Avocado Tree Description
The avocado tree may be erect, usually to 30 ft (9 m) but sometimes to 60 ft (18 m) or more, with a trunk 12 to 24 in (30-60 cm) in diameter, (greater in very old trees) or it may be short and spreading with branches beginning close to the ground.
Almost evergreen, being shed briefly in dry seasons at blooming time, the leaves are alternate, dark-green and glossy on the upper surface, whitish on the underside; variable in shape (lanceolate, elliptic, oval, ovate or obovate), 3 to 16 in (7.5-40 cm) long. Those of the Mexican race are strongly anise-scented. Small, pale-green or yellow-green flowers are borne profusely in racemes near the branch tips. They lack petals but have 2 whorls of 3 perianth lobes, more or less pubescent, and 9 stamens with 2 basal orange nectar glands.
The fruit, pear-shaped, often more or less necked, oval, or nearly round, may be 3 to 13 in (7.5-33 cm) long and up to 6 in (15 cm) wide. The skin may be yellow-green, deep-green or very dark- green, reddish-purple, or so dark a purple as to appear almost black, and is sometimes speckled with tiny yellow dots, it may be smooth or pebbled, glossy or dull, thin or leathery and up to 1/4 in (6 mm) thick, pliable or granular and brittle.
In some fruits, immediately beneath the skin there is a thin layer of soft, bright-green flesh, but generally the flesh is entirely pale to rich-yellow, buttery and bland or nutlike in flavor. The single seed is oblate, round, conical or ovoid, 2 to 2 1\2 in (5-6.4 cm) long, hard and heavy, ivory in color but enclosed in two brown, thin, papery seedcoats often adhering to the flesh cavity, while the seed slips out readily. Some fruits are seedless because of lack of pollination or other factors.
Avocado Tree Origin and Distribution
The avocado may have originated in southern Mexico but was cultivated from the Rio Grande to central Peru long before the arrival of Europeans. Thereafter, it was carried not only to the West Indies (where it was first reported in Jamaica in 1696), but to nearly all parts of the tropical and subtropical world with suitable environmental conditions. It was taken to the Philippines near the end of the 16th Century; to the Dutch East Indies by 1750 and Mauritius in 1780; was first brought to Singapore between 1830 and 1840 but has never become common in Malaya.
It reached India in 1892 and is grown especially around Madras and Bangalore but has never become very popular because of the preference for sweet fruits. It was planted in Hawaii in 1825 and was common throughout the islands by 1910; it was introduced into Florida from Mexico by Dr. Henry Perrine in 1833 and into California, also from Mexico, in 1871. Vegetative propagation began in 1890 and stimulated the importation of budwood of various types, primarily to extend the season of fruiting. Some came from Hawaii in 1904 (S. P. I. Nos. 19377-19380).
Avocado Tree Pollination
Many isolated avocado trees fail to fruit from lack of pollination. Commercial growers are careful to match Class A cultivars whose flowers will receive pollen in the morning with Class B cultivars that release pollen in the morning and every grower must be sure to include compatible pollinators in his grove. Bulletin 29 (1971) of the Ministry of Agriculture in Guatemala tabulates the flowering periods (varying from August to April) of 48 introduced and locally selected cultivars, and the hours of the day when each is receptive to or shedding pollen.
Avocado Tree Climate
The West Indian race requires a tropical or near tropical (southern Florida) climate and high atmospheric humidity especially during flowering and fruitsetting. The Guatemalan race is somewhat hardier, having arisen in subtropical highlands of tropical America, and it is successful in coastal California. The Mexican race is the hardiest and the source of most of California avocados. It is not suited to southern Florida, Puerto Rico or other areas of similar climate. Temperatures as low as 25ºF (-4ºC) do it little harm. In areas of strong winds, wind-breaks are necessary. Wind reduces humidity, debydrates the flowers and interferes with pollination, and also causes many fruits to fall prematurely.
Avocado Tree Soil
The avocado tree is remarkably versatile as to soil adaptability, doing well on such diverse types as red clay, sand, volcanic loam, lateritic soils, or limestone. In Puerto Rico, it has been found healthier on nearly neutral or slightly alkaline soils than on moderately or highly acid soils. The desirable pH level is generally considered to be between 6 and 7, but, in southern Florida, avocados are grown on limestone soils ranging from 7.2 to 8.3. Mexican and Guatemalan cultivars have shown chlorosis on calcareous soils in Israel. The tree's primary requirement is good drainage. It cannot stand excessive soil moisture or even temporary water- logging. Sites with underlying hardpan must be avoided. The water table should be at least 3 ft (.9 m) below the surface. Salinity is prejudicial but certain cultivars (see 'Fuchs-20' and 'Maoz') have shown considerable salt-tolerance in Israel.
