Avocado

Persea americana

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Description

Avocado, Persea americana, is an evergreen tree in the family Lauraceae which grown for its nutritious fruit, the avocado. The avocado tree is large and dome shaped with oval or elliptical leaves arranged in a spiral on the tips of branches. The leaves have a red pigmentation when they first emerge and turn green as they mature. Avocado trees produce clusters of small, green-yellow flowers at the end of twigs and a large, fleshy, pear-shaped fruit with a single large seed. The fruits can be purple to green in color with smooth or warty skin depending on variety. The flesh of the fruit is yellow-green in color and has the consistency of butter. Each fruit contains one large seed. Avocado trees grown from seed can take 4–6 years to produce fruit whereas grafted plants may produce fruit within 1–2 years. The tree can reach a height of 20 m (65.6 ft) and originated in the rainforests of Central America.

Uses

The avocado is usually consumed fresh as a fruit or as an ingredient in salads or savory dishes. It has a markedly higher fat content than other fruits and is a staple in diets that have limited access to foods with high contents of monounsaturated fats. It is the main ingredient in guacamole, a popular Mexican dip. In Asia, avocados are used in desserts and dessert drinks.

Propagation


Basic requirements
Avocados thrive in subtropical or tropical climates but can also be grown successfully in cooler areas of the world. The optimum temperature for growing avocado is between 25 and 33°C (77–91.4°F) with moderate humidity levels. Once established, trees can tolerate temperatures down to around -2°C (28°F) with minimal damage but young trees will not tolerate freezing temperatures. Avocado requires a well draining, aerated soil and they produce a shallow root system which require a warm soil for efficient water and nutrient uptake. Although trees will tolerate low rainfall, irrigation, particularly during flowering and fruit set, will ensure high fruit yields.

Propagation
Avocados are commonly propagated from seeds but the seeds will not breed true to type and this should be taken into consideration before planting. Clonal plants are obtained from budding and grafting from a parent tree to ensure the offspring are of the same high quality of the parent. In plantations, seeds can be sown directly in the soil. 2 to 3 seeds are usually sown and thinned later to leave the strongest seedling for grafting. Seeds may also be sown in containers and grown for 2 to 3 months before planting at the final site.

Planting
Avocado seedling should ideally be planted in the Spring when the soil has warmed through. Choose a location that receives full sun and has protection from the wind. The trees should be planted by digging a hole a little wider than the root ball and gently easing the tree into the hole. Slow release fertilizer can be added to the hole at planting but is not necessary. Care should be taken not to disturb the roots as much as possible and the hole should be carefully backfilled and the soil and tamped to prevent dislodging. Trees should be planted 4.5 to 6 m (15–20 ft) apart in rows spaced 6 m (20 ft) apart.

General care and maintenance
Newly planted young trees should be mulched after planting with several inches of straw or woodchips. Young trees will also benefit from staking which will help to prevent wind damage. Wood stakes should be driven into the ground outside the root ball allowing 2 stakes per tree. The tree should then be tied loosely to the stakes to provide support while it establishes. Young trees should be irrigated and the root ball should not be allowed to dry out. Trees should be watered every few days. Water trees at and around the base to ensure the root ball is wetted. Trees are usually fertilized at around 4 weeks after planting. Add half a cup of urea every 4 to 6 weeks during the growing season.

Diseases

Algal leaf spot Alga Cephaleuros virescens

Symptoms

Raised, orange-red spots on both upper and lower surfaces of leaves; spots may coalesce to form irregularly shaped patches; spots may also be present on twigs and branches; when the surface of the spot is scraped away, a gray to dark necrotic crust is visible

Comments

Disease affects many fruit trees in the tropics; infection is unsightly but often harmless

Management

Ensure that trees are properly pruned and fertilized to promote vigor; remove all weeds from around tree bases; employ a wider tree spacing to increase air circulation around the trees; badly infested trees can be treated with copper containing fungicides

Anthracnose Fungus Glomerella cingulata

Symptoms

Chlorotic and necrotic spots; dead leaf tip; defoliation; brown or purple lesions on new shoots

Comments

Spores spread by rain splash; high moisture and warm temperatures encourage spread

Management

Prune dead twigs and branches from tree before fungi produce spores; knock dead leaves out of canopy; keep harvested fruit dry and cool

Avocado thrips Insect Scirtothrips perseae

Symptoms

Obvious feeding scars on fruit; scars begin as scabs or leathery patches and spread across fruit; adult insect is orange-yellow in color with distinct brown bands and reaches 0.7 mm (0.03 in) in length

Comments

Insect thrives in cooler temperatures; insect may undergo 6 or more generations per year

Management

Addition of coarse organic mulch about 6 inches thick below trees may help to reduce survival of thrips pupating in soil; if insecticides are to be applied, a selective insecticide should be selected to reduce damage to populations of natural enemies; Sprays of Entrust are organically acceptable

Bacterial soft rot Bacteria Erwinia herbicola
Erwinia carotovora

Symptoms

Gray to black, mushy, foul smelling rot on fruit; fruti has darkened metallic sheen

Comments

Bacteria may be present on leaves but do not cause damage unless plant is stressed or bacteria enter the plant through a wound

Management

No treatment for disease; use certified seed; disinfected tools and cuttings; employ crop rotation; remove plant debris from soil

Black streak Unknown Avocado black streak

Symptoms

Elongated black streaks on bark; cankers parallel to growth of limbs; black blotches with distinct margins on green wood; cankers on bark cause cracks which ooze sap; removal of bark over cankers reveals dark discoloration underneath

Comments

Disease emergence favors adverse growing conditions for avocado; more common in Guatemalan cultivars

Management

Avoid stressing trees by following good fertilization and irrigation practices

Persea mites Arachnid Oligonychus perseae

Symptoms

Tree dropping leaves and becoming defoliated causing sunburn damage to exposed bark and fruit; mites cause the development of circular chlorotic to brown spots on the undersides of the leaves and fruit surface; dense colonies of mites produce silk webbing which may appear as a silvery spot; large mite populations can cause the entire tree canopy to appear lighter in color

Comments

Persea mites are most damaging on Hass and Gwen varieties

Management

Ensure the tree is adequately fertilized, pruned properly and irrigated to avoid unnecessary stress to the tree which can make them more susceptible to mite attack; persistent infestations may require treatment with an appropriate chemical; organic controls include several types of horticultural oil

Phytophthora root rot Oomycete Phytophthora cinnamomi

Symptoms

Black lesions on roots; black, brittle roots; small, yellow leaves; premature leaf drop; decline in fruit yield

Comments

Wet soils encourage Phytophthora infection; plant in a well draining soil.

Management

Minimize water splash between trees by not working in a wet orchard; prune out dead limbs and twigs; remove fruit from the ground; dispose of dead wood and fruit away from trees

Scab Fungus Sphaceloma perseae

Symptoms

Oval or irregular brown or purple spots on fruit with rough texture

Comments

High humidity encourages scab growth and spread

Management

Plant tolerant varieties; spray with copper containing fungicides

Stem-end rot Fungi Many different fungal species. Depends on growing region. Mainly Botryosphaeria dothidea in the US.

Symptoms

Shrivelled tissue at stem end; dark brown or black lesions at stem end and eventually over entire fruit; fruit covered in mycelium

Comments

Environmental conditions may determine which fungal species is most common; spores can spread by wind or rain.

Management

Prune dead limbs and twigs; prune and harvest in dry conditions; provide trees with sufficient irrigation; apply a thick layer of mulch

Sunblotch Viroid Avocado sunblotch viroid (ASBVd)

Symptoms

Red, yellow, pink or white streaks running the length of young stems; white, yellow or red blotches on fruit

Comments

Can be transmitted by grafting

Management

Frequently sanitize all pruning equipment with disinfectant; plant only certified nursery stock

Western avocado leafroller (Amorbia) Insect Amorbia cuneana

Symptoms

Upper surface of leaves consumed, leaving thin brown membrane or leaves skeletonized; defoliation of tree; terminal leaves joined together by silk webbing; scarred fruit; young larvae are yellow-green and mature to dark green and a short, dark horizontal line on the side of their thorax; adult is a orange or tan moth with dark markings and bell-shaped wings

Comments

Females can lay 150-200 eggs during her lifetime; insect generally undergoes 3 generations per year

Management

Healthy avocado trees can tolerate feeding damage well but insect may become problematic if defoliation causes sunburn on fruit; applications of selective insecticides such as Bacillus thurengiensis help to conserve populations of natural enemies; pruning trees so that terminal foliage does not touch helps to prevent leafroller movement between trees

References

CABI Crop Protection Compendium. (2010). Persea americana datasheet. Available at: http://www.cabi.org/cpc/datasheet/39380. [Accessed 06 November 14]. Paid subscription required.

Crane, J. H., Balerdi, C. F. & Maguire, I. (1998). Avocado Growing in the Florida Home Landscape. University of Florida IFAS Extension. Available at: http://edis.ifas.ufl.edu/pdffiles/MG/.... [Accessed 06 November 14]. Free to access.

Ploetz, R. C., Zentmyer, G. A., Nishijima, W. T., Rohrbach, K. G. & Ohr, H. D. (Eds) (1994). Compendium of Tropical Fruit Diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.