Growing Food: A Guide to Food Production

Day length: long days favour vine (stem) development, short days favour tuber
growth.
Temperature: yams do not tolerate frost and grow poorly below 25C. D. opposita
is the most tolerant species to cold. Should not be stored below about 10 C.
Rainfall: optimum is 1500 mm per year or more, though some species can survive
with 1000 mm. There should ideally be at least 2–4 months of dry season.
Pests: in West Africa Yam Beetles are the worst; adults feed on the tubers, making
round 1–2 cm lesions and may also damage the growing points of newly planted
setts. Other pests include the Yam Weevil and Scale Insects. The Yam Nematode,
which is also a storage pest, can be controlled with rotation and by only planting
clean tubers.
Diseases: rarely a serious problem. Leaf Spot is the most common, causing brown
or black spots on the leaves and stems. Anthracnose and Witches Broom occur in
West Africa, and Rust occurs everywhere. Mosaic virus diseases have been reported
in West Africa and the Caribbean, controlled by destroying all infected plants.
Storage rots can cause major losses, though the severity depends on the species,
variety, degree of injury of the tubers and the number of yam nematodes that are
present.

YIELD

crop management, species and variety.
The global average is about 10 MT/ha (9.1 MT/ha in 2004, according to FAO),
but yields of about 20–30 MT/ha are common. National average yields in 2004
ranged from a high of 22.3 MT/ha in the Solomon Islands to 2.7 MT/ha in Ruanda.
The most commonly grown yam species produce 1, 2 or 3 tubers, each weighing
from 5 to 15 kg. Some plants produce up to 50 kg of tubers.

UTILISATION

• Yams are grown almost exclusively for the human consumption of the tubers.
  Small tubers can be peeled, then boiled or roasted; larger tubers are also peeled,
  cut into smaller pieces, then either fried in oil, roasted or boiled. They are
  sometimes added to stews. The tubers can be stored for longer periods if dried
  up and ground into flour. In West Africa, they are normally eaten as “fufu”,
  peeled, cut up and boiled tubers pounded in a wooden mortar.


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  proportions of diosgenin, a precursor for the synthesis of fertility and anti-
  inflammatory drugs.

  
  

LIMITATIONS

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220 TONY WINCH

Some wild species such as D. floribunda and D. villosa contain usable

Yams are very labour intensive, for both cultivation and food preparation.

The tubers are often rather expensive to buy.

The plants require high rainfall, or irrigation, and fertile and well drained soil.

The plants are also sensitive to frost.

Fungi and/or nematodes can often cause large storage losses.

Some species contain toxic alkaloids.

Modest energy value of about 100 calories per 100mg edible portion.

There is enormous variation in yields of yam, according to the growing conditions,