Avocados grafted onto 'Fuch-20' rootstocks and irrigated with water containing 380 to 400 ppm C1 performed well in a commercial orchard. In the Rio Grande Valley of Texas, cultivars of the Mexican race must be grafted onto salt tolerant West Indian rootstocks.
Avocado Tree Propagation
Normally, avocado seeds lose viability within a month. 'Lula' seeds can be stored up to 5 months if placed in non-perforated polyethylene bags and kept at 40ºF (4.4ºC), thus indicating that it may be possible to successfully store seeds of other cultivars ripening at different seasons for later simultaneous planting. Fresh seeds germinate in 4 to 6 weeks, and many people in metropolitan areas grow avocado trees as novelty house plants by piercing the seed partway through with toothpicks on both sides to hold it on the top of a tumbler with water just covering 1/2 in (1.25 cm) of the base.
When roots and leaves are well formed (in 2 to 6 weeks), the plant is set in potting soil. Of course, it must be given adequate light and ventilation. In nurseries, seeds that have been in contact with the soil are disinfected with hot water. Experiments with gibberellic acid and cutting of both ends of the seed with a view to achieving more uniform germination have not produced encouraging results. Seedlings will begin to bear in 4 or 5 years and the avocado tree will continue to bear for 50 years or more. Some bearing trees have been judged to be more than 100 years old.
In Australia, seeds planted in early fall germinate in 4 to 6 weeks; if planted later, they may remain dormant all winter and germinate in early spring. Seedlings should be kept in partial shade and not overwatered. While many important selections have originated from seeds, vegetative propagation is essential to early fruiting and the perpetuation of desirable cultivars. However, seedlings are grown for rootstocks.
For many years, shield budding was commonly practiced in Florida, but this method requires considerable skill and experience and is not successful with all cultivars. Therefore, it was largely replaced by whip, side-, or cleft-grafting, all of which make a stronger union than budding.
In the past, seedlings were grafted when 18 to 36 in (45-90 cm) high. It is now considered far better to graft when 6 to 9 in (15-23 cm) high, making the graft 1 to 3 in (2.5-7.5 cm) above ground level. West Indian rootstocks are desirable for overcoming chlorosis in avocados in Israel.
Avocado cuttings are generally difficult to root. Cuttings of West Indian cultivars will generally root only if they are taken from the tops or side shoots of young seed rings. But etiolated cuttings (new shoots) from gibberellin treated hardwood and semi hardwood cuttings of 'Pollock' as well as 'Lula' have been rooted with 50-60% success and, when treated with IBA, 66-83% success under mist in Trinidad. Cuttings of 'Fuchs-20' have rooted under mist with 40 to 50 or even 70% in Israel. Cuttings of 'Maoz' have rooted at the rate of 60% by a special technique developed in California.
An Israeli selection, 'G.A. 13' has given 70 to 90% success in rooting cuttings under mist for the purpose of utilizing them as rootstocks in saline and high lime situations. Air- layering is sometimes done to obtain uniform material uninfluenced by rootstock, for research on specific problems. Degree of success depends on the cultivar (those of the Mexican race rooting most quickly), and air-layering is best done in spring and early summer.
At times, mature avocado groves are top worked to change from an unsatisfactory cultivar, or one declining in popularity, to a more profitable one, or an assortment of cultivars for different markets. In 1957, 2,700 ";obsolete"; avocado trees in Ventura, California, were being grafted (top-worked) to mainly 'Hass', some to 'Bacon' and 'Rincon'. This procedure may involve thousands of trees in a given region. It is done in December and January in Florida.
Inasmuch as avocado roots are sensitive to transplanting, it is now considered advisable to raise planting material in plastic bags which can be slit and set in the field without disturbing the root system.
Avocado Tree Spacing
Spacing is determined by the habit of the cultivar and the character of the soil. In light soil, 25 x 25 ft (7.5x7.5 m) may be sufficient. In deep, rich soil, the tree makes its maximum growth and a spacing of 30 or 35 ft (9.1 or 10.7 m) may be necessary. If trees are planted so close that they will ultimately touch each other, the branches will die back. Some growers plant 10 to 15 ft (3-4.5 m) apart initially and remove every other tree at 7 to 8 years of age. If the surplus trees are not bulldozed but just cut down leaving a stump, application of herbicide may be needed to prevent regrowth. Ammonium sulfamate has been proven effective. In modernized plantings, space between rows is necessary for mechanical operations.
Holes at least 2 ft (0. 6 m) deep and wide are prepared well in advance with enriched soil formed into a mound.
After the young plant is put in place a mulch is beneficial, weeds should be controlled, and watering is necessary until the roots are well established. Generally small amounts of fertilizer are given every 2 months with the amount gradually increasing until fruiting begins. Bearing trees need, on the average, 3 to 4 lbs (1 1/2-2 kg) 3 times a year, beginning when the tree is making vegetative growth. No fertilizer should be given at blooming time; one must wait until the fruits are firmly set. Nitrogen has the greatest influence on tree growth, its resistance to cold temperatures, and on fruit size and yield. Fertilizer mixes vary greatly with the type of soil.
Mineral deficiencies determined by leaf analysis, are usually remedied by foliar spraying. Magnesium deficiency was formerly a serious handicap to avocado growers in Florida and Kenya. In California, zinc deficiency has been corrected by applying zinc chelates or zinc sulfate to the soil instead of spraying the foliage. Keeping the upper soil moist has been greatly facilitated by drip irrigation, which also may carry 80% of the fertilizer requirement.
Because some cultivars tend to grow too tall for practical purposes, commercial growers cut trees back to 16 or 18 ft (4.8-5.4 m), let them grow back to 30 ft (9.1 m) and top them again. But decapitation is not a perfect remedy because the tendency of the avocado tree is to grow a new top very quickly. Recently it has been found that the growth-inhibiting chemical, TIBA (triiodobenzoic acid) slows down terminal growth and encourages lateral shoots.
A system of pruning to encourage lower branching is being tried on 'Lula' in Martinique. Avocado branches frequently need propping to avoid breaking with the weight of the developing fruits. Some growers find it profitable to interplant bananas until the avocado trees reach bearing age.
Avocado Tree Maturity and Harvesting
Avocados will not ripen while they are still attached to the tree, apparently because of an inhibitor in the fruit stem. Homeowners usually consider the entire crop pickable when a few mature (full grown) fruits have fallen. This is not a dependable guide because the prolonged flowering of the avocado results in fruits in varying stages of development on the tree at the same time. The largest fruits, of course, should be picked first but the problem is to determine when the largest are full grown (perfectly mature for later perfect ripening). If picked when full grown and firm, avocados will ripen in 1 to 2 weeks at room temperature. If allowed to remain too long on the tree, the fruits may be blown down by wind and they will be bruised or broken by the fall.
Florida maturity standards for marketing have been determined by weight and time of year for each commercial cultivar so that immature fruits will not reach the market. Immature fruits do not ripen but become rubbery, shriveled and discolored. Most West Indian cultivars will ripen properly if picked when the specific gravity becomes 0. 96 or lower, but 'Waldin' is fully mature when the specific gravity is still above 0.98. Guatemalan and Guatemalan X West Indian cultivars generally are harvest-mature when the specific gravity is 0.98 or lower. In California, physiological maturity of 'Bacon', 'Fuerte,' 'Hess' and 'Zutano' has been determined by measurement of length, diameter and volume, but dry weight, correlating with oil content, is considered a better maturity index. California law has, since 1925, required a minimum of 8 % oil, but oil content varies greatly among cultivars and also the climatic region where the fruit is grown. Some people complain that the 8% standard is too low for some cultivars.
Maximum flavor of 'Fuerte' develops when the fruit is harvested at an oil content of 16%. Therefore, a minimum dry weight standard of 21 % has been recommended. Formerly, avocados were detached by means of a forked stick and allowed to fall, but this causes much damage and loss. Nowadays harvesters usually use clippers for lowhanging fruits and for those higher up a long handled picking pole with a sharp ";V"; on the metal rim to cut the stem and a strong cloth bag to catch the fruit. Gloves are worn to avoid fingernail scratches on the fruit. In California, studies have been made of the effects of hand clipping (leaving stem on), hand snapping (which removes the stem), tree- shaking, and limb shaking (which removes the stem from some of the fruits).
All methods are acceptable if the stem scar is waxed on stemless fruits to avoid weight loss before ripening at which time the stem detaches naturally. In Australia, some growers are using hydraulic lifts to facilitate hand- picking. A tractor fitted with a triple-decked picking platform has been adopted by some large growers in Chile. Efforts to develop dwarf avocado trees by means of sandwich interstocks from low growing types have been going on in California since 1964. Avocados must be handled with care and are packed and padded in single or double-layer boxes or cartons for shipment. A special ";Bruce box";, holding 32 lbs (14.5 kg) is used for large fruit. The fruits may be held in position in molded trays.
Avocado Tree Pests and Diseases
Avocados have no major insect enemies in Florida but migrating cedar waxwings feed on leaves, flowers and very young fruits and the fruits are commonly attacked by squirrels, rats and mice. The avocado red mite, Oligonychus yothersi; is the most common predator on the leaves in some groves and not in others. Red-banded thrips, Selenothrips rubrocinctus, the greenhouse thrips, Heliothrips haemorrhoidalis, and red-spider, Tetranychus mytilaspidis, may feed on avocado leaves and blemish the fruits from time to time.
There are several scales also which may feed on foliage, especially the Florida wax scale, Ceroplastes floridensis, the pyriform, or soft white, scale, Protopulvinaria pyriformis, Dictyospermum scale, Chrysomphalus dictyospermi; and the black scale, Saissetia oleae. Among two dozen other minor pests in Florida are the citrus mealybug, Pseudococcus citri and avocado mealybug, P. nipae. Stinkbugs may prick the fruits leaving little dents in the skin coupled with gritty areas at the same locations inside.
In California, 2 lepidopterous pests, Amorbia cuneana and the omnivorous looper, Sabulodes aegrotata, when present in large numbers, cause severe defoliation and fruit-scarring. Biological control is being achieved by release of the egg parasite, Trichogramma platneri; which is now commercially available to growers. Since 1949, the orange tortrix (a leaf roller), Argyrotaenia citrana, has been increasing as a menace to the avocado in California, the larvae feeding on twigs, terminal buds and foliage, flowers, and fruits. Since the pest requires shaded areas, it is best controlled by thinning out a close-planted grove or top-working to less susceptible cultivars.
The fruit-spotting bug, Amblypelta nitida, and banana spotting bug, A. Iutescens, are important pests requiring control in Queensland. The Mediterranean fruit fly is a major hazard in Israel, but very thick- skinned fruits such as 'Anaheim' are not attacked. The Queensland fruit fly, Dacus tryoni; seriously damages only Mexican cultivars or Guatemalan X Mexican hybrids in Australia. In 1971, a nematode survey in Bahia, Brazil, revealed 9 genera of known or suspected parasitic nematodes associated with avocado tree decline. Israeli avocado growers are seeking and testing means of biological control of the more serious of the 3 dozen insects and mites preying on the crop in that country. In Mexico, the avocado weevil, Heilipus lauri; tunnels into the seeds.
The major disease of avocados in South and Central America and some islands of the West Indies, in California, Hawaii, and various other areas, is root-rot caused by the fungus, Phytophthora cinnamomi, which is being combatted by the use of strict sanitary procedures and resistant rootstocks, especially 'Duke'. At the University of California, Riverside, over 750 seedlings and cuttings were being tested for root- rot resistance in 1976 and 1977 and the most promising tried out for grafting compatibility with commercial cultivars.
Also, soil fumigation experiments with methyl bromide and newly developed chemicals were being carried forward. The disease has been so devastating in the high rainfall areas of New South Wales and Queensland that plantings have expanded into the semi-arid Murray Valley in the hope of avoiding it. In New Zealand, it is not a problem on deep, volcanic soils, but occurs on shallow, heavier soils. It was allegedly introduced into Chile with balled trees from California and vigorous measures are being taken to control it.
Mushroom root-rot from Clitocybe tabescens may occasionally occur. Cercospora spot (brown spots on the leaves and fruits), caused by the fungus, Cercospora purpurea, may cause cracks in affected areas of the skin and thus allow entrance of the anthracnose fungus, Colletotrichum gloeosporioides, which invades and spoils the flesh. Glomerella cingulata is an important source of anthracnose in Queensland. Some cultivars are subject to scab which is readily controlled by copper sprays.
More than 30 other pathogens are variously responsible for wood rot, collar rot, dieback, leafspot, stem-and rot of fruit, branch canker, and powdery mildew. Sunblotch viroid cripples young trees and damages fruits in California and Israel. So far, it is unknown in New Zealand. Stems of young trees may be affected by sunburm, and hot, dry winds cause tipburm of leaves. The avocado tree may show copper or zinc deficiency or tipburm from an excess of mineral salts.
Article Source Purdue University Horticulture Department
http://www.hort.purdue.edu/newcrop/Indices/index_ab.html
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