Clove

Description

Clove, Syzygium aromaticum, is a monoecious (both male and female flowers on the same plant) evergreen tree in the family Myrtaceae grown for its aromatic flowers. The many branches of the tree are semi-erect with smooth oval shaped leaves. The branches end with a 3–4 flowers near the tip with one terminal flower and the others opening below it. The leaves, flowers and bark all have a distinct smell. The clove is the unopened flower buds. The tree grows 8–15 m (26–50 ft) tall and can live to be more than 100 years old. The clove tree may also be referred to as tropical myrtle and are native to the Molucca Islands.


Uses

The vast majority of commercially cultivated cloves are used by the tobacco industry to flavor cigarettes. Cloves are also used commonly as spices, either in their whole form or after first grinding into powder.


Propagation


Basic requirements
Clove trees require a warm, tropical climate with an average rainfall of at least 1500 mm per year. Clove trees are very susceptible to stress. Areas that undergo a dry season are good for flowering but the tree must be planted in an area with deep, fertile soil to limit water stress. Clove trees grow optimally at temperatures between 16 and 27°C (65–80°F) in rich loamy, well draining soil.

Propagation
Clove is commercially propagated from seeds which are planted soon after harvest. Seeds should be collected and extracted from the fruits of healthy mother plants exhibiting desirable characteristics. The seeds are extracted by soaking the fruits in water and peeling the skin from the fruit. The seeds can be planted in prepared nursery beds or polyethylene bags containing a mixture of soil and aged manure and should be planted to a depth of 2 to 5 cm (0.8-2.0 in) and spaced 12 to 15 cm (4.7-5.9 in) apart. Germination usually occurs within 1 to 6 weeks. The seedlings should be shaded to protect them from harsh sunlight. The seedlings should be kept moist through regular watering and can be transplanted when they reach at least 30 cm (11.8 in) in height. The seedlings should be hardened off by exposing them to increasing amounts of sunlight before the are transplanted to the field.

Transplanting
Young clove trees should be planted in pre-dug pits which are approximately 60 × 60 ×60 cm (24 × 24 × 24 in), or large enough to accommodate the root ball. The recommended spacing for clove trees is 8 m (26 ft) but closer spacings are commonly used. Trees planted in the field should be provided with temporary shading to alleviate stress. Shade can be provided through intercropping with other crops such as banana, cassava or coconut but trees such as Gliricidia are also used as these can be pruned to alter the amount of light reaching the cloves throughout the year.

General care and maintenance
Once the temporary shade plants are removed, the plantation should be kept free from weeds by weeding once or twice each year or by applying a layer of mulch around the trees. Mulch helps prevent the roots being damaged by the physical removal of the weeds. Trees may require additional irrigation during dry periods to prevent them becoming stressed which harms their production. The trees should also be provided with nutrients in the form of fertilizer or manure. The composition and amount of fertilizer required is dependent on the region and soil type.

Harvesting
The complete inflorescence (flower) should be picked just before the first buds open to ensure maximum size and oil content of the buds. The harvest is often conducted over 3 to 8 pickings during the season as buds mature. After harvesting, the buds are laid out to dry in the sun for several days.


References

CABI Crop Protection Compendium. (2008). Syzygium aromaticum datasheet. Available at: http://www.cabi.org/cpc/datasheet/52412. [Accessed 11 November 14]. Paid subscription required.

KAU Agri. (2013). Clove (Syzygium aromaticum). Kerala Agricultural University. Available at: http://www.celkau.in/Crops/Spices/Clo.... [Accessed 11 November 14]. Free to access.


Common Pests and Diseases

Sumatra disease
Ralstonia syzygii

Symptoms
Dieback of trees which begins in the crown and leads to tree death within 3 years of initial infection; leaves turn chlorotic and drop from the tree or may wilt and remain attached; discoloration of vascular tissues evident as gray-brown streaks in new wood; bacterial exudate may ooze out of tissue when cut
Cause
Bacterium
Comments
Bacteria are limited to the water-carrying vessels in the tree (xylem); the disease is thought to be transmitted by Hindola striata and Hindola fulva, both sucking insect species
Management
An antibiotic - oxytetracycline - can be injected into the tree to slow the decline of infected trees but there is currently no known cure for the disease; several insecticides can give control of Hindola insect species which are believed to spread the disease

Dieback (Eucalyptus canker)
Cryphonectria cubensis

Symptoms
Once fungus enters a wound on a branch it moves downward until it reaches the branch junction; all branches above the junction die back
Cause
Fungus
Comments
Fungus attacks wounded areas of the plant
Management
Avoid causing damage to trees with machinery and tools; protect pruning wounds with appropriate fungicide; infected parts of trees should be pruned out and burned; any resultant wounds should be treated with fungicide paste

Coconut scale
Aspidiotus destructor

Symptoms
Pale yellow spots on leaves; entire leaves yellowing; leaves turning brown and dropping prematurely; adult insect is a flattened oval, resembling a scale, which is red-brown in color
Cause
Insect
Comments
Insect also attacks other crops such as coconut, tea and mango
Management
May be possible to control coconut scale by pruning infested parts of trees and destroying by burning; chemical control may be necessary

Oriental fruit fly
Bactrocera dorsalis

Symptoms
Dark brown to black lesions on foliage; adult insect is a small fly with a bright yellow abdomen with a distinctive dark "T-shape" pattern; larvae are cream-white maggots which burrow into fruit
Cause
Insect
Comments
Oriental fruit flies are serious pests of many crops including mango, avocado, guava, pineapple and papaya
Management
One of the most effective methods of controlling the oriental fruit fly on many crops is to bag the fruit in paper bags or polythene sleeves to prevent oviposition by female flies; chemical sprays are most effective when combined with a protein bait to attract the insect

Soft scale
Ceroplastes floridensis

Symptoms
Reduction in plant vigor and viability; wilting leaves with abnormal coloration; stick residue on leaves; insect is a a rounded scale covered in white or pink-white waxy substance
Cause
Insect
Comments
Large populations of soft scale promote the development of sooty mold
Management
Natural enemies of the soft scale can generally provide good control of the pest; application of horticultural oil may be applied if scales become problematic; healthy plants are less susceptible to scale damage than those which are stressed so adequate irrigation and fertilization are important in scale management

Nematodes (Ring nematode, Reniform nematode, Common spiral nematode)
Criconemoides spp.
Rotylenchulus reniformis
Helicotylenchus dihystera

Symptoms
Dark lesions on roots; stunted plant growth; yellow leaves; slow growth and low yield

Cause
Nematode
Comments
Damage from nematodes often promotes root infection by bacteria and fungi
Management
Solarizing soil can reduce nematode populations in the soil and levels of inoculum of many other pathogens