsuitable for export market but preferred by local consumers.

All three horticultural races adapted to tropical and sub-tropical conditions. The West Indian race is well-adapted in the humid tropics, but its hybrids with Guatemalan (e.g.

Booth selection) perform well and are considered valuable for extending the harvest season.

In subtropical regions, hybrids of Guatemalan and Mexican races predominate since they combine the cold hardiness of the latter with the superior horticultural traits of both while bringing the two seasons of maturity.

Breeding programs include the universally desired objectives (yield, fruit quality, vigour, adaptability etc.) and some specific objectives for avocado. In avocado rootstock breeding, the need of Phytophthora-resistance is recognized as the most important objectives. Duke 7 shows moderate resistance. In many tropical countries, locally collected seeds are used to raise seedlings as rootstocks.

There is a large number of avocado cultivars. Hass and Fuerte are the most widely grown cultivars. Other important cultivars are Bacon, Zutano, Booth 7, Booth 8, Sharwil, Ettinger etc...

Table 10:

Comparison of Selected Characteristic of three Avocado Races

Trait West Indian Guatemalan Mexican

Climate Tropical Subtropical Semi-tropical

Cold tolerance Least Intermediate Most

Salt tolerance Most Intermediate Least

Disease resistance Higher Susceptible to Susceptible to diseases Verticillium wilt due to very thin skin

Fruit bloom 5-9 months 9-12 months 6-8 months

to maturity or more

Fruit size Variable Variable Small

Fruit color Green or reddish Green Green to purple or black

Skin thickness Medium Thick Very thin

Skin surface Shiny, leathery Rough and warty Waxy bloom

Seed cavity Variable Tight Loose

Oil content Low (3-10 %) High (8-15 %) Highest (up to 30 %) Pulp flavor Sweet, mild, watery Rich Anise-like, rich SOURCE: NAKASONE ET AL. 1998

Flowering Types and Flowering Induction The flowering habit of avocados is unique in that the flowers are perfect, having both male and female organs, but the parts do not function together. Flowers of type A varieties (e.g. Hass, Guatemala, Simmonds, Nadir, Lula) open in the morning as receptive females, then close in the afternoon until the following afternoon when they reopen for pollen shed. In contrast, flowers of type B avocados (e.g. Fuerte, Bacon, Zutano, Hardee) open in the afternoon as receptive females, close overnight and reopen the follow- ing morning to shed pollen. If an avocado plan- tation is to be established in a relatively new area, the varieties to be selected for planting should belong to both A and B groups and their flowering must overlap to assure good pollina- tion. The proportion of A and B group varieties can be 1:1 or 2:1. Under some conditions, there is suff icient overlap between the phases of a flower type that pollination and fruit set are rarely a problem. Flowering occurs from late autumn to spring, depending upon the cultivar and climate. Dry conditions during flowering and fruit set can cause flower and young-fruit drop.

Propagation and Planting

Planting materials may come in the form of grafted plants or seedlings for rootstock use. The commonly used and preferred method for large-scale propagation is grafting.

This method is less demanding, faster and economical in the use of scion materials. In the case of cleft-grafting, 6- 12 month old seedlings are used as rootstocks. Budwood sticks are obtained from the season’s mature growth with well-developed terminal buds. New shoots are formed within three to four weeks. While in conventional agricul- ture many avocado trees are produced in containers of soil- less media, organic growers use compost/earth-mixtures as media.

The use of grafted plants is limited, especially in countries that do not have specialized avocado nurseries.

There, propagation of avocado is mainly carried out by seed. The seeds taken from mature fruits are sown directly in the nursery or in polyethylene bags. When 6-8 months old, the seedlings are ready for transplanting. In many cases, the resulting seedlings are used as rootstocks. Such seedling trees produce 300 to 400 fruits at 10-15 years.

When providing trees, it is of crucial importance to order certified trees where the absence of diseases, pests, viruses and authenticy of rootstock and cultivar are guar- anteed. In different organic label regulations the use of trees from certified organic nurseries is compulsory from 2004 on. At present time, the detailed regulations e.g. in cases where organic trees are not available are very dynamic. Therefore, the actual regulations should be checked carefully with the certification body before ordering any plantlets.

Establishment of Avocado Orchards

For orchard planting in flat to gently rolling terrain, the land is cleared, ploughed deeply to break the hard subsurface soil layer and harrowed two or three times to achieve the desired soil tilt and to level the field. A few months before planting avocado trees, some organic growers saw in vigorous legumes (e.g. Canavalia sp. or Cayanur cayan) and later mulch them shortly before planting the avocado trees. By this measure, the soil will be enriched with organic matter and nitrogen that in turn stimulate soil microbial activity.

For rolling land and steep slopes, plowing and harrowing are not practiced in organic agriculture. Instead, hand forking and hoeing are carried out so as to minimize erosion. Stakes are then set, following the desired planting distance.

Depending upon the cultivar, the plants are set at 8-10 m apart to give a population of 100-156 trees per hectare (see below).

Holes are dug that are deep and wide enough to accom- modate the root system (60 x 60 x 60 cm to 90 x 90 x 60 cm)

in early spring. The holes are filled with compost or farm- yard manure and top soil (1:1 ratio) before planting. As most of the wet zone soils are acidic, pH adjustment (to 5.5 - 6.5) is achieved by liming, according to the organic standards. The places are then occupied by the stakes.

Planting is done in summer.

The plants should be subjected to hardening under full sunlight before transferring to the field. Before planting, the leaves of the planting materials are pruned in half to reduce transpiration. After removal from the container or polyethylene bag, the plants are set in the prepared holes, with soil mixture filled and firmed to ensure good contact with roots. The holes are then filled up with top soil that is packed firmly around the stem. The plants are then watered immediately after planting. After establishment, the young plants are shaded by bamboo sheet, banana leaves, etc. Tree basins are mulched by dried weed and straw during the next dry season. The plants are protected from wind by planting windbreak trees as well as by using stakes for supporting newly planted trees.

Where avocados are vegetatively propagated by means of budding or grafting, it is common practice to plant the tree deeper than normal so that the graft is at or below ground level. In addition, soil is mounded around the trunk as the tree grows to assure that the graft union is below ground. Thus, trees killed to the ground by severe cold will regenerate from varietal wood rather than from rootstock.

In areas prone to excess water, they should be planted on mounds as avocados cannot withstand waterlogging.

Planting usually takes place at the beginning of the rainy season to minimize the need for frequent watering of the newly set plants in the field. However, in areas where there is a uniform distribution of rainfall or where irrigation water is readily available, planting takes place at any time of the year.

Design of New Organic Avocado Orchards

Avocado is planted out to a distance of 6 to 12 meters depending on the vigour of variety and its growth habit.

For varieties having a spreading type of growth, like Fuerte, a wider spacing should be given. Most organic growers generally prefer a wider spacing compared to their conventional colleagues in order to guarantee good ventilation and to prevent fungal diseases. Pollinizer trees are placed so that adequate pollinization is guar- anteed (e.g. one pollinator for every nine trees). To limit the erosion on sloping land and to collect water from rain- fall, organic growers install water traps surrounding the tree basins. On hills slopes planting is preferred on half- moon terraces in order to prevent erosion. The shallow, triangular path connecting to the tree pit is dug with its open mouth on the reverse slope direction to collect rainwater.

Avocado in Agroforestry Systems

Some organic growers plant avocado as one of the mixed crops in agroforestry systems. In such systems, avocado is a good shade tree for tea and coffee production. In other cases, organic avocado is inter-cropped with other perennial crops such as coconut, banana, jackfruit, rambutan, mangosteen and other fruit trees. Avocado are also planted along fences.

Such production systems are of great importance in organic agriculture because they can be effective in limiting pests and diseases, thereby enhancing production efficiency.

Another advantage of inter-mixing: avocado trees benefit indirectly from the cultural practices applied to the main crop. Therefore, in many cases no special attention is necessary after planting or throughout the lifetime of avocado trees. Generally, fertilizer application, weeding and irrigation regimes for avocado trees are practices to be varied from orchard to orchard, depending on what main crop is grown in mixed orchards.

Conversion of orchards to agroforestry systems

In order to convert monocrop orchards to organic agri- culture, some organic growers reduced the original high density of avocado trees (156 trees/ha) by thinning to allow for the development of other crops like coffee, durian, mulberry, rambutan, etc. Such avocado orchards have a density of 50-80 trees/ha. Organic growers prefer taller and bigger trees as they are more convenient in mixed gardens.