Cocoyam

Description

Cocoyams are herbaceous perennial plants belonging to the family Araceae and are grown primarily for their edible roots, although all parts of the plant are edible. Cocoyams that are cultivated as food crops belong to either the genus Colocasia or the genus Xanthosoma and are generally comprised of a large spherical corm (swollen underground storage stem), from which a few large leaves emerge. The petioles of the leaves stand erect and can reach lengths in excess of 1 m (3.3 ft). The leaf blades are large and heart-shaped and can reach 50 cm (15.8 in) in length. The corm produces lateral buds which give rise to tubers or cormels and suckers or stolons. Cocoyams commonly reach in excess of 1 m (3.3 ft) in height and although they are perennials, they are often grown as annuals, harvested after one season. Colocasia species may also be referred to as taro, old cocoyam, arrowroot, eddoe, macabo or dasheen and originates from Southeast or Central Asia. Xanthosoma species may be referred to as tannia, yautia, new cocoyam or Chinese taro and originates from Central and South America.


Uses

Cocoyam is most commonly grown for its starchy edible roots. Colocasia is grown for its corm which is consumed after boiling, frying or roasting. The corms can be dried and used to make flour or sliced and fried to make chips. The leaves of the plant are also edible and are usually consumed as a vegetable after cooking in dishes such as stews. Xanthosoma species produce tubers much like potato and are boiled, baked, steamed or fried prior to consumption. The corm of some varieties is also consumed. Young leaves are eaten as a vegetable.


Propagation


Basic requirements
Cocoyam grows best in fertile, well-draining, sandy loam soil with a pH between 4.2–7.5.It can be grown in a wide variety of conditions including paddies in wetland areas using a system similar to that of rice. Xanthosoma species require temperatures above 21°C (69.8°F) to grow properly. Unlike Colocasia spp, they will not tolerate waterlogging and grow best in deep, well-draining loams with a pH between 5.5 and 6.5 in partial shade. Cocoyam will thrive when planted in full sunlight or partial shade. The plants can survive for short periods at temperatures of 10°C (50°F) but will be damaged or killed by lower temperatures.

Propagation
Cocoyam is vegetatively propagated from headsetts (“tops”) or suckers which establish quickly and give the highest rate of survival. Larger headsetts and suckers tend to produce larger corms and bigger yeilds but the size of the planting material may be determined by the particular cultivar being grown e.g. some varieties will produce two heads from the same corm if the sucker that is planted is too big and therefore medium sized suckers are selected when growing tubers for export. Headsetts and suckers should only be taken from healthy plants in order to protect yields and prevent the spread of diseases.

Planting
Cocoyam is planted in such a way as to encourage sucker growth e.g. the use of larger plant spacing and shallow planting depths. The planting material (sucker or headsett) is set in furrows or ridges and plant spacing can be anywhere between 30 to 100 cm (11.8–39.4 in) between plants depending on the prevailing soil and climatic conditions. A narrow spacing helps to control weeds whereas a wider spacing is preferred for good growth.


References

Anon. Growing Cocoyam in Nigeria. International Institute for Tropical Agriculture. Commercial Crop Production Series. Available at: http://www.cassavabiz.org/agroenterpr.... [Accessed 13 November 14]. Free to access.

CABI Crop Protection Compendium. (2008). Xanthosoma datasheet. Available at: http://www.cabi.org/cpc/datasheet/56986. [Accessed 13 November 14]. Paid subscription required.

FAO. Pacific Root Crops. Pacific Food Security Toolkit. Available at: http://www.fao.org/docrep/013/am014e/.... [Accessed 13 November 14]. Free to access.

Wilson, J. W. (1987). Taro planting material. USP Institute for Research, Extension and Training in Agriculture (IRETA). Available at: http://www.adap.hawaii.edu/adap/Publi.... [Accessed 13 November 14]. Free to access.



Common Pests and Diseases

Pythium root and corm rot
Pythium spp.

Symptoms
Soft, mushy rot on corm with foul odor; root system may be completely destroyed; plants stunted with shortened leaf stalks; leaves curled, crinkled and yellowish in color; when corm is cut open rotted area is sharply delineated from healthy tissue
Cause
Fungi
Comments
Disease emergence appears to favor the presence of warm, stagnant water where cocoyam is grown in wetlands; poor field sanitation practices aid in the spread of the disease
Management
Some varieties are more resistant to the disease than others (e.g. Hawaiian varieties Kai Kea and Kai uliuli) and should be planted if disease is known to be a problem in the area; root rot severity can be reduced in acid soils by the addition of the fungicide Captan prior to planting

Phyllosticta leaf spot
Phyllosticta colocasiophila

Symptoms
Oval or irregular beige to reddish brown spots on leaves; dark brown spots with chlorotic area around lesion; holes in leaves where lesion centers have dried and dropped out
Cause
Fungus
Comments
Disease emergence favored by cloudy, rainy weather
Management
No control is needed unless the disease is causing severe defoliation of the plants; remove and destroy any diseased leaves

Taro beetle
Papuana spp.
Eucopidocaulus spp.

Symptoms
Tunnels bored in corms; tunnels may join together to form large cavities; large populations of insects may bcause the plants to wilt and die; adult insect is a shiny black scarab beetle
Cause
Insect
Comments
Beetle is a major pest of taro in Papua New Guinea and Fiji
Management
The lack of chemical pesticides registered for use on taro beetles means that control relies largely on populations of natural enemies and cultural control methods; taro gardens in Papua New Guinea are often flooded in an attempt to control the pest; farmers may also apply wood ash to the crop

Pink hibiscus mealybug
Maconellicoccus hirsutus

Symptoms
Stunted growth of plants at growing tips; leaves may be distorted; insects are soft bodied elongated ovals which are pink in color and covered in a white waxy substance; adults are approximately 2-4 mm in length
Cause
Insect
Comments
Pink hibiscus mealybugs have a very wide host range and may also be found on other crops such as cassava and mango as well as leguminous plants
Management
Pesticides are often largely ineffective at controlling the insects as they hide in crevices on the plants and avoid contact with the chemical; in the Caribbean, predatory beetles have been very successful at controlling populations of mealy bugs

Phytophthora leaf blight
Phytophthora colocasiae

Symptoms
Small purple or dark brown, circular, water-soaked lesions on leaves which enlarge, turn purple-brown and may coalesce; lesions may exude clear amber fluid from center; lesions may from concentric pattern and develop white fuzz on both sides of leaves; holes may form in leaves where lesions have dried and dropped out; leaves collapse and die; disease may occur as a post-harvest rot of corms and causes large areas of dark gray to blue rot with indistinct margins
Cause
Oomycete
Comments
Disease emergence favored by rainy overcast weather with low night temperatures; disease spread primarily by splashing rain water
Management
Planting resistant varieties is the most effective method of controlling the disease; if plants do become infected, symptomatic leaves should be removed and destroyed to reduce the level of inoculum; plant cocoyam in well-draining soils; disease may also be controlled by protective sprays of appropriate systemic or non-systemic fungicides where available

Dasheen mosaic
Dasheen mosaic virus (DsMV)

Symptoms
Mosaic pattern on leaves; mild to moderately distorted leaves
Cause
Virus
Comments
Transmitted by aphids; symptoms more pronounced in cooler temperatures; infected plants may show no symptoms of disease fro several months after infection
Management
The best method of control appears to be planting resistant varieties

Alomae-bobone
Colocasia bobone disease virus (CBDV)

Symptoms
Leaves thickened, rolled and often brittle; leaf veins are prominent and develop enations; leaf petioles are shortened and also develop enations; plant may recover (bobone disease)

Leaves thickened and dark green in color; distorted areas may develop on leaves and leaves may be cupped (but not rolled as in bobone disease); usually only one to three leaves affected (alomae disease)
Cause
Virus
Comments
Taro plants have been traditionally classified as 'male' or 'female' based on their susceptibility to two diseases, both of which are caused by Colocasia bobone disease virus (CBDV), which are known as bobone and alomae; 'male' plants are susceptible to alomae and die as a result of the infection, but are resistant to bobone; 'female' plants are resistant to alomae but susceptible to bobone from which they recover; the virus is transmitted by leafhoppers
Management
Remove any plants showing symptoms of disease and destroy by burning; removal of outer leaf sheaths may control the disease by reducing the number of leafhopper eggs