Starfruit (Carambola)

Description

The Carambola tree, Averrhoa carambola, is a woody plant in the family Oxalidaceae grown for its fruit known as starfruit. Carambola trees are small, bushy evergreens with drooping branches. The tree possesses soft compound leaves composed of 2–5 leaflets which are green in color and arranged in a spiral around the branches. Each leaf is 15–20 cm (5.9–7.9 in) long and each leaflet 3.8–9 cm (1.5–3.5 in) long. The leaflets are oval or ovate in shape. The tree produces small lilac or purple flowers and The waxy fruit is orange-yellow in color, 7.5–12.5 cm (3.0–4.9 in) in length, oval and deeply ribbed. The Carambola tree has a lifespan of 40 years and reaches a height of 7–10 m (22–33 ft), spreading 6–7.6 m (20–25 ft) in diameter. Carambola may also be referred to as averrhoa, starfruit, five corner or coromandel gooseberry and originates from Southeast Asia.


Uses

Carambola fruit can be eaten fresh or cooked and the juice can be used in iced beverages. The fruit is also used in relishes or as a seasoning.


Propagation


Basic requirements
Carambola is a tropical to subtropical tree which grows best in warm to hot temperatures between 20 and 35°C (68–95°F). Trees will generally cease to grow at temperatures below 18.3°C (65°F) although established trees can withstand short periods at -2.8°C (27°F). Carambola can be grown successfully in many soil types, from heavy clay to sandy soils but will perform optimally in a rich, well-draining loam with a pH between 4.5 to 7.0 Trees will not tolerate waterlogging but do require a moist soil for optimum production.

Propagation
Carambola is commonly grown from seed although veneer grafting onto suitable rootstock yields the best results for commercial production. Seeds only remain viable for a few days and should be plump and fully developed for propagation. Seeds should be planted in a well-draining potting media such as peat moss and will germinate in approximately 7 days depending on temperature. Once seeds have germinated, they should be transplanted into individual containers containing sandy loam soil. The seedlings should be kept in these containers until they are transplanted. Carambola trees should be planted in full sun and spaced 9 m (30 ft) apart.

General care and maintenance
Carambola trees require moist soil for optimum production. Young trees should be irrigated regularly after planting to promote establishment in the soil. Older trees should be watered regularly from flowering to fruit set. A layer of mulch around the trees will help to conserve moisture in the soil but should not be mounded up against the trunks. Young carambola will benefit from applications of fertilizer containing nitrogen, phosphorous, potassium and magnesium every 30 to 60 days. Established, mature trees should be fertilized 4 to 6 times per year. Young trees should be pruned in the first 1–2 years after planting by cutting the tips off of branches which are greater than 60–90 cm (2–3 ft) in length to encourage branching. Older trees should be pruned to maintain a manageable height.

Harvesting
Carambola fruit does not ripen off the tree and should be harvested when fully mature. Fruits are ready to harvest when they have turned from green to yellow in color, with the tips of the ribs remaining green. Fully ripe fruit naturally fall from the tree but can be picked earlier if desired.


References

Crane, J. H. (2007). Carambola growing in the Florida home landscape. University of Florida IFAS Extension. Available at: http://edis.ifas.ufl.edu/mg269. [Accessed 15 April 15]. Free to access.

Morton, J. (1987). Carambola. In: Fruits of warm climates. Julia F. Morton, Miami, FL. Available at: https://www.hort.purdue.edu/newcrop/m.... [Accessed 15 April 15]. Free to access.


Common Pests and Diseases

Algal disease
Cephaleuros virescens

Symptoms
Orange, rusty pustules on leaves, stems, twigs and fruit; swelling tissue; leaves on infected twigs wilting and turning yellow; dieback of shoots
Cause
Alga
Comments
Not usually a major problem but can be damaging when it emerges in unmanaged plantations; spread by water splash; disease emergence favors wet, humid conditions and poor air circulation
Management
Maintain proper irrigation, pruning and fertilization regimes in carambola plantations; appropriate copper based fungicides may be required to control the disease in severely infected plantations

Pythium root rot
Pythium splendens
Pythium ultimum

Symptoms
Canopy has sparse appearance; wilting during periods of water stress; foliage may show symptoms of nutrient deficiencies
Cause
Fungi
Comments
Disease most damaging during cool weather which is not optimum for the tree
Management
Plant only disease-free nursery stock; plant in areas with no history of the disease; avoid planting trees in low lying areas

Cercospora leaf spot
Cercospora averrhoae

Symptoms
Tiny necrotic or chlorotic spots on leaflets; spots grow larger and developgray-white centers, reddish-brown margins and chlorotic halos; spots coalesce to form large lesions; leaves turn yellow and drop from palm
Cause
Fungus
Comments
Spores transmitted by rainsplash, wind, insects and irrigation water
Management
Plant carambola varieties that are more tolerant of the disease in areas where disease is present; disease can be controlled with regular applications of appropriate foliar fungicides

Alternaria black spot (Brown spot)
Alternaria alternata

Symptoms
Small, circular light brown or black spots on skin of fruits; lesions develop sunken centres and olive-brown spores
Cause
Fungus
Comments
Fungi spread via wind and rain and enter plant via wounds
Management
Avoid wounding fruits during harvest

Anthracnose
Colletotrichum gleosporoides

Symptoms
Tiny, slightly depressed light to dark brown spots which expand and make fruit soft; spots coalesce to form large irregular lesions; oranges spore masses may develop; dark brown, oval shaped lesionunguss may develop on leaves
Cause
Fungus
Comments
Fungus is spread by wind, rain splash and insects
Management
Avoid wounding fruits during harvest

Flyspeck
Zygophiala jamaicensis

Symptoms
Small black dots in roughly circular pattern on surface of fruit which can be rubbed off
Cause
Fungus
Comments
Symptoms resemble fly feeding damage; disease emerges in warm, wet conditions
Management
No specific treatments are currently recommended