Yams

Description

Yam is the name given to several plant species in the genus Dioscorea including Dioscorea alata (white yam), Dioscorea bulbifera (potato yam), Dioscorea cayenensis (yellow yam), Dioscorea esculenta (Asiatic yam) and Dioscorea batatas (Chinese yam) that are grown for their edible tubers. These species are not to be confused with the sweet potato, Ipomoea batatas, which is often referred to as a yam in the US. Yam plants are herbaceous annual or perennials with climbing or trailing vines. The vines can be smooth or prickly, reaching 10 m (32.8 ft) or more in length depending on the variety. The leaves of the plant are simple and usually oval to heart-shaped with petioles which are the same length, or slightly longer, than the leaf blade itself. Some varieties possess spikes at the bases of the leaves. The plant can produce one singular tuber or several tubers which extend from stolons from a central corm (up to 20) depending on the species. The tubers can be cylindrical, curved or lobed, with brown, grey, black or pink skin and white, orange or purplish flesh. Most yams are annual plants, harvested after one season, but some are perennial with tubers increasing in size each year with the vines dying back at the end of the growing season and regrowing on the return of favorable conditions. The origin of yams is uncertain and genetic information suggests that there may be more than one point of origin.


Uses

Yams are used differently in different parts of the world. They are consumed after cooking by frying, boiling or roasting. The green parts of some plants can be cooked and consumed as a vegetable. Yams may also be used to produce flour or starch.


Propagation


Basic requirements
Yams are mainly grown in tropical and subtropical climates and they do not grow well at temperatures below 22°C (71.6°F) and are killed by frost.. The optimum temperature for the growth of yams is between 25 and 30°C (77–86°F). They grow optimally in well-draining fertile soils with a pH between 5.5 and 6.5 in full sun or part shade. Very wet soils should be avoided as this promotes tuber rot.

Propagation
Yams are propagated vegetatively from small tubers. Land should be prepared for planting by plowing and harrowing. Tubers should be planted in trenches to a depth of 15 cm (6 in) allowing at least 30 cm (12 in) between individual plants and 1.5 m (5 ft) between rows. The soil is often mounded around plants or ridged to aid drainage. It is common practice to stake plants with a 2–4 m (6.6–13.2 ft) support to allow them to climb and ensure that all parts of the plant receive adequate sunlight.

General care and maintenance
Yams require 100 cm of water distributed evenly throughout the growing season. Yam plants should be mulched after planting to prevent plants from drying out. Failure to mulch the plants will result in drastic decreases in yield.

Harvesting
Yams can be harvested at any time after the leaves have started to yellow. The soil should be carefully dug around the tuber and the the tuber cut from the vine. Harvesting is best carried out on sunny, dry days to prevent tuber rot.


References

Farmer's bookshelf. Yam. A University of Hawaii Department of Tropical Plant & Soil Sciences University of Hawaii at Manoa. Available at: http://www.ctahr.hawaii.edu/fb/yam/ya.... [Accessed 21 April 15]. Free to access


Common Pests and Diseases

Anthracnose (Scorch)
Colletotrichum gleosporoides

Symptoms
Small, dark brown spots or black lesions on leaves which may be surrounded by a chlorotic halo; leaf necrosis; dieback of stem; withered leaves and scorched appearance
Cause
Fungus
Comments
Disease overwinters in plant debris; occurs worldwide
Management
The most effective method of controlling the disease is to plant yam varieties that are resistant to anthracnose such as TDA 291 or TDA 297

Mealybugs
Rastrococcus spp.

Symptoms
Flattened oval to round disc-like insect covered in waxy substance on tree branches; insects attract ants which may also be present; insect colony may also be associated with growth of sooty mold due to fungal colonization of sugary honeydew excreted by the insect
Cause
Insect
Comments
Insects have a wide host range; often tended by ants which farm them for their sugary honeydew secretions
Management
Prune out heavily infested branches; mealybugs can potentially be controlled by natural enemies; chemical pesticides may decrease populations of natural enemies leading to mealybug outbreaks; horticultural oils or soapy solutions can be used to treat heavy infestations

White Scale insects
Aspidiella hartii

Symptoms
The leaves and tubers are covered with small white scales from field to storage. Even though it won't effect yield sometimes foliage cause poor growth and tubers may show delay in germination or even stopped. Severe infestation may leads to tuber shrivel.
Cause
Insect
Comments

Root Knot Nematode
Meloidogyne incognita(Kofoid & White)

Symptoms
The infected plants are stunted with poor growth. The leaves turn yellow in color. Tubers and feeder roots are galled. Tubers are deformed and develop abnormal rootlets. Reduction in edible portion of tubers.
Cause
Nematode
Comments
It reduces market value and quality of tubers (up to 32 - 59 % in Nigeria). The actual yield loss is estimated up to 27 to 55 %.
Management
Deep summer ploughing to expose and kill nematode. Follow crop rotation with non host crops like groundnut and maize.

Dry rot disease (caused by yam nematode)
Scutellonema bradys

Symptoms
The infected tubers show dry rot of 1 to 2 cm. Initially this dry rot is of cream and light yellow lesions appear just below the outer skin without any external symptom. With progress in disease lesion spreads deeper (maximum up to 2 cm). At later stage the rot become light and dark brown to black in color and tubers may show external cracks. Entry of fungus through this wounds causes further decay of tubers in storage.
There is no above ground symptom with yam nematode infestation.
Cause
Nematode
Comments
Some time the infected tubers may not show external cracking which make it difficult to diagnose. In that case scrap out the external layer of tuber to check the disease incidence.
Management
Use disease free tubers/setts for planting. Treating tubers with hot water for 40 min at 50-55 C before sowing and after harvest to reduce disease both in field and storage. In Africa smearing tubers with wood ash or cow dung shows reduced nematode infection in field. Follow crop rotation with non host or antagonist crops like ground nut, sorghum, maize, chill pepper etc.

Yam mosaic disease
Yam mosaic potyvirus

Symptoms
The common symptoms are infected leaves show yellow and green patterns (called mosaics) between the veins or may show a narrow green strips bordering the veins (called vein banding). If the disease is severe the leaves become long, thin and strap shape (called shoe-string symptom) and whole plant become stunted. Plant may produce few small tubers with less starch content.
Some plants may recover from the virus infection soon after first symptom but virus may survive in plant and reduce the vigour.
Cause
Virus
Comments
The virus is transmitted by aphids and tubers/setts. It may cause up to 40% loss in yield. Yam mosaic virus is always associated with yam mild mosaic virus, yam badnaviruses and cucumber mosaic virus in Africa making this disease more complex.
Disease is reported in West Africa , South America and Caribbean.
Management
Use healthy and disease free tubers or setts for planting. Select healthy and large tubers for planting instead of small tubers. Keep fields free from weeds. Collect crop debris and destroy them